First pass at implementation and testing of reusable builders.

36 files changed, 10475 insertions, 2404 deletions

diff --git a/src/AddressBook/AddressBookProtos.cs b/src/AddressBook/AddressBookProtos.cs
index 4d391f0f..8bb19df6 100644
--- a/src/AddressBook/AddressBookProtos.cs
+++ b/src/AddressBook/AddressBookProtos.cs
@@ -223,7 +223,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(PhoneNumber prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -233,21 +233,48 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new PhoneNumber();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(PhoneNumber cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

           

-          PhoneNumber result = new PhoneNumber();

+          bool builderIsReadOnly;

+          PhoneNumber result;

+          

+          private PhoneNumber PrepareBuilder() {

+            if (builderIsReadOnly) {

+              PhoneNumber original = result;

+              result = new PhoneNumber();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override PhoneNumber MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new PhoneNumber();

+            result = DefaultInstance ?? new PhoneNumber();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -259,12 +286,11 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
           }

           

           public override PhoneNumber BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            PhoneNumber returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -278,6 +304,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
           

           public override Builder MergeFrom(PhoneNumber other) {

             if (other == global::Google.ProtocolBuffers.Examples.AddressBook.Person.Types.PhoneNumber.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasNumber) {

               Number = other.Number;

             }

@@ -293,6 +320,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -361,11 +389,13 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
           }

           public Builder SetNumber(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasNumber = true;

             result.number_ = value;

             return this;

           }

           public Builder ClearNumber() {

+            PrepareBuilder();

             result.hasNumber = false;

             result.number_ = "";

             return this;

@@ -379,11 +409,13 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
             set { SetType(value); }

           }

           public Builder SetType(global::Google.ProtocolBuffers.Examples.AddressBook.Person.Types.PhoneType value) {

+            PrepareBuilder();

             result.hasType = true;

             result.type_ = value;

             return this;

           }

           public Builder ClearType() {

+            PrepareBuilder();

             result.hasType = false;

             result.type_ = global::Google.ProtocolBuffers.Examples.AddressBook.Person.Types.PhoneType.HOME;

             return this;

@@ -527,7 +559,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(Person prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -537,21 +569,48 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new Person();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(Person cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      Person result = new Person();

+      bool builderIsReadOnly;

+      Person result;

+      

+      private Person PrepareBuilder() {

+        if (builderIsReadOnly) {

+          Person original = result;

+          result = new Person();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override Person MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new Person();

+        result = DefaultInstance ?? new Person();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -563,13 +622,12 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }

       

       public override Person BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.phone_.MakeReadOnly();

-        Person returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -583,6 +641,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       

       public override Builder MergeFrom(Person other) {

         if (other == global::Google.ProtocolBuffers.Examples.AddressBook.Person.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasName) {

           Name = other.Name;

         }

@@ -604,6 +663,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -672,11 +732,13 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }

       public Builder SetName(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasName = true;

         result.name_ = value;

         return this;

       }

       public Builder ClearName() {

+        PrepareBuilder();

         result.hasName = false;

         result.name_ = "";

         return this;

@@ -690,11 +752,13 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
         set { SetId(value); }

       }

       public Builder SetId(int value) {

+        PrepareBuilder();

         result.hasId = true;

         result.id_ = value;

         return this;

       }

       public Builder ClearId() {

+        PrepareBuilder();

         result.hasId = false;

         result.id_ = 0;

         return this;

@@ -709,18 +773,20 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }

       public Builder SetEmail(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasEmail = true;

         result.email_ = value;

         return this;

       }

       public Builder ClearEmail() {

+        PrepareBuilder();

         result.hasEmail = false;

         result.email_ = "";

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.Examples.AddressBook.Person.Types.PhoneNumber> PhoneList {

-        get { return result.phone_; }

+        get { return PrepareBuilder().phone_; }

       }

       public int PhoneCount {

         get { return result.PhoneCount; }

@@ -730,29 +796,35 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }

       public Builder SetPhone(int index, global::Google.ProtocolBuffers.Examples.AddressBook.Person.Types.PhoneNumber value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.phone_[index] = value;

         return this;

       }

       public Builder SetPhone(int index, global::Google.ProtocolBuffers.Examples.AddressBook.Person.Types.PhoneNumber.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.phone_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddPhone(global::Google.ProtocolBuffers.Examples.AddressBook.Person.Types.PhoneNumber value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.phone_.Add(value);

         return this;

       }

       public Builder AddPhone(global::Google.ProtocolBuffers.Examples.AddressBook.Person.Types.PhoneNumber.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.phone_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangePhone(scg::IEnumerable<global::Google.ProtocolBuffers.Examples.AddressBook.Person.Types.PhoneNumber> values) {

+        PrepareBuilder();

         base.AddRange(values, result.phone_);

         return this;

       }

       public Builder ClearPhone() {

+        PrepareBuilder();

         result.phone_.Clear();

         return this;

       }

@@ -869,7 +941,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(AddressBook prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -879,21 +951,48 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new AddressBook();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(AddressBook cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      AddressBook result = new AddressBook();

+      bool builderIsReadOnly;

+      AddressBook result;

+      

+      private AddressBook PrepareBuilder() {

+        if (builderIsReadOnly) {

+          AddressBook original = result;

+          result = new AddressBook();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override AddressBook MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new AddressBook();

+        result = DefaultInstance ?? new AddressBook();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -905,13 +1004,12 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }

       

       public override AddressBook BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.person_.MakeReadOnly();

-        AddressBook returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -925,6 +1023,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       

       public override Builder MergeFrom(AddressBook other) {

         if (other == global::Google.ProtocolBuffers.Examples.AddressBook.AddressBook.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.person_.Count != 0) {

           base.AddRange(other.person_, result.person_);

         }

@@ -937,6 +1036,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -985,7 +1085,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.Examples.AddressBook.Person> PersonList {

-        get { return result.person_; }

+        get { return PrepareBuilder().person_; }

       }

       public int PersonCount {

         get { return result.PersonCount; }

@@ -995,29 +1095,35 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }

       public Builder SetPerson(int index, global::Google.ProtocolBuffers.Examples.AddressBook.Person value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.person_[index] = value;

         return this;

       }

       public Builder SetPerson(int index, global::Google.ProtocolBuffers.Examples.AddressBook.Person.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.person_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddPerson(global::Google.ProtocolBuffers.Examples.AddressBook.Person value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.person_.Add(value);

         return this;

       }

       public Builder AddPerson(global::Google.ProtocolBuffers.Examples.AddressBook.Person.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.person_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangePerson(scg::IEnumerable<global::Google.ProtocolBuffers.Examples.AddressBook.Person> values) {

+        PrepareBuilder();

         base.AddRange(values, result.person_);

         return this;

       }

       public Builder ClearPerson() {

+        PrepareBuilder();

         result.person_.Clear();

         return this;

       }

diff --git a/src/ProtoGen/EnumFieldGenerator.cs b/src/ProtoGen/EnumFieldGenerator.cs
index 8b56b9e9..84e677ed 100644
--- a/src/ProtoGen/EnumFieldGenerator.cs
+++ b/src/ProtoGen/EnumFieldGenerator.cs
@@ -70,11 +70,13 @@ namespace Google.ProtocolBuffers.ProtoGen
             writer.WriteLine("}");

             AddClsComplianceCheck(writer);

             writer.WriteLine("public Builder Set{0}({1} value) {{", PropertyName, TypeName);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.has{0} = true;", PropertyName);

             writer.WriteLine("  result.{0}_ = value;", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             writer.WriteLine("public Builder Clear{0}() {{", PropertyName);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.has{0} = false;", PropertyName);

             writer.WriteLine("  result.{0}_ = {1};", Name, DefaultValue);

             writer.WriteLine("  return this;");

diff --git a/src/ProtoGen/MessageFieldGenerator.cs b/src/ProtoGen/MessageFieldGenerator.cs
index b86f2c9a..fab0f29e 100644
--- a/src/ProtoGen/MessageFieldGenerator.cs
+++ b/src/ProtoGen/MessageFieldGenerator.cs
@@ -68,18 +68,21 @@ namespace Google.ProtocolBuffers.ProtoGen
             writer.WriteLine("}");

             writer.WriteLine("public Builder Set{0}({1} value) {{", PropertyName, TypeName);

             AddNullCheck(writer);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.has{0} = true;", PropertyName);

             writer.WriteLine("  result.{0}_ = value;", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             writer.WriteLine("public Builder Set{0}({1}.Builder builderForValue) {{", PropertyName, TypeName);

             AddNullCheck(writer, "builderForValue");

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.has{0} = true;", PropertyName);

             writer.WriteLine("  result.{0}_ = builderForValue.Build();", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             writer.WriteLine("public Builder Merge{0}({1} value) {{", PropertyName, TypeName);

             AddNullCheck(writer);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  if (result.has{0} &&", PropertyName);

             writer.WriteLine("      result.{0}_ != {1}) {{", Name, DefaultValue);

             writer.WriteLine("      result.{0}_ = {1}.CreateBuilder(result.{0}_).MergeFrom(value).BuildPartial();", Name,

@@ -91,6 +94,7 @@ namespace Google.ProtocolBuffers.ProtoGen
             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             writer.WriteLine("public Builder Clear{0}() {{", PropertyName);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.has{0} = false;", PropertyName);

             writer.WriteLine("  result.{0}_ = {1};", Name, DefaultValue);

             writer.WriteLine("  return this;");

diff --git a/src/ProtoGen/MessageGenerator.cs b/src/ProtoGen/MessageGenerator.cs
index 01d47ce2..f191da4f 100644
--- a/src/ProtoGen/MessageGenerator.cs
+++ b/src/ProtoGen/MessageGenerator.cs
@@ -551,7 +551,7 @@ namespace Google.ProtocolBuffers.ProtoGen
             writer.WriteLine("public override Builder ToBuilder() { return CreateBuilder(this); }");

             writer.WriteLine("public override Builder CreateBuilderForType() { return new Builder(); }");

             writer.WriteLine("public static Builder CreateBuilder({0} prototype) {{", ClassName);

-            writer.WriteLine("  return (Builder) new Builder().MergeFrom(prototype);");

+            writer.WriteLine("  return new Builder(prototype);");

             writer.WriteLine("}");

             writer.WriteLine();

             writer.WriteLine("[global::System.Diagnostics.DebuggerNonUserCodeAttribute()]");

@@ -582,21 +582,52 @@ namespace Google.ProtocolBuffers.ProtoGen
 

         private void GenerateCommonBuilderMethods(TextGenerator writer)

         {

-            writer.WriteLine("public Builder() {{}}", ClassAccessLevel);

+            //default constructor

+            writer.WriteLine("public Builder() {");

+            //Durring static initialization of message, DefaultInstance is expected to return null.

+            writer.WriteLine("  result = DefaultInstance ?? new {0}();", ClassName);

+            writer.WriteLine("  builderIsReadOnly = result == DefaultInstance;");

+            writer.WriteLine("}");

+            //clone constructor

+            writer.WriteLine("internal Builder({0} cloneFrom) {{", ClassName);

+            writer.WriteLine("  result = cloneFrom;");

+            writer.WriteLine("  builderIsReadOnly = true;");

+            writer.WriteLine("}");

+            writer.WriteLine();

+            writer.WriteLine("bool builderIsReadOnly;");

+            writer.WriteLine("{0} result;", ClassName);

+            writer.WriteLine();

+            writer.WriteLine("private {0} PrepareBuilder() {{", ClassName);

+            writer.WriteLine("  if (builderIsReadOnly) {");

+            writer.WriteLine("    {0} original = result;", ClassName);

+            writer.WriteLine("    result = new {0}();", ClassName);

+            writer.WriteLine("    builderIsReadOnly = false;");

+            writer.WriteLine("    MergeFrom(original);");

+            writer.WriteLine("  }");

+            writer.WriteLine("  return result;");

+            writer.WriteLine("}");

             writer.WriteLine();

-            writer.WriteLine("{0} result = new {0}();", ClassName);

+            writer.WriteLine("public override bool IsInitialized {");

+            writer.WriteLine("  get { return result.IsInitialized; }");

+            writer.WriteLine("}");

             writer.WriteLine();

             writer.WriteLine("protected override {0} MessageBeingBuilt {{", ClassName);

-            writer.WriteLine("  get { return result; }");

+            writer.WriteLine("  get { return PrepareBuilder(); }");

             writer.WriteLine("}");

             writer.WriteLine();

+            //Not actually expecting that DefaultInstance would ever be null here; however, we will ensure it does not break

             writer.WriteLine("public override Builder Clear() {");

-            writer.WriteLine("  result = new {0}();", ClassName);

+            writer.WriteLine("  result = DefaultInstance ?? new {0}();", ClassName);

+            writer.WriteLine("  builderIsReadOnly = true;");

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             writer.WriteLine();

             writer.WriteLine("public override Builder Clone() {");

-            writer.WriteLine("  return new Builder().MergeFrom(result);");

+            writer.WriteLine("  if (builderIsReadOnly) {");

+            writer.WriteLine("    return new Builder(result);");

+            writer.WriteLine("  } else {");

+            writer.WriteLine("    return new Builder().MergeFrom(result);");

+            writer.WriteLine("  }");

             writer.WriteLine("}");

             writer.WriteLine();

             if (!UseLiteRuntime)

@@ -613,17 +644,15 @@ namespace Google.ProtocolBuffers.ProtoGen
 

             writer.WriteLine("public override {0} BuildPartial() {{", ClassName);

             writer.Indent();

-            writer.WriteLine("if (result == null) {");

-            writer.WriteLine(

-                "  throw new global::System.InvalidOperationException(\"build() has already been called on this Builder\");");

+            writer.WriteLine("if (builderIsReadOnly) {");

+            writer.WriteLine("  return result;");

             writer.WriteLine("}");

             foreach (FieldDescriptor field in Descriptor.Fields)

             {

                 CreateFieldGenerator(field).GenerateBuildingCode(writer);

             }

-            writer.WriteLine("{0} returnMe = result;", ClassName);

-            writer.WriteLine("result = null;");

-            writer.WriteLine("return returnMe;");

+            writer.WriteLine("builderIsReadOnly = true;");

+            writer.WriteLine("return result;");

             writer.Outdent();

             writer.WriteLine("}");

             writer.WriteLine();

@@ -644,6 +673,7 @@ namespace Google.ProtocolBuffers.ProtoGen
                 // fields are set so we can skip the merge.

                 writer.Indent();

                 writer.WriteLine("if (other == {0}.DefaultInstance) return this;", FullClassName);

+                writer.WriteLine("PrepareBuilder();");

                 foreach (FieldDescriptor field in Descriptor.Fields)

                 {

                     CreateFieldGenerator(field).GenerateMergingCode(writer);

@@ -676,6 +706,7 @@ namespace Google.ProtocolBuffers.ProtoGen
             writer.WriteLine(

                 "public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {");

             writer.Indent();

+            writer.WriteLine("PrepareBuilder();");

             if (!UseLiteRuntime)

             {

                 writer.WriteLine("pb::UnknownFieldSet.Builder unknownFields = null;");

diff --git a/src/ProtoGen/PrimitiveFieldGenerator.cs b/src/ProtoGen/PrimitiveFieldGenerator.cs
index e945bf87..b9aee865 100644
--- a/src/ProtoGen/PrimitiveFieldGenerator.cs
+++ b/src/ProtoGen/PrimitiveFieldGenerator.cs
@@ -72,11 +72,13 @@ namespace Google.ProtocolBuffers.ProtoGen
             AddClsComplianceCheck(writer);

             writer.WriteLine("public Builder Set{0}({1} value) {{", PropertyName, TypeName);

             AddNullCheck(writer);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.has{0} = true;", PropertyName);

             writer.WriteLine("  result.{0}_ = value;", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             writer.WriteLine("public Builder Clear{0}() {{", PropertyName);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.has{0} = false;", PropertyName);

             writer.WriteLine("  result.{0}_ = {1};", Name, DefaultValue);

             writer.WriteLine("  return this;");

diff --git a/src/ProtoGen/RepeatedEnumFieldGenerator.cs b/src/ProtoGen/RepeatedEnumFieldGenerator.cs
index 5880390b..ae1ed240 100644
--- a/src/ProtoGen/RepeatedEnumFieldGenerator.cs
+++ b/src/ProtoGen/RepeatedEnumFieldGenerator.cs
@@ -71,7 +71,7 @@ namespace Google.ProtocolBuffers.ProtoGen
             // Note:  We can return the original list here, because we make it unmodifiable when we build

             // We return it via IPopsicleList so that collection initializers work more pleasantly.

             writer.WriteLine("public pbc::IPopsicleList<{0}> {1}List {{", TypeName, PropertyName);

-            writer.WriteLine("  get {{ return result.{0}_; }}", Name);

+            writer.WriteLine("  get {{ return PrepareBuilder().{0}_; }}", Name);

             writer.WriteLine("}");

             writer.WriteLine("public int {0}Count {{", PropertyName);

             writer.WriteLine("  get {{ return result.{0}Count; }}", PropertyName);

@@ -80,18 +80,22 @@ namespace Google.ProtocolBuffers.ProtoGen
             writer.WriteLine("  return result.Get{0}(index);", PropertyName);

             writer.WriteLine("}");

             writer.WriteLine("public Builder Set{0}(int index, {1} value) {{", PropertyName, TypeName);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.{0}_[index] = value;", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             writer.WriteLine("public Builder Add{0}({1} value) {{", PropertyName, TypeName);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.{0}_.Add(value);", Name, TypeName);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             writer.WriteLine("public Builder AddRange{0}(scg::IEnumerable<{1}> values) {{", PropertyName, TypeName);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  base.AddRange(values, result.{0}_);", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             writer.WriteLine("public Builder Clear{0}() {{", PropertyName);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.{0}_.Clear();", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

diff --git a/src/ProtoGen/RepeatedMessageFieldGenerator.cs b/src/ProtoGen/RepeatedMessageFieldGenerator.cs
index bfa0763f..c184b345 100644
--- a/src/ProtoGen/RepeatedMessageFieldGenerator.cs
+++ b/src/ProtoGen/RepeatedMessageFieldGenerator.cs
@@ -67,7 +67,7 @@ namespace Google.ProtocolBuffers.ProtoGen
             // Note:  We can return the original list here, because we make it unmodifiable when we build

             // We return it via IPopsicleList so that collection initializers work more pleasantly.

             writer.WriteLine("public pbc::IPopsicleList<{0}> {1}List {{", TypeName, PropertyName);

-            writer.WriteLine("  get {{ return result.{0}_; }}", Name);

+            writer.WriteLine("  get {{ return PrepareBuilder().{0}_; }}", Name);

             writer.WriteLine("}");

             writer.WriteLine("public int {0}Count {{", PropertyName);

             writer.WriteLine("  get {{ return result.{0}Count; }}", PropertyName);

@@ -77,31 +77,37 @@ namespace Google.ProtocolBuffers.ProtoGen
             writer.WriteLine("}");

             writer.WriteLine("public Builder Set{0}(int index, {1} value) {{", PropertyName, TypeName);

             AddNullCheck(writer);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.{0}_[index] = value;", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             // Extra overload for builder (just on messages)

             writer.WriteLine("public Builder Set{0}(int index, {1}.Builder builderForValue) {{", PropertyName, TypeName);

             AddNullCheck(writer, "builderForValue");

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.{0}_[index] = builderForValue.Build();", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             writer.WriteLine("public Builder Add{0}({1} value) {{", PropertyName, TypeName);

             AddNullCheck(writer);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.{0}_.Add(value);", Name, TypeName);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             // Extra overload for builder (just on messages)

             writer.WriteLine("public Builder Add{0}({1}.Builder builderForValue) {{", PropertyName, TypeName);

             AddNullCheck(writer, "builderForValue");

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.{0}_.Add(builderForValue.Build());", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             writer.WriteLine("public Builder AddRange{0}(scg::IEnumerable<{1}> values) {{", PropertyName, TypeName);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  base.AddRange(values, result.{0}_);", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             writer.WriteLine("public Builder Clear{0}() {{", PropertyName);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.{0}_.Clear();", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

diff --git a/src/ProtoGen/RepeatedPrimitiveFieldGenerator.cs b/src/ProtoGen/RepeatedPrimitiveFieldGenerator.cs
index 9e7f5f64..3bbcff52 100644
--- a/src/ProtoGen/RepeatedPrimitiveFieldGenerator.cs
+++ b/src/ProtoGen/RepeatedPrimitiveFieldGenerator.cs
@@ -74,7 +74,7 @@ namespace Google.ProtocolBuffers.ProtoGen
             // We return it via IPopsicleList so that collection initializers work more pleasantly.

             AddClsComplianceCheck(writer);

             writer.WriteLine("public pbc::IPopsicleList<{0}> {1}List {{", TypeName, PropertyName);

-            writer.WriteLine("  get {{ return result.{0}_; }}", Name);

+            writer.WriteLine("  get {{ return PrepareBuilder().{0}_; }}", Name);

             writer.WriteLine("}");

             writer.WriteLine("public int {0}Count {{", PropertyName);

             writer.WriteLine("  get {{ return result.{0}Count; }}", PropertyName);

@@ -86,21 +86,25 @@ namespace Google.ProtocolBuffers.ProtoGen
             AddClsComplianceCheck(writer);

             writer.WriteLine("public Builder Set{0}(int index, {1} value) {{", PropertyName, TypeName);

             AddNullCheck(writer);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.{0}_[index] = value;", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             AddClsComplianceCheck(writer);

             writer.WriteLine("public Builder Add{0}({1} value) {{", PropertyName, TypeName);

             AddNullCheck(writer);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.{0}_.Add(value);", Name, TypeName);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             AddClsComplianceCheck(writer);

             writer.WriteLine("public Builder AddRange{0}(scg::IEnumerable<{1}> values) {{", PropertyName, TypeName);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  base.AddRange(values, result.{0}_);", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             writer.WriteLine("public Builder Clear{0}() {{", PropertyName);

+            writer.WriteLine("  PrepareBuilder();");

             writer.WriteLine("  result.{0}_.Clear();", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

diff --git a/src/ProtocolBuffers.Test/GeneratedMessageTest.cs b/src/ProtocolBuffers.Test/GeneratedMessageTest.cs
index 410452d3..12224e9b 100644
--- a/src/ProtocolBuffers.Test/GeneratedMessageTest.cs
+++ b/src/ProtocolBuffers.Test/GeneratedMessageTest.cs
@@ -112,22 +112,6 @@ namespace Google.ProtocolBuffers
         }

 

         [Test]

-        public void DoubleBuildError()

-        {

-            TestAllTypes.Builder builder = new TestAllTypes.Builder();

-            builder.Build();

-            try

-            {

-                builder.Build();

-                Assert.Fail("Should have thrown exception.");

-            }

-            catch (InvalidOperationException)

-            {

-                // Success.

-            }

-        }

-

-        [Test]

         public void DefaultInstance()

         {

             Assert.AreSame(TestAllTypes.DefaultInstance, TestAllTypes.DefaultInstance.DefaultInstanceForType);

diff --git a/src/ProtocolBuffers.Test/ProtocolBuffers.Test.csproj b/src/ProtocolBuffers.Test/ProtocolBuffers.Test.csproj
index 95ab0b9c..b1a7e559 100644
--- a/src/ProtocolBuffers.Test/ProtocolBuffers.Test.csproj
+++ b/src/ProtocolBuffers.Test/ProtocolBuffers.Test.csproj
@@ -105,6 +105,7 @@
     <Compile Include="NameHelpersTest.cs" />

     <Compile Include="Properties\AssemblyInfo.cs" />

     <Compile Include="ReflectionTester.cs" />

+    <Compile Include="ReusableBuilderTest.cs" />

     <Compile Include="ServiceTest.cs" />

     <Compile Include="TestProtos\UnitTestCSharpOptionsProtoFile.cs" />

     <Compile Include="TestProtos\UnitTestCustomOptionsProtoFile.cs" />

diff --git a/src/ProtocolBuffers.Test/ReusableBuilderTest.cs b/src/ProtocolBuffers.Test/ReusableBuilderTest.cs
new file mode 100644
index 00000000..4184971f
--- /dev/null
+++ b/src/ProtocolBuffers.Test/ReusableBuilderTest.cs
@@ -0,0 +1,117 @@
+using System;

+using System.Collections.Generic;

+using System.Text;

+using NUnit.Framework;

+using Google.ProtocolBuffers.TestProtos;

+

+namespace Google.ProtocolBuffers

+{

+    [TestFixture]

+    public class ReusableBuilderTest

+    {

+        [Test]

+        public void TestUnmodifiedDefaultInstance()

+        {

+            //Simply calling ToBuilder().Build() no longer creates a copy of the message

+            TestAllTypes.Builder builder = TestAllTypes.DefaultInstance.ToBuilder();

+            Assert.IsTrue(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+        }

+

+        [Test]

+        public void BuildMultipleWithoutChange()

+        {

+            //Calling Build() or BuildPartial() does not require a copy of the message

+            TestAllTypes.Builder builder = TestAllTypes.DefaultInstance.ToBuilder();

+            builder.SetDefaultBool(true);

+

+            TestAllTypes first = builder.BuildPartial();

+            //Still the same instance?

+            Assert.IsTrue(ReferenceEquals(first, builder.Build()));

+            //Still the same instance?

+            Assert.IsTrue(ReferenceEquals(first, builder.BuildPartial().ToBuilder().Build()));

+        }

+

+        [Test]

+        public void MergeFromDefaultInstance()

+        {

+            TestAllTypes.Builder builder = TestAllTypes.DefaultInstance.ToBuilder();

+            Assert.IsTrue(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+            builder.MergeFrom(TestAllTypes.DefaultInstance);

+            Assert.IsTrue(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+        }

+

+        [Test]

+        public void BuildNewBuilderIsDefaultInstance()

+        {

+            Assert.IsTrue(ReferenceEquals(TestAllTypes.DefaultInstance, new TestAllTypes.Builder().Build()));

+            Assert.IsTrue(ReferenceEquals(TestAllTypes.DefaultInstance, TestAllTypes.CreateBuilder().Build()));

+            //last test, if you clear a builder it reverts to default instance

+            Assert.IsTrue(ReferenceEquals(TestAllTypes.DefaultInstance,

+                TestAllTypes.CreateBuilder().SetOptionalBool(true).Build().ToBuilder().Clear().Build()));

+        }

+

+        [Test]

+        public void CloneOnChangePrimitive()

+        {

+            TestAllTypes.Builder builder = TestAllTypes.DefaultInstance.ToBuilder();

+            Assert.IsTrue(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+            builder.SetDefaultBool(true);

+            Assert.IsFalse(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+        }

+

+        [Test]

+        public void CloneOnAddRepeatedBool()

+        {

+            TestAllTypes.Builder builder = TestAllTypes.DefaultInstance.ToBuilder();

+            Assert.IsTrue(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+            builder.AddRepeatedBool(true);

+            Assert.IsFalse(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+        }

+

+        [Test]

+        public void CloneOnChangeMessage()

+        {

+            TestAllTypes.Builder builder = TestAllTypes.DefaultInstance.ToBuilder();

+            Assert.IsTrue(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+            builder.SetOptionalForeignMessage(new ForeignMessage.Builder());

+            Assert.IsFalse(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+        }

+

+        [Test]

+        public void CloneOnClearMessage()

+        {

+            TestAllTypes.Builder builder = TestAllTypes.DefaultInstance.ToBuilder();

+            Assert.IsTrue(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+            builder.ClearOptionalForeignMessage();

+            Assert.IsFalse(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+        }

+

+        [Test]

+        public void CloneOnAddRepeatedForeignMessage()

+        {

+            TestAllTypes.Builder builder = TestAllTypes.DefaultInstance.ToBuilder();

+            Assert.IsTrue(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+            builder.AddRepeatedForeignMessage(ForeignMessage.DefaultInstance);

+            Assert.IsFalse(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+        }

+

+        [Test]

+        public void CloneOnChangeEnumValue()

+        {

+            TestAllTypes.Builder builder = TestAllTypes.DefaultInstance.ToBuilder();

+            Assert.IsTrue(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+            builder.SetOptionalForeignEnum(ForeignEnum.FOREIGN_BAR);

+            Assert.IsFalse(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+        }

+

+        [Test]

+        public void CloneOnAddRepeatedForeignEnum()

+        {

+            TestAllTypes.Builder builder = TestAllTypes.DefaultInstance.ToBuilder();

+            Assert.IsTrue(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+            builder.AddRepeatedForeignEnum(ForeignEnum.FOREIGN_BAR);

+            Assert.IsFalse(ReferenceEquals(TestAllTypes.DefaultInstance, builder.Build()));

+        }

+

+    }

+}

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestCSharpOptionsProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestCSharpOptionsProtoFile.cs
index ff726d65..215b9b4f 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestCSharpOptionsProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestCSharpOptionsProtoFile.cs
@@ -190,7 +190,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(OptionsMessage prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -200,21 +200,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new OptionsMessage();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(OptionsMessage cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      OptionsMessage result;

       

-      OptionsMessage result = new OptionsMessage();

+      private OptionsMessage PrepareBuilder() {

+        if (builderIsReadOnly) {

+          OptionsMessage original = result;

+          result = new OptionsMessage();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override OptionsMessage MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new OptionsMessage();

+        result = DefaultInstance ?? new OptionsMessage();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -226,12 +253,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OptionsMessage BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        OptionsMessage returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -245,6 +271,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(OptionsMessage other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.OptionsMessage.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasNormal) {

           Normal = other.Normal;

         }

@@ -263,6 +290,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -327,11 +355,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetNormal(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasNormal = true;

         result.normal_ = value;

         return this;

       }

       public Builder ClearNormal() {

+        PrepareBuilder();

         result.hasNormal = false;

         result.normal_ = "";

         return this;

@@ -346,11 +376,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionsMessage_(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionsMessage_ = true;

         result.optionsMessage_ = value;

         return this;

       }

       public Builder ClearOptionsMessage_() {

+        PrepareBuilder();

         result.hasOptionsMessage_ = false;

         result.optionsMessage_ = "";

         return this;

@@ -365,11 +397,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetCustomName(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasCustomName = true;

         result.customized_ = value;

         return this;

       }

       public Builder ClearCustomName() {

+        PrepareBuilder();

         result.hasCustomName = false;

         result.customized_ = "";

         return this;

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestCustomOptionsProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestCustomOptionsProtoFile.cs
index 0b1be422..bc29da26 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestCustomOptionsProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestCustomOptionsProtoFile.cs
@@ -513,7 +513,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestMessageWithCustomOptions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -523,21 +523,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestMessageWithCustomOptions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestMessageWithCustomOptions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestMessageWithCustomOptions result;

+      

+      private TestMessageWithCustomOptions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestMessageWithCustomOptions original = result;

+          result = new TestMessageWithCustomOptions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestMessageWithCustomOptions result = new TestMessageWithCustomOptions();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestMessageWithCustomOptions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestMessageWithCustomOptions();

+        result = DefaultInstance ?? new TestMessageWithCustomOptions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -549,12 +576,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMessageWithCustomOptions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestMessageWithCustomOptions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -568,6 +594,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestMessageWithCustomOptions other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestMessageWithCustomOptions.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasField1) {

           Field1 = other.Field1;

         }

@@ -580,6 +607,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -636,11 +664,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField1(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField1 = true;

         result.field1_ = value;

         return this;

       }

       public Builder ClearField1() {

+        PrepareBuilder();

         result.hasField1 = false;

         result.field1_ = "";

         return this;

@@ -737,7 +767,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(CustomOptionFooRequest prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -747,21 +777,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new CustomOptionFooRequest();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(CustomOptionFooRequest cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      CustomOptionFooRequest result;

+      

+      private CustomOptionFooRequest PrepareBuilder() {

+        if (builderIsReadOnly) {

+          CustomOptionFooRequest original = result;

+          result = new CustomOptionFooRequest();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      CustomOptionFooRequest result = new CustomOptionFooRequest();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override CustomOptionFooRequest MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new CustomOptionFooRequest();

+        result = DefaultInstance ?? new CustomOptionFooRequest();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -773,12 +830,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override CustomOptionFooRequest BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        CustomOptionFooRequest returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -792,6 +848,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(CustomOptionFooRequest other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.CustomOptionFooRequest.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -801,6 +858,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -935,7 +993,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(CustomOptionFooResponse prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -945,21 +1003,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new CustomOptionFooResponse();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(CustomOptionFooResponse cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      CustomOptionFooResponse result = new CustomOptionFooResponse();

+      bool builderIsReadOnly;

+      CustomOptionFooResponse result;

+      

+      private CustomOptionFooResponse PrepareBuilder() {

+        if (builderIsReadOnly) {

+          CustomOptionFooResponse original = result;

+          result = new CustomOptionFooResponse();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override CustomOptionFooResponse MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new CustomOptionFooResponse();

+        result = DefaultInstance ?? new CustomOptionFooResponse();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -971,12 +1056,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override CustomOptionFooResponse BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        CustomOptionFooResponse returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -990,6 +1074,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(CustomOptionFooResponse other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.CustomOptionFooResponse.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -999,6 +1084,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -1148,7 +1234,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(DummyMessageContainingEnum prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1158,21 +1244,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new DummyMessageContainingEnum();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(DummyMessageContainingEnum cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      DummyMessageContainingEnum result = new DummyMessageContainingEnum();

+      bool builderIsReadOnly;

+      DummyMessageContainingEnum result;

+      

+      private DummyMessageContainingEnum PrepareBuilder() {

+        if (builderIsReadOnly) {

+          DummyMessageContainingEnum original = result;

+          result = new DummyMessageContainingEnum();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override DummyMessageContainingEnum MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new DummyMessageContainingEnum();

+        result = DefaultInstance ?? new DummyMessageContainingEnum();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1184,12 +1297,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override DummyMessageContainingEnum BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        DummyMessageContainingEnum returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -1203,6 +1315,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(DummyMessageContainingEnum other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.DummyMessageContainingEnum.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -1212,6 +1325,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -1346,7 +1460,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(DummyMessageInvalidAsOptionType prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1356,21 +1470,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new DummyMessageInvalidAsOptionType();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(DummyMessageInvalidAsOptionType cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      DummyMessageInvalidAsOptionType result = new DummyMessageInvalidAsOptionType();

+      bool builderIsReadOnly;

+      DummyMessageInvalidAsOptionType result;

+      

+      private DummyMessageInvalidAsOptionType PrepareBuilder() {

+        if (builderIsReadOnly) {

+          DummyMessageInvalidAsOptionType original = result;

+          result = new DummyMessageInvalidAsOptionType();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override DummyMessageInvalidAsOptionType MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new DummyMessageInvalidAsOptionType();

+        result = DefaultInstance ?? new DummyMessageInvalidAsOptionType();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1382,12 +1523,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override DummyMessageInvalidAsOptionType BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        DummyMessageInvalidAsOptionType returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -1401,6 +1541,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(DummyMessageInvalidAsOptionType other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.DummyMessageInvalidAsOptionType.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -1410,6 +1551,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -1544,7 +1686,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(CustomOptionMinIntegerValues prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1554,21 +1696,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new CustomOptionMinIntegerValues();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(CustomOptionMinIntegerValues cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      CustomOptionMinIntegerValues result = new CustomOptionMinIntegerValues();

+      bool builderIsReadOnly;

+      CustomOptionMinIntegerValues result;

+      

+      private CustomOptionMinIntegerValues PrepareBuilder() {

+        if (builderIsReadOnly) {

+          CustomOptionMinIntegerValues original = result;

+          result = new CustomOptionMinIntegerValues();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override CustomOptionMinIntegerValues MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new CustomOptionMinIntegerValues();

+        result = DefaultInstance ?? new CustomOptionMinIntegerValues();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1580,12 +1749,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override CustomOptionMinIntegerValues BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        CustomOptionMinIntegerValues returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -1599,6 +1767,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(CustomOptionMinIntegerValues other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.CustomOptionMinIntegerValues.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -1608,6 +1777,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -1742,7 +1912,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(CustomOptionMaxIntegerValues prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1752,21 +1922,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new CustomOptionMaxIntegerValues();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(CustomOptionMaxIntegerValues cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      CustomOptionMaxIntegerValues result;

+      

+      private CustomOptionMaxIntegerValues PrepareBuilder() {

+        if (builderIsReadOnly) {

+          CustomOptionMaxIntegerValues original = result;

+          result = new CustomOptionMaxIntegerValues();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      CustomOptionMaxIntegerValues result = new CustomOptionMaxIntegerValues();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override CustomOptionMaxIntegerValues MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new CustomOptionMaxIntegerValues();

+        result = DefaultInstance ?? new CustomOptionMaxIntegerValues();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1778,12 +1975,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override CustomOptionMaxIntegerValues BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        CustomOptionMaxIntegerValues returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -1797,6 +1993,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(CustomOptionMaxIntegerValues other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.CustomOptionMaxIntegerValues.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -1806,6 +2003,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -1940,7 +2138,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(CustomOptionOtherValues prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1950,21 +2148,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new CustomOptionOtherValues();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(CustomOptionOtherValues cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      CustomOptionOtherValues result;

       

-      CustomOptionOtherValues result = new CustomOptionOtherValues();

+      private CustomOptionOtherValues PrepareBuilder() {

+        if (builderIsReadOnly) {

+          CustomOptionOtherValues original = result;

+          result = new CustomOptionOtherValues();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override CustomOptionOtherValues MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new CustomOptionOtherValues();

+        result = DefaultInstance ?? new CustomOptionOtherValues();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1976,12 +2201,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override CustomOptionOtherValues BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        CustomOptionOtherValues returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -1995,6 +2219,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(CustomOptionOtherValues other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.CustomOptionOtherValues.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -2004,6 +2229,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -2138,7 +2364,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(SettingRealsFromPositiveInts prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -2148,21 +2374,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new SettingRealsFromPositiveInts();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(SettingRealsFromPositiveInts cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      SettingRealsFromPositiveInts result;

       

-      SettingRealsFromPositiveInts result = new SettingRealsFromPositiveInts();

+      private SettingRealsFromPositiveInts PrepareBuilder() {

+        if (builderIsReadOnly) {

+          SettingRealsFromPositiveInts original = result;

+          result = new SettingRealsFromPositiveInts();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override SettingRealsFromPositiveInts MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new SettingRealsFromPositiveInts();

+        result = DefaultInstance ?? new SettingRealsFromPositiveInts();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -2174,12 +2427,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SettingRealsFromPositiveInts BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        SettingRealsFromPositiveInts returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -2193,6 +2445,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(SettingRealsFromPositiveInts other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.SettingRealsFromPositiveInts.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -2202,6 +2455,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -2336,7 +2590,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(SettingRealsFromNegativeInts prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -2346,21 +2600,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new SettingRealsFromNegativeInts();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(SettingRealsFromNegativeInts cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      SettingRealsFromNegativeInts result = new SettingRealsFromNegativeInts();

+      bool builderIsReadOnly;

+      SettingRealsFromNegativeInts result;

+      

+      private SettingRealsFromNegativeInts PrepareBuilder() {

+        if (builderIsReadOnly) {

+          SettingRealsFromNegativeInts original = result;

+          result = new SettingRealsFromNegativeInts();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override SettingRealsFromNegativeInts MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new SettingRealsFromNegativeInts();

+        result = DefaultInstance ?? new SettingRealsFromNegativeInts();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -2372,12 +2653,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SettingRealsFromNegativeInts BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        SettingRealsFromNegativeInts returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -2391,6 +2671,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(SettingRealsFromNegativeInts other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.SettingRealsFromNegativeInts.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -2400,6 +2681,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -2586,7 +2868,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(ComplexOptionType1 prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -2596,21 +2878,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new ComplexOptionType1();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(ComplexOptionType1 cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      ComplexOptionType1 result;

+      

+      private ComplexOptionType1 PrepareBuilder() {

+        if (builderIsReadOnly) {

+          ComplexOptionType1 original = result;

+          result = new ComplexOptionType1();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      ComplexOptionType1 result = new ComplexOptionType1();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override ComplexOptionType1 MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new ComplexOptionType1();

+        result = DefaultInstance ?? new ComplexOptionType1();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -2622,12 +2931,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ComplexOptionType1 BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        ComplexOptionType1 returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -2641,6 +2949,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(ComplexOptionType1 other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.ComplexOptionType1.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasFoo) {

           Foo = other.Foo;

         }

@@ -2660,6 +2969,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -2723,11 +3033,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetFoo(value); }

       }

       public Builder SetFoo(int value) {

+        PrepareBuilder();

         result.hasFoo = true;

         result.foo_ = value;

         return this;

       }

       public Builder ClearFoo() {

+        PrepareBuilder();

         result.hasFoo = false;

         result.foo_ = 0;

         return this;

@@ -2741,11 +3053,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetFoo2(value); }

       }

       public Builder SetFoo2(int value) {

+        PrepareBuilder();

         result.hasFoo2 = true;

         result.foo2_ = value;

         return this;

       }

       public Builder ClearFoo2() {

+        PrepareBuilder();

         result.hasFoo2 = false;

         result.foo2_ = 0;

         return this;

@@ -2759,11 +3073,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetFoo3(value); }

       }

       public Builder SetFoo3(int value) {

+        PrepareBuilder();

         result.hasFoo3 = true;

         result.foo3_ = value;

         return this;

       }

       public Builder ClearFoo3() {

+        PrepareBuilder();

         result.hasFoo3 = false;

         result.foo3_ = 0;

         return this;

@@ -2910,7 +3226,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(ComplexOptionType4 prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -2920,21 +3236,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new ComplexOptionType4();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(ComplexOptionType4 cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

           

-          ComplexOptionType4 result = new ComplexOptionType4();

+          bool builderIsReadOnly;

+          ComplexOptionType4 result;

+          

+          private ComplexOptionType4 PrepareBuilder() {

+            if (builderIsReadOnly) {

+              ComplexOptionType4 original = result;

+              result = new ComplexOptionType4();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override ComplexOptionType4 MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new ComplexOptionType4();

+            result = DefaultInstance ?? new ComplexOptionType4();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -2946,12 +3289,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override ComplexOptionType4 BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            ComplexOptionType4 returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -2965,6 +3307,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(ComplexOptionType4 other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.ComplexOptionType2.Types.ComplexOptionType4.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasWaldo) {

               Waldo = other.Waldo;

             }

@@ -2977,6 +3320,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -3032,11 +3376,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetWaldo(value); }

           }

           public Builder SetWaldo(int value) {

+            PrepareBuilder();

             result.hasWaldo = true;

             result.waldo_ = value;

             return this;

           }

           public Builder ClearWaldo() {

+            PrepareBuilder();

             result.hasWaldo = false;

             result.waldo_ = 0;

             return this;

@@ -3164,7 +3510,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(ComplexOptionType2 prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -3174,21 +3520,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new ComplexOptionType2();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(ComplexOptionType2 cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      ComplexOptionType2 result;

       

-      ComplexOptionType2 result = new ComplexOptionType2();

+      private ComplexOptionType2 PrepareBuilder() {

+        if (builderIsReadOnly) {

+          ComplexOptionType2 original = result;

+          result = new ComplexOptionType2();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override ComplexOptionType2 MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new ComplexOptionType2();

+        result = DefaultInstance ?? new ComplexOptionType2();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -3200,12 +3573,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ComplexOptionType2 BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        ComplexOptionType2 returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -3219,6 +3591,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(ComplexOptionType2 other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.ComplexOptionType2.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasBar) {

           MergeBar(other.Bar);

         }

@@ -3238,6 +3611,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -3312,18 +3686,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetBar(global::Google.ProtocolBuffers.TestProtos.ComplexOptionType1 value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasBar = true;

         result.bar_ = value;

         return this;

       }

       public Builder SetBar(global::Google.ProtocolBuffers.TestProtos.ComplexOptionType1.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasBar = true;

         result.bar_ = builderForValue.Build();

         return this;

       }

       public Builder MergeBar(global::Google.ProtocolBuffers.TestProtos.ComplexOptionType1 value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasBar &&

             result.bar_ != global::Google.ProtocolBuffers.TestProtos.ComplexOptionType1.DefaultInstance) {

             result.bar_ = global::Google.ProtocolBuffers.TestProtos.ComplexOptionType1.CreateBuilder(result.bar_).MergeFrom(value).BuildPartial();

@@ -3334,6 +3711,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearBar() {

+        PrepareBuilder();

         result.hasBar = false;

         result.bar_ = global::Google.ProtocolBuffers.TestProtos.ComplexOptionType1.DefaultInstance;

         return this;

@@ -3347,11 +3725,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetBaz(value); }

       }

       public Builder SetBaz(int value) {

+        PrepareBuilder();

         result.hasBaz = true;

         result.baz_ = value;

         return this;

       }

       public Builder ClearBaz() {

+        PrepareBuilder();

         result.hasBaz = false;

         result.baz_ = 0;

         return this;

@@ -3366,18 +3746,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetFred(global::Google.ProtocolBuffers.TestProtos.ComplexOptionType2.Types.ComplexOptionType4 value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasFred = true;

         result.fred_ = value;

         return this;

       }

       public Builder SetFred(global::Google.ProtocolBuffers.TestProtos.ComplexOptionType2.Types.ComplexOptionType4.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasFred = true;

         result.fred_ = builderForValue.Build();

         return this;

       }

       public Builder MergeFred(global::Google.ProtocolBuffers.TestProtos.ComplexOptionType2.Types.ComplexOptionType4 value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasFred &&

             result.fred_ != global::Google.ProtocolBuffers.TestProtos.ComplexOptionType2.Types.ComplexOptionType4.DefaultInstance) {

             result.fred_ = global::Google.ProtocolBuffers.TestProtos.ComplexOptionType2.Types.ComplexOptionType4.CreateBuilder(result.fred_).MergeFrom(value).BuildPartial();

@@ -3388,6 +3771,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearFred() {

+        PrepareBuilder();

         result.hasFred = false;

         result.fred_ = global::Google.ProtocolBuffers.TestProtos.ComplexOptionType2.Types.ComplexOptionType4.DefaultInstance;

         return this;

@@ -3532,7 +3916,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(ComplexOptionType5 prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -3542,21 +3926,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new ComplexOptionType5();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(ComplexOptionType5 cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          ComplexOptionType5 result;

+          

+          private ComplexOptionType5 PrepareBuilder() {

+            if (builderIsReadOnly) {

+              ComplexOptionType5 original = result;

+              result = new ComplexOptionType5();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

           

-          ComplexOptionType5 result = new ComplexOptionType5();

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override ComplexOptionType5 MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new ComplexOptionType5();

+            result = DefaultInstance ?? new ComplexOptionType5();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -3568,12 +3979,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override ComplexOptionType5 BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            ComplexOptionType5 returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -3587,6 +3997,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(ComplexOptionType5 other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.ComplexOptionType3.Types.ComplexOptionType5.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasPlugh) {

               Plugh = other.Plugh;

             }

@@ -3599,6 +4010,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -3654,11 +4066,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetPlugh(value); }

           }

           public Builder SetPlugh(int value) {

+            PrepareBuilder();

             result.hasPlugh = true;

             result.plugh_ = value;

             return this;

           }

           public Builder ClearPlugh() {

+            PrepareBuilder();

             result.hasPlugh = false;

             result.plugh_ = 0;

             return this;

@@ -3763,7 +4177,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(ComplexOptionType3 prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -3773,21 +4187,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new ComplexOptionType3();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(ComplexOptionType3 cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      ComplexOptionType3 result = new ComplexOptionType3();

+      bool builderIsReadOnly;

+      ComplexOptionType3 result;

+      

+      private ComplexOptionType3 PrepareBuilder() {

+        if (builderIsReadOnly) {

+          ComplexOptionType3 original = result;

+          result = new ComplexOptionType3();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override ComplexOptionType3 MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new ComplexOptionType3();

+        result = DefaultInstance ?? new ComplexOptionType3();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -3799,12 +4240,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ComplexOptionType3 BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        ComplexOptionType3 returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -3818,6 +4258,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(ComplexOptionType3 other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.ComplexOptionType3.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasQux) {

           Qux = other.Qux;

         }

@@ -3833,6 +4274,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -3897,11 +4339,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetQux(value); }

       }

       public Builder SetQux(int value) {

+        PrepareBuilder();

         result.hasQux = true;

         result.qux_ = value;

         return this;

       }

       public Builder ClearQux() {

+        PrepareBuilder();

         result.hasQux = false;

         result.qux_ = 0;

         return this;

@@ -3916,18 +4360,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetComplexOptionType5(global::Google.ProtocolBuffers.TestProtos.ComplexOptionType3.Types.ComplexOptionType5 value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasComplexOptionType5 = true;

         result.complexOptionType5_ = value;

         return this;

       }

       public Builder SetComplexOptionType5(global::Google.ProtocolBuffers.TestProtos.ComplexOptionType3.Types.ComplexOptionType5.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasComplexOptionType5 = true;

         result.complexOptionType5_ = builderForValue.Build();

         return this;

       }

       public Builder MergeComplexOptionType5(global::Google.ProtocolBuffers.TestProtos.ComplexOptionType3.Types.ComplexOptionType5 value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasComplexOptionType5 &&

             result.complexOptionType5_ != global::Google.ProtocolBuffers.TestProtos.ComplexOptionType3.Types.ComplexOptionType5.DefaultInstance) {

             result.complexOptionType5_ = global::Google.ProtocolBuffers.TestProtos.ComplexOptionType3.Types.ComplexOptionType5.CreateBuilder(result.complexOptionType5_).MergeFrom(value).BuildPartial();

@@ -3938,6 +4385,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearComplexOptionType5() {

+        PrepareBuilder();

         result.hasComplexOptionType5 = false;

         result.complexOptionType5_ = global::Google.ProtocolBuffers.TestProtos.ComplexOptionType3.Types.ComplexOptionType5.DefaultInstance;

         return this;

@@ -4050,7 +4498,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(ComplexOpt6 prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -4060,21 +4508,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new ComplexOpt6();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(ComplexOpt6 cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      ComplexOpt6 result;

+      

+      private ComplexOpt6 PrepareBuilder() {

+        if (builderIsReadOnly) {

+          ComplexOpt6 original = result;

+          result = new ComplexOpt6();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      ComplexOpt6 result = new ComplexOpt6();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override ComplexOpt6 MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new ComplexOpt6();

+        result = DefaultInstance ?? new ComplexOpt6();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -4086,12 +4561,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ComplexOpt6 BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        ComplexOpt6 returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -4105,6 +4579,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(ComplexOpt6 other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.ComplexOpt6.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasXyzzy) {

           Xyzzy = other.Xyzzy;

         }

@@ -4117,6 +4592,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -4172,11 +4648,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetXyzzy(value); }

       }

       public Builder SetXyzzy(int value) {

+        PrepareBuilder();

         result.hasXyzzy = true;

         result.xyzzy_ = value;

         return this;

       }

       public Builder ClearXyzzy() {

+        PrepareBuilder();

         result.hasXyzzy = false;

         result.xyzzy_ = 0;

         return this;

@@ -4273,7 +4751,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(VariousComplexOptions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -4283,21 +4761,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new VariousComplexOptions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(VariousComplexOptions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      VariousComplexOptions result;

       

-      VariousComplexOptions result = new VariousComplexOptions();

+      private VariousComplexOptions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          VariousComplexOptions original = result;

+          result = new VariousComplexOptions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override VariousComplexOptions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new VariousComplexOptions();

+        result = DefaultInstance ?? new VariousComplexOptions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -4309,12 +4814,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override VariousComplexOptions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        VariousComplexOptions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -4328,6 +4832,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(VariousComplexOptions other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.VariousComplexOptions.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -4337,6 +4842,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestEmbedOptimizeForProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestEmbedOptimizeForProtoFile.cs
index 09ecdd63..466cce73 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestEmbedOptimizeForProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestEmbedOptimizeForProtoFile.cs
@@ -186,7 +186,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestEmbedOptimizedForSize prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -196,21 +196,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestEmbedOptimizedForSize();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestEmbedOptimizedForSize cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestEmbedOptimizedForSize result;

       

-      TestEmbedOptimizedForSize result = new TestEmbedOptimizedForSize();

+      private TestEmbedOptimizedForSize PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestEmbedOptimizedForSize original = result;

+          result = new TestEmbedOptimizedForSize();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestEmbedOptimizedForSize MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestEmbedOptimizedForSize();

+        result = DefaultInstance ?? new TestEmbedOptimizedForSize();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -222,13 +249,12 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestEmbedOptimizedForSize BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.repeatedMessage_.MakeReadOnly();

-        TestEmbedOptimizedForSize returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -242,6 +268,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestEmbedOptimizedForSize other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestEmbedOptimizedForSize.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasOptionalMessage) {

           MergeOptionalMessage(other.OptionalMessage);

         }

@@ -257,6 +284,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -322,18 +350,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalMessage(global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalMessage = true;

         result.optionalMessage_ = value;

         return this;

       }

       public Builder SetOptionalMessage(global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalMessage = true;

         result.optionalMessage_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalMessage(global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalMessage &&

             result.optionalMessage_ != global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize.DefaultInstance) {

             result.optionalMessage_ = global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize.CreateBuilder(result.optionalMessage_).MergeFrom(value).BuildPartial();

@@ -344,13 +375,14 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalMessage() {

+        PrepareBuilder();

         result.hasOptionalMessage = false;

         result.optionalMessage_ = global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize.DefaultInstance;

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize> RepeatedMessageList {

-        get { return result.repeatedMessage_; }

+        get { return PrepareBuilder().repeatedMessage_; }

       }

       public int RepeatedMessageCount {

         get { return result.RepeatedMessageCount; }

@@ -360,29 +392,35 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedMessage(int index, global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedMessage_[index] = value;

         return this;

       }

       public Builder SetRepeatedMessage(int index, global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedMessage_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedMessage(global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedMessage_.Add(value);

         return this;

       }

       public Builder AddRepeatedMessage(global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedMessage_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedMessage(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedMessage_);

         return this;

       }

       public Builder ClearRepeatedMessage() {

+        PrepareBuilder();

         result.repeatedMessage_.Clear();

         return this;

       }

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestExtrasIssuesProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestExtrasIssuesProtoFile.cs
index db56e8b9..7791759a 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestExtrasIssuesProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestExtrasIssuesProtoFile.cs
@@ -175,7 +175,7 @@ namespace UnitTest.Issues.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(A prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -185,21 +185,48 @@ namespace UnitTest.Issues.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new A();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(A cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      A result;

       

-      A result = new A();

+      private A PrepareBuilder() {

+        if (builderIsReadOnly) {

+          A original = result;

+          result = new A();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override A MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new A();

+        result = DefaultInstance ?? new A();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -211,12 +238,11 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override A BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        A returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -230,6 +256,7 @@ namespace UnitTest.Issues.TestProtos {
       

       public override Builder MergeFrom(A other) {

         if (other == global::UnitTest.Issues.TestProtos.A.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA_) {

           A_ = other.A_;

         }

@@ -242,6 +269,7 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -297,11 +325,13 @@ namespace UnitTest.Issues.TestProtos {
         set { SetA_(value); }

       }

       public Builder SetA_(int value) {

+        PrepareBuilder();

         result.hasA_ = true;

         result.a_ = value;

         return this;

       }

       public Builder ClearA_() {

+        PrepareBuilder();

         result.hasA_ = false;

         result.a_ = 0;

         return this;

@@ -414,7 +444,7 @@ namespace UnitTest.Issues.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(B prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -424,21 +454,48 @@ namespace UnitTest.Issues.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new B();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(B cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      B result;

       

-      B result = new B();

+      private B PrepareBuilder() {

+        if (builderIsReadOnly) {

+          B original = result;

+          result = new B();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override B MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new B();

+        result = DefaultInstance ?? new B();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -450,12 +507,11 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override B BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        B returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -469,6 +525,7 @@ namespace UnitTest.Issues.TestProtos {
       

       public override Builder MergeFrom(B other) {

         if (other == global::UnitTest.Issues.TestProtos.B.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasB_) {

           B_ = other.B_;

         }

@@ -481,6 +538,7 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -536,11 +594,13 @@ namespace UnitTest.Issues.TestProtos {
         set { SetB_(value); }

       }

       public Builder SetB_(int value) {

+        PrepareBuilder();

         result.hasB_ = true;

         result.b_ = value;

         return this;

       }

       public Builder ClearB_() {

+        PrepareBuilder();

         result.hasB_ = false;

         result.b_ = 0;

         return this;

@@ -653,7 +713,7 @@ namespace UnitTest.Issues.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(AB prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -663,21 +723,48 @@ namespace UnitTest.Issues.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new AB();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(AB cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      AB result;

       

-      AB result = new AB();

+      private AB PrepareBuilder() {

+        if (builderIsReadOnly) {

+          AB original = result;

+          result = new AB();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override AB MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new AB();

+        result = DefaultInstance ?? new AB();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -689,12 +776,11 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override AB BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        AB returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -708,6 +794,7 @@ namespace UnitTest.Issues.TestProtos {
       

       public override Builder MergeFrom(AB other) {

         if (other == global::UnitTest.Issues.TestProtos.AB.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasAB_) {

           AB_ = other.AB_;

         }

@@ -720,6 +807,7 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -775,11 +863,13 @@ namespace UnitTest.Issues.TestProtos {
         set { SetAB_(value); }

       }

       public Builder SetAB_(int value) {

+        PrepareBuilder();

         result.hasAB_ = true;

         result.aB_ = value;

         return this;

       }

       public Builder ClearAB_() {

+        PrepareBuilder();

         result.hasAB_ = false;

         result.aB_ = 0;

         return this;

@@ -894,7 +984,7 @@ namespace UnitTest.Issues.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(NumberField prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -904,21 +994,48 @@ namespace UnitTest.Issues.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new NumberField();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(NumberField cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      NumberField result;

       

-      NumberField result = new NumberField();

+      private NumberField PrepareBuilder() {

+        if (builderIsReadOnly) {

+          NumberField original = result;

+          result = new NumberField();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override NumberField MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new NumberField();

+        result = DefaultInstance ?? new NumberField();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -930,12 +1047,11 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override NumberField BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        NumberField returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -949,6 +1065,7 @@ namespace UnitTest.Issues.TestProtos {
       

       public override Builder MergeFrom(NumberField other) {

         if (other == global::UnitTest.Issues.TestProtos.NumberField.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.Has_01) {

           _01 = other._01;

         }

@@ -961,6 +1078,7 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -1018,11 +1136,13 @@ namespace UnitTest.Issues.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder Set_01(int value) {

+        PrepareBuilder();

         result.has_01 = true;

         result._01_ = value;

         return this;

       }

       public Builder Clear_01() {

+        PrepareBuilder();

         result.has_01 = false;

         result._01_ = 0;

         return this;

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSizeProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSizeProtoFile.cs
index 77635f21..64046eeb 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSizeProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSizeProtoFile.cs
@@ -606,7 +606,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(SizeMessage1 prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -616,21 +616,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new SizeMessage1();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(SizeMessage1 cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      SizeMessage1 result;

+      

+      private SizeMessage1 PrepareBuilder() {

+        if (builderIsReadOnly) {

+          SizeMessage1 original = result;

+          result = new SizeMessage1();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      SizeMessage1 result = new SizeMessage1();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override SizeMessage1 MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new SizeMessage1();

+        result = DefaultInstance ?? new SizeMessage1();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -642,13 +669,12 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SizeMessage1 BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.field5_.MakeReadOnly();

-        SizeMessage1 returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       

@@ -661,11 +687,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField1(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField1 = true;

         result.field1_ = value;

         return this;

       }

       public Builder ClearField1() {

+        PrepareBuilder();

         result.hasField1 = false;

         result.field1_ = "";

         return this;

@@ -680,11 +708,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField9(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField9 = true;

         result.field9_ = value;

         return this;

       }

       public Builder ClearField9() {

+        PrepareBuilder();

         result.hasField9 = false;

         result.field9_ = "";

         return this;

@@ -699,11 +729,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField18(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField18 = true;

         result.field18_ = value;

         return this;

       }

       public Builder ClearField18() {

+        PrepareBuilder();

         result.hasField18 = false;

         result.field18_ = "";

         return this;

@@ -717,11 +749,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField80(value); }

       }

       public Builder SetField80(bool value) {

+        PrepareBuilder();

         result.hasField80 = true;

         result.field80_ = value;

         return this;

       }

       public Builder ClearField80() {

+        PrepareBuilder();

         result.hasField80 = false;

         result.field80_ = false;

         return this;

@@ -735,11 +769,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField81(value); }

       }

       public Builder SetField81(bool value) {

+        PrepareBuilder();

         result.hasField81 = true;

         result.field81_ = value;

         return this;

       }

       public Builder ClearField81() {

+        PrepareBuilder();

         result.hasField81 = false;

         result.field81_ = true;

         return this;

@@ -753,11 +789,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField2(value); }

       }

       public Builder SetField2(int value) {

+        PrepareBuilder();

         result.hasField2 = true;

         result.field2_ = value;

         return this;

       }

       public Builder ClearField2() {

+        PrepareBuilder();

         result.hasField2 = false;

         result.field2_ = 0;

         return this;

@@ -771,11 +809,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField3(value); }

       }

       public Builder SetField3(int value) {

+        PrepareBuilder();

         result.hasField3 = true;

         result.field3_ = value;

         return this;

       }

       public Builder ClearField3() {

+        PrepareBuilder();

         result.hasField3 = false;

         result.field3_ = 0;

         return this;

@@ -789,11 +829,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField280(value); }

       }

       public Builder SetField280(int value) {

+        PrepareBuilder();

         result.hasField280 = true;

         result.field280_ = value;

         return this;

       }

       public Builder ClearField280() {

+        PrepareBuilder();

         result.hasField280 = false;

         result.field280_ = 0;

         return this;

@@ -807,11 +849,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField6(value); }

       }

       public Builder SetField6(int value) {

+        PrepareBuilder();

         result.hasField6 = true;

         result.field6_ = value;

         return this;

       }

       public Builder ClearField6() {

+        PrepareBuilder();

         result.hasField6 = false;

         result.field6_ = 0;

         return this;

@@ -825,11 +869,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField22(value); }

       }

       public Builder SetField22(long value) {

+        PrepareBuilder();

         result.hasField22 = true;

         result.field22_ = value;

         return this;

       }

       public Builder ClearField22() {

+        PrepareBuilder();

         result.hasField22 = false;

         result.field22_ = 0L;

         return this;

@@ -844,11 +890,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField4(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField4 = true;

         result.field4_ = value;

         return this;

       }

       public Builder ClearField4() {

+        PrepareBuilder();

         result.hasField4 = false;

         result.field4_ = "";

         return this;

@@ -856,7 +904,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> Field5List {

-        get { return result.field5_; }

+        get { return PrepareBuilder().field5_; }

       }

       public int Field5Count {

         get { return result.Field5Count; }

@@ -867,20 +915,24 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetField5(int index, ulong value) {

+        PrepareBuilder();

         result.field5_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddField5(ulong value) {

+        PrepareBuilder();

         result.field5_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeField5(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.field5_);

         return this;

       }

       public Builder ClearField5() {

+        PrepareBuilder();

         result.field5_.Clear();

         return this;

       }

@@ -893,11 +945,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField59(value); }

       }

       public Builder SetField59(bool value) {

+        PrepareBuilder();

         result.hasField59 = true;

         result.field59_ = value;

         return this;

       }

       public Builder ClearField59() {

+        PrepareBuilder();

         result.hasField59 = false;

         result.field59_ = false;

         return this;

@@ -912,11 +966,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField7(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField7 = true;

         result.field7_ = value;

         return this;

       }

       public Builder ClearField7() {

+        PrepareBuilder();

         result.hasField7 = false;

         result.field7_ = "";

         return this;

@@ -930,11 +986,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField16(value); }

       }

       public Builder SetField16(int value) {

+        PrepareBuilder();

         result.hasField16 = true;

         result.field16_ = value;

         return this;

       }

       public Builder ClearField16() {

+        PrepareBuilder();

         result.hasField16 = false;

         result.field16_ = 0;

         return this;

@@ -948,11 +1006,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField130(value); }

       }

       public Builder SetField130(int value) {

+        PrepareBuilder();

         result.hasField130 = true;

         result.field130_ = value;

         return this;

       }

       public Builder ClearField130() {

+        PrepareBuilder();

         result.hasField130 = false;

         result.field130_ = 0;

         return this;

@@ -966,11 +1026,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField12(value); }

       }

       public Builder SetField12(bool value) {

+        PrepareBuilder();

         result.hasField12 = true;

         result.field12_ = value;

         return this;

       }

       public Builder ClearField12() {

+        PrepareBuilder();

         result.hasField12 = false;

         result.field12_ = true;

         return this;

@@ -984,11 +1046,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField17(value); }

       }

       public Builder SetField17(bool value) {

+        PrepareBuilder();

         result.hasField17 = true;

         result.field17_ = value;

         return this;

       }

       public Builder ClearField17() {

+        PrepareBuilder();

         result.hasField17 = false;

         result.field17_ = true;

         return this;

@@ -1002,11 +1066,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField13(value); }

       }

       public Builder SetField13(bool value) {

+        PrepareBuilder();

         result.hasField13 = true;

         result.field13_ = value;

         return this;

       }

       public Builder ClearField13() {

+        PrepareBuilder();

         result.hasField13 = false;

         result.field13_ = true;

         return this;

@@ -1020,11 +1086,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField14(value); }

       }

       public Builder SetField14(bool value) {

+        PrepareBuilder();

         result.hasField14 = true;

         result.field14_ = value;

         return this;

       }

       public Builder ClearField14() {

+        PrepareBuilder();

         result.hasField14 = false;

         result.field14_ = true;

         return this;

@@ -1038,11 +1106,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField104(value); }

       }

       public Builder SetField104(int value) {

+        PrepareBuilder();

         result.hasField104 = true;

         result.field104_ = value;

         return this;

       }

       public Builder ClearField104() {

+        PrepareBuilder();

         result.hasField104 = false;

         result.field104_ = 0;

         return this;

@@ -1056,11 +1126,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField100(value); }

       }

       public Builder SetField100(int value) {

+        PrepareBuilder();

         result.hasField100 = true;

         result.field100_ = value;

         return this;

       }

       public Builder ClearField100() {

+        PrepareBuilder();

         result.hasField100 = false;

         result.field100_ = 0;

         return this;

@@ -1074,11 +1146,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField101(value); }

       }

       public Builder SetField101(int value) {

+        PrepareBuilder();

         result.hasField101 = true;

         result.field101_ = value;

         return this;

       }

       public Builder ClearField101() {

+        PrepareBuilder();

         result.hasField101 = false;

         result.field101_ = 0;

         return this;

@@ -1093,11 +1167,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField102(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField102 = true;

         result.field102_ = value;

         return this;

       }

       public Builder ClearField102() {

+        PrepareBuilder();

         result.hasField102 = false;

         result.field102_ = "";

         return this;

@@ -1112,11 +1188,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField103(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField103 = true;

         result.field103_ = value;

         return this;

       }

       public Builder ClearField103() {

+        PrepareBuilder();

         result.hasField103 = false;

         result.field103_ = "";

         return this;

@@ -1130,11 +1208,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField29(value); }

       }

       public Builder SetField29(int value) {

+        PrepareBuilder();

         result.hasField29 = true;

         result.field29_ = value;

         return this;

       }

       public Builder ClearField29() {

+        PrepareBuilder();

         result.hasField29 = false;

         result.field29_ = 0;

         return this;

@@ -1148,11 +1228,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField30(value); }

       }

       public Builder SetField30(bool value) {

+        PrepareBuilder();

         result.hasField30 = true;

         result.field30_ = value;

         return this;

       }

       public Builder ClearField30() {

+        PrepareBuilder();

         result.hasField30 = false;

         result.field30_ = false;

         return this;

@@ -1166,11 +1248,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField60(value); }

       }

       public Builder SetField60(int value) {

+        PrepareBuilder();

         result.hasField60 = true;

         result.field60_ = value;

         return this;

       }

       public Builder ClearField60() {

+        PrepareBuilder();

         result.hasField60 = false;

         result.field60_ = -1;

         return this;

@@ -1184,11 +1268,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField271(value); }

       }

       public Builder SetField271(int value) {

+        PrepareBuilder();

         result.hasField271 = true;

         result.field271_ = value;

         return this;

       }

       public Builder ClearField271() {

+        PrepareBuilder();

         result.hasField271 = false;

         result.field271_ = -1;

         return this;

@@ -1202,11 +1288,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField272(value); }

       }

       public Builder SetField272(int value) {

+        PrepareBuilder();

         result.hasField272 = true;

         result.field272_ = value;

         return this;

       }

       public Builder ClearField272() {

+        PrepareBuilder();

         result.hasField272 = false;

         result.field272_ = -1;

         return this;

@@ -1220,11 +1308,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField150(value); }

       }

       public Builder SetField150(int value) {

+        PrepareBuilder();

         result.hasField150 = true;

         result.field150_ = value;

         return this;

       }

       public Builder ClearField150() {

+        PrepareBuilder();

         result.hasField150 = false;

         result.field150_ = 0;

         return this;

@@ -1238,11 +1328,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField23(value); }

       }

       public Builder SetField23(int value) {

+        PrepareBuilder();

         result.hasField23 = true;

         result.field23_ = value;

         return this;

       }

       public Builder ClearField23() {

+        PrepareBuilder();

         result.hasField23 = false;

         result.field23_ = 0;

         return this;

@@ -1256,11 +1348,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField24(value); }

       }

       public Builder SetField24(bool value) {

+        PrepareBuilder();

         result.hasField24 = true;

         result.field24_ = value;

         return this;

       }

       public Builder ClearField24() {

+        PrepareBuilder();

         result.hasField24 = false;

         result.field24_ = false;

         return this;

@@ -1274,11 +1368,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField25(value); }

       }

       public Builder SetField25(int value) {

+        PrepareBuilder();

         result.hasField25 = true;

         result.field25_ = value;

         return this;

       }

       public Builder ClearField25() {

+        PrepareBuilder();

         result.hasField25 = false;

         result.field25_ = 0;

         return this;

@@ -1293,18 +1389,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField15(global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField15 = true;

         result.field15_ = value;

         return this;

       }

       public Builder SetField15(global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasField15 = true;

         result.field15_ = builderForValue.Build();

         return this;

       }

       public Builder MergeField15(global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasField15 &&

             result.field15_ != global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage.DefaultInstance) {

             result.field15_ = global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage.CreateBuilder(result.field15_).MergeFrom(value).BuildPartial();

@@ -1315,6 +1414,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearField15() {

+        PrepareBuilder();

         result.hasField15 = false;

         result.field15_ = global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage.DefaultInstance;

         return this;

@@ -1328,11 +1428,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField78(value); }

       }

       public Builder SetField78(bool value) {

+        PrepareBuilder();

         result.hasField78 = true;

         result.field78_ = value;

         return this;

       }

       public Builder ClearField78() {

+        PrepareBuilder();

         result.hasField78 = false;

         result.field78_ = false;

         return this;

@@ -1346,11 +1448,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField67(value); }

       }

       public Builder SetField67(int value) {

+        PrepareBuilder();

         result.hasField67 = true;

         result.field67_ = value;

         return this;

       }

       public Builder ClearField67() {

+        PrepareBuilder();

         result.hasField67 = false;

         result.field67_ = 0;

         return this;

@@ -1364,11 +1468,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField68(value); }

       }

       public Builder SetField68(int value) {

+        PrepareBuilder();

         result.hasField68 = true;

         result.field68_ = value;

         return this;

       }

       public Builder ClearField68() {

+        PrepareBuilder();

         result.hasField68 = false;

         result.field68_ = 0;

         return this;

@@ -1382,11 +1488,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField128(value); }

       }

       public Builder SetField128(int value) {

+        PrepareBuilder();

         result.hasField128 = true;

         result.field128_ = value;

         return this;

       }

       public Builder ClearField128() {

+        PrepareBuilder();

         result.hasField128 = false;

         result.field128_ = 0;

         return this;

@@ -1401,11 +1509,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField129(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField129 = true;

         result.field129_ = value;

         return this;

       }

       public Builder ClearField129() {

+        PrepareBuilder();

         result.hasField129 = false;

         result.field129_ = "xxxxxxxxxxxxxxxxxxxxx";

         return this;

@@ -1419,11 +1529,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField131(value); }

       }

       public Builder SetField131(int value) {

+        PrepareBuilder();

         result.hasField131 = true;

         result.field131_ = value;

         return this;

       }

       public Builder ClearField131() {

+        PrepareBuilder();

         result.hasField131 = false;

         result.field131_ = 0;

         return this;

@@ -1697,7 +1809,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(SizeMessage1SubMessage prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1707,21 +1819,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new SizeMessage1SubMessage();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(SizeMessage1SubMessage cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      SizeMessage1SubMessage result = new SizeMessage1SubMessage();

+      bool builderIsReadOnly;

+      SizeMessage1SubMessage result;

+      

+      private SizeMessage1SubMessage PrepareBuilder() {

+        if (builderIsReadOnly) {

+          SizeMessage1SubMessage original = result;

+          result = new SizeMessage1SubMessage();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override SizeMessage1SubMessage MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new SizeMessage1SubMessage();

+        result = DefaultInstance ?? new SizeMessage1SubMessage();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1733,12 +1872,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SizeMessage1SubMessage BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        SizeMessage1SubMessage returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       

@@ -1750,11 +1888,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField1(value); }

       }

       public Builder SetField1(int value) {

+        PrepareBuilder();

         result.hasField1 = true;

         result.field1_ = value;

         return this;

       }

       public Builder ClearField1() {

+        PrepareBuilder();

         result.hasField1 = false;

         result.field1_ = 0;

         return this;

@@ -1768,11 +1908,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField2(value); }

       }

       public Builder SetField2(int value) {

+        PrepareBuilder();

         result.hasField2 = true;

         result.field2_ = value;

         return this;

       }

       public Builder ClearField2() {

+        PrepareBuilder();

         result.hasField2 = false;

         result.field2_ = 0;

         return this;

@@ -1786,11 +1928,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField3(value); }

       }

       public Builder SetField3(int value) {

+        PrepareBuilder();

         result.hasField3 = true;

         result.field3_ = value;

         return this;

       }

       public Builder ClearField3() {

+        PrepareBuilder();

         result.hasField3 = false;

         result.field3_ = 0;

         return this;

@@ -1805,11 +1949,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField15(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField15 = true;

         result.field15_ = value;

         return this;

       }

       public Builder ClearField15() {

+        PrepareBuilder();

         result.hasField15 = false;

         result.field15_ = "";

         return this;

@@ -1823,11 +1969,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField12(value); }

       }

       public Builder SetField12(bool value) {

+        PrepareBuilder();

         result.hasField12 = true;

         result.field12_ = value;

         return this;

       }

       public Builder ClearField12() {

+        PrepareBuilder();

         result.hasField12 = false;

         result.field12_ = true;

         return this;

@@ -1841,11 +1989,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField13(value); }

       }

       public Builder SetField13(long value) {

+        PrepareBuilder();

         result.hasField13 = true;

         result.field13_ = value;

         return this;

       }

       public Builder ClearField13() {

+        PrepareBuilder();

         result.hasField13 = false;

         result.field13_ = 0L;

         return this;

@@ -1859,11 +2009,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField14(value); }

       }

       public Builder SetField14(long value) {

+        PrepareBuilder();

         result.hasField14 = true;

         result.field14_ = value;

         return this;

       }

       public Builder ClearField14() {

+        PrepareBuilder();

         result.hasField14 = false;

         result.field14_ = 0L;

         return this;

@@ -1877,11 +2029,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField16(value); }

       }

       public Builder SetField16(int value) {

+        PrepareBuilder();

         result.hasField16 = true;

         result.field16_ = value;

         return this;

       }

       public Builder ClearField16() {

+        PrepareBuilder();

         result.hasField16 = false;

         result.field16_ = 0;

         return this;

@@ -1895,11 +2049,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField19(value); }

       }

       public Builder SetField19(int value) {

+        PrepareBuilder();

         result.hasField19 = true;

         result.field19_ = value;

         return this;

       }

       public Builder ClearField19() {

+        PrepareBuilder();

         result.hasField19 = false;

         result.field19_ = 2;

         return this;

@@ -1913,11 +2069,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField20(value); }

       }

       public Builder SetField20(bool value) {

+        PrepareBuilder();

         result.hasField20 = true;

         result.field20_ = value;

         return this;

       }

       public Builder ClearField20() {

+        PrepareBuilder();

         result.hasField20 = false;

         result.field20_ = true;

         return this;

@@ -1931,11 +2089,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField28(value); }

       }

       public Builder SetField28(bool value) {

+        PrepareBuilder();

         result.hasField28 = true;

         result.field28_ = value;

         return this;

       }

       public Builder ClearField28() {

+        PrepareBuilder();

         result.hasField28 = false;

         result.field28_ = true;

         return this;

@@ -1951,11 +2111,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetField21(ulong value) {

+        PrepareBuilder();

         result.hasField21 = true;

         result.field21_ = value;

         return this;

       }

       public Builder ClearField21() {

+        PrepareBuilder();

         result.hasField21 = false;

         result.field21_ = 0;

         return this;

@@ -1969,11 +2131,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField22(value); }

       }

       public Builder SetField22(int value) {

+        PrepareBuilder();

         result.hasField22 = true;

         result.field22_ = value;

         return this;

       }

       public Builder ClearField22() {

+        PrepareBuilder();

         result.hasField22 = false;

         result.field22_ = 0;

         return this;

@@ -1987,11 +2151,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField23(value); }

       }

       public Builder SetField23(bool value) {

+        PrepareBuilder();

         result.hasField23 = true;

         result.field23_ = value;

         return this;

       }

       public Builder ClearField23() {

+        PrepareBuilder();

         result.hasField23 = false;

         result.field23_ = false;

         return this;

@@ -2005,11 +2171,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField206(value); }

       }

       public Builder SetField206(bool value) {

+        PrepareBuilder();

         result.hasField206 = true;

         result.field206_ = value;

         return this;

       }

       public Builder ClearField206() {

+        PrepareBuilder();

         result.hasField206 = false;

         result.field206_ = false;

         return this;

@@ -2025,11 +2193,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetField203(uint value) {

+        PrepareBuilder();

         result.hasField203 = true;

         result.field203_ = value;

         return this;

       }

       public Builder ClearField203() {

+        PrepareBuilder();

         result.hasField203 = false;

         result.field203_ = 0;

         return this;

@@ -2043,11 +2213,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField204(value); }

       }

       public Builder SetField204(int value) {

+        PrepareBuilder();

         result.hasField204 = true;

         result.field204_ = value;

         return this;

       }

       public Builder ClearField204() {

+        PrepareBuilder();

         result.hasField204 = false;

         result.field204_ = 0;

         return this;

@@ -2062,11 +2234,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField205(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField205 = true;

         result.field205_ = value;

         return this;

       }

       public Builder ClearField205() {

+        PrepareBuilder();

         result.hasField205 = false;

         result.field205_ = "";

         return this;

@@ -2082,11 +2256,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetField207(ulong value) {

+        PrepareBuilder();

         result.hasField207 = true;

         result.field207_ = value;

         return this;

       }

       public Builder ClearField207() {

+        PrepareBuilder();

         result.hasField207 = false;

         result.field207_ = 0UL;

         return this;

@@ -2102,11 +2278,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetField300(ulong value) {

+        PrepareBuilder();

         result.hasField300 = true;

         result.field300_ = value;

         return this;

       }

       public Builder ClearField300() {

+        PrepareBuilder();

         result.hasField300 = false;

         result.field300_ = 0UL;

         return this;

@@ -2373,7 +2551,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(Group1 prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -2383,21 +2561,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new Group1();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(Group1 cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          Group1 result;

           

-          Group1 result = new Group1();

+          private Group1 PrepareBuilder() {

+            if (builderIsReadOnly) {

+              Group1 original = result;

+              result = new Group1();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override Group1 MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new Group1();

+            result = DefaultInstance ?? new Group1();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -2409,15 +2614,14 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Group1 BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

             result.field14_.MakeReadOnly();

             result.field22_.MakeReadOnly();

             result.field73_.MakeReadOnly();

-            Group1 returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           

@@ -2429,11 +2633,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetField11(value); }

           }

           public Builder SetField11(float value) {

+            PrepareBuilder();

             result.hasField11 = true;

             result.field11_ = value;

             return this;

           }

           public Builder ClearField11() {

+            PrepareBuilder();

             result.hasField11 = false;

             result.field11_ = 0F;

             return this;

@@ -2447,11 +2653,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetField26(value); }

           }

           public Builder SetField26(float value) {

+            PrepareBuilder();

             result.hasField26 = true;

             result.field26_ = value;

             return this;

           }

           public Builder ClearField26() {

+            PrepareBuilder();

             result.hasField26 = false;

             result.field26_ = 0F;

             return this;

@@ -2466,11 +2674,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField12(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasField12 = true;

             result.field12_ = value;

             return this;

           }

           public Builder ClearField12() {

+            PrepareBuilder();

             result.hasField12 = false;

             result.field12_ = "";

             return this;

@@ -2485,18 +2695,20 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField13(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasField13 = true;

             result.field13_ = value;

             return this;

           }

           public Builder ClearField13() {

+            PrepareBuilder();

             result.hasField13 = false;

             result.field13_ = "";

             return this;

           }

           

           public pbc::IPopsicleList<string> Field14List {

-            get { return result.field14_; }

+            get { return PrepareBuilder().field14_; }

           }

           public int Field14Count {

             get { return result.Field14Count; }

@@ -2506,19 +2718,23 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField14(int index, string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.field14_[index] = value;

             return this;

           }

           public Builder AddField14(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.field14_.Add(value);

             return this;

           }

           public Builder AddRangeField14(scg::IEnumerable<string> values) {

+            PrepareBuilder();

             base.AddRange(values, result.field14_);

             return this;

           }

           public Builder ClearField14() {

+            PrepareBuilder();

             result.field14_.Clear();

             return this;

           }

@@ -2533,11 +2749,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           [global::System.CLSCompliant(false)]

           public Builder SetField15(ulong value) {

+            PrepareBuilder();

             result.hasField15 = true;

             result.field15_ = value;

             return this;

           }

           public Builder ClearField15() {

+            PrepareBuilder();

             result.hasField15 = false;

             result.field15_ = 0UL;

             return this;

@@ -2551,11 +2769,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetField5(value); }

           }

           public Builder SetField5(int value) {

+            PrepareBuilder();

             result.hasField5 = true;

             result.field5_ = value;

             return this;

           }

           public Builder ClearField5() {

+            PrepareBuilder();

             result.hasField5 = false;

             result.field5_ = 0;

             return this;

@@ -2570,11 +2790,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField27(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasField27 = true;

             result.field27_ = value;

             return this;

           }

           public Builder ClearField27() {

+            PrepareBuilder();

             result.hasField27 = false;

             result.field27_ = "";

             return this;

@@ -2588,11 +2810,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetField28(value); }

           }

           public Builder SetField28(int value) {

+            PrepareBuilder();

             result.hasField28 = true;

             result.field28_ = value;

             return this;

           }

           public Builder ClearField28() {

+            PrepareBuilder();

             result.hasField28 = false;

             result.field28_ = 0;

             return this;

@@ -2607,11 +2831,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField29(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasField29 = true;

             result.field29_ = value;

             return this;

           }

           public Builder ClearField29() {

+            PrepareBuilder();

             result.hasField29 = false;

             result.field29_ = "";

             return this;

@@ -2626,18 +2852,20 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField16(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasField16 = true;

             result.field16_ = value;

             return this;

           }

           public Builder ClearField16() {

+            PrepareBuilder();

             result.hasField16 = false;

             result.field16_ = "";

             return this;

           }

           

           public pbc::IPopsicleList<string> Field22List {

-            get { return result.field22_; }

+            get { return PrepareBuilder().field22_; }

           }

           public int Field22Count {

             get { return result.Field22Count; }

@@ -2647,25 +2875,29 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField22(int index, string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.field22_[index] = value;

             return this;

           }

           public Builder AddField22(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.field22_.Add(value);

             return this;

           }

           public Builder AddRangeField22(scg::IEnumerable<string> values) {

+            PrepareBuilder();

             base.AddRange(values, result.field22_);

             return this;

           }

           public Builder ClearField22() {

+            PrepareBuilder();

             result.field22_.Clear();

             return this;

           }

           

           public pbc::IPopsicleList<int> Field73List {

-            get { return result.field73_; }

+            get { return PrepareBuilder().field73_; }

           }

           public int Field73Count {

             get { return result.Field73Count; }

@@ -2674,18 +2906,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             return result.GetField73(index);

           }

           public Builder SetField73(int index, int value) {

+            PrepareBuilder();

             result.field73_[index] = value;

             return this;

           }

           public Builder AddField73(int value) {

+            PrepareBuilder();

             result.field73_.Add(value);

             return this;

           }

           public Builder AddRangeField73(scg::IEnumerable<int> values) {

+            PrepareBuilder();

             base.AddRange(values, result.field73_);

             return this;

           }

           public Builder ClearField73() {

+            PrepareBuilder();

             result.field73_.Clear();

             return this;

           }

@@ -2698,11 +2934,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetField20(value); }

           }

           public Builder SetField20(int value) {

+            PrepareBuilder();

             result.hasField20 = true;

             result.field20_ = value;

             return this;

           }

           public Builder ClearField20() {

+            PrepareBuilder();

             result.hasField20 = false;

             result.field20_ = 0;

             return this;

@@ -2717,11 +2955,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField24(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasField24 = true;

             result.field24_ = value;

             return this;

           }

           public Builder ClearField24() {

+            PrepareBuilder();

             result.hasField24 = false;

             result.field24_ = "";

             return this;

@@ -2736,18 +2976,21 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField31(global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasField31 = true;

             result.field31_ = value;

             return this;

           }

           public Builder SetField31(global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage.Builder builderForValue) {

             pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+            PrepareBuilder();

             result.hasField31 = true;

             result.field31_ = builderForValue.Build();

             return this;

           }

           public Builder MergeField31(global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             if (result.hasField31 &&

                 result.field31_ != global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage.DefaultInstance) {

                 result.field31_ = global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage.CreateBuilder(result.field31_).MergeFrom(value).BuildPartial();

@@ -2758,6 +3001,7 @@ namespace Google.ProtocolBuffers.TestProtos {
             return this;

           }

           public Builder ClearField31() {

+            PrepareBuilder();

             result.hasField31 = false;

             result.field31_ = global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage.DefaultInstance;

             return this;

@@ -3113,7 +3357,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(SizeMessage2 prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -3123,21 +3367,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new SizeMessage2();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(SizeMessage2 cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      SizeMessage2 result = new SizeMessage2();

+      bool builderIsReadOnly;

+      SizeMessage2 result;

+      

+      private SizeMessage2 PrepareBuilder() {

+        if (builderIsReadOnly) {

+          SizeMessage2 original = result;

+          result = new SizeMessage2();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override SizeMessage2 MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new SizeMessage2();

+        result = DefaultInstance ?? new SizeMessage2();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -3149,16 +3420,15 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SizeMessage2 BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.group1_.MakeReadOnly();

         result.field128_.MakeReadOnly();

         result.field127_.MakeReadOnly();

         result.field130_.MakeReadOnly();

-        SizeMessage2 returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       

@@ -3171,11 +3441,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField1(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField1 = true;

         result.field1_ = value;

         return this;

       }

       public Builder ClearField1() {

+        PrepareBuilder();

         result.hasField1 = false;

         result.field1_ = "";

         return this;

@@ -3189,11 +3461,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField3(value); }

       }

       public Builder SetField3(long value) {

+        PrepareBuilder();

         result.hasField3 = true;

         result.field3_ = value;

         return this;

       }

       public Builder ClearField3() {

+        PrepareBuilder();

         result.hasField3 = false;

         result.field3_ = 0L;

         return this;

@@ -3207,11 +3481,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField4(value); }

       }

       public Builder SetField4(long value) {

+        PrepareBuilder();

         result.hasField4 = true;

         result.field4_ = value;

         return this;

       }

       public Builder ClearField4() {

+        PrepareBuilder();

         result.hasField4 = false;

         result.field4_ = 0L;

         return this;

@@ -3225,11 +3501,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField30(value); }

       }

       public Builder SetField30(long value) {

+        PrepareBuilder();

         result.hasField30 = true;

         result.field30_ = value;

         return this;

       }

       public Builder ClearField30() {

+        PrepareBuilder();

         result.hasField30 = false;

         result.field30_ = 0L;

         return this;

@@ -3243,11 +3521,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField75(value); }

       }

       public Builder SetField75(bool value) {

+        PrepareBuilder();

         result.hasField75 = true;

         result.field75_ = value;

         return this;

       }

       public Builder ClearField75() {

+        PrepareBuilder();

         result.hasField75 = false;

         result.field75_ = false;

         return this;

@@ -3262,11 +3542,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField6(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField6 = true;

         result.field6_ = value;

         return this;

       }

       public Builder ClearField6() {

+        PrepareBuilder();

         result.hasField6 = false;

         result.field6_ = "";

         return this;

@@ -3281,11 +3563,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField2(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField2 = true;

         result.field2_ = value;

         return this;

       }

       public Builder ClearField2() {

+        PrepareBuilder();

         result.hasField2 = false;

         result.field2_ = pb::ByteString.Empty;

         return this;

@@ -3299,11 +3583,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField21(value); }

       }

       public Builder SetField21(int value) {

+        PrepareBuilder();

         result.hasField21 = true;

         result.field21_ = value;

         return this;

       }

       public Builder ClearField21() {

+        PrepareBuilder();

         result.hasField21 = false;

         result.field21_ = 0;

         return this;

@@ -3317,11 +3603,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField71(value); }

       }

       public Builder SetField71(int value) {

+        PrepareBuilder();

         result.hasField71 = true;

         result.field71_ = value;

         return this;

       }

       public Builder ClearField71() {

+        PrepareBuilder();

         result.hasField71 = false;

         result.field71_ = 0;

         return this;

@@ -3335,11 +3623,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField25(value); }

       }

       public Builder SetField25(float value) {

+        PrepareBuilder();

         result.hasField25 = true;

         result.field25_ = value;

         return this;

       }

       public Builder ClearField25() {

+        PrepareBuilder();

         result.hasField25 = false;

         result.field25_ = 0F;

         return this;

@@ -3353,11 +3643,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField109(value); }

       }

       public Builder SetField109(int value) {

+        PrepareBuilder();

         result.hasField109 = true;

         result.field109_ = value;

         return this;

       }

       public Builder ClearField109() {

+        PrepareBuilder();

         result.hasField109 = false;

         result.field109_ = 0;

         return this;

@@ -3371,11 +3663,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField210(value); }

       }

       public Builder SetField210(int value) {

+        PrepareBuilder();

         result.hasField210 = true;

         result.field210_ = value;

         return this;

       }

       public Builder ClearField210() {

+        PrepareBuilder();

         result.hasField210 = false;

         result.field210_ = 0;

         return this;

@@ -3389,11 +3683,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField211(value); }

       }

       public Builder SetField211(int value) {

+        PrepareBuilder();

         result.hasField211 = true;

         result.field211_ = value;

         return this;

       }

       public Builder ClearField211() {

+        PrepareBuilder();

         result.hasField211 = false;

         result.field211_ = 0;

         return this;

@@ -3407,11 +3703,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField212(value); }

       }

       public Builder SetField212(int value) {

+        PrepareBuilder();

         result.hasField212 = true;

         result.field212_ = value;

         return this;

       }

       public Builder ClearField212() {

+        PrepareBuilder();

         result.hasField212 = false;

         result.field212_ = 0;

         return this;

@@ -3425,11 +3723,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField213(value); }

       }

       public Builder SetField213(int value) {

+        PrepareBuilder();

         result.hasField213 = true;

         result.field213_ = value;

         return this;

       }

       public Builder ClearField213() {

+        PrepareBuilder();

         result.hasField213 = false;

         result.field213_ = 0;

         return this;

@@ -3443,11 +3743,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField216(value); }

       }

       public Builder SetField216(int value) {

+        PrepareBuilder();

         result.hasField216 = true;

         result.field216_ = value;

         return this;

       }

       public Builder ClearField216() {

+        PrepareBuilder();

         result.hasField216 = false;

         result.field216_ = 0;

         return this;

@@ -3461,11 +3763,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField217(value); }

       }

       public Builder SetField217(int value) {

+        PrepareBuilder();

         result.hasField217 = true;

         result.field217_ = value;

         return this;

       }

       public Builder ClearField217() {

+        PrepareBuilder();

         result.hasField217 = false;

         result.field217_ = 0;

         return this;

@@ -3479,11 +3783,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField218(value); }

       }

       public Builder SetField218(int value) {

+        PrepareBuilder();

         result.hasField218 = true;

         result.field218_ = value;

         return this;

       }

       public Builder ClearField218() {

+        PrepareBuilder();

         result.hasField218 = false;

         result.field218_ = 0;

         return this;

@@ -3497,11 +3803,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField220(value); }

       }

       public Builder SetField220(int value) {

+        PrepareBuilder();

         result.hasField220 = true;

         result.field220_ = value;

         return this;

       }

       public Builder ClearField220() {

+        PrepareBuilder();

         result.hasField220 = false;

         result.field220_ = 0;

         return this;

@@ -3515,11 +3823,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField221(value); }

       }

       public Builder SetField221(int value) {

+        PrepareBuilder();

         result.hasField221 = true;

         result.field221_ = value;

         return this;

       }

       public Builder ClearField221() {

+        PrepareBuilder();

         result.hasField221 = false;

         result.field221_ = 0;

         return this;

@@ -3533,11 +3843,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField222(value); }

       }

       public Builder SetField222(float value) {

+        PrepareBuilder();

         result.hasField222 = true;

         result.field222_ = value;

         return this;

       }

       public Builder ClearField222() {

+        PrepareBuilder();

         result.hasField222 = false;

         result.field222_ = 0F;

         return this;

@@ -3551,18 +3863,20 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField63(value); }

       }

       public Builder SetField63(int value) {

+        PrepareBuilder();

         result.hasField63 = true;

         result.field63_ = value;

         return this;

       }

       public Builder ClearField63() {

+        PrepareBuilder();

         result.hasField63 = false;

         result.field63_ = 0;

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.SizeMessage2.Types.Group1> Group1List {

-        get { return result.group1_; }

+        get { return PrepareBuilder().group1_; }

       }

       public int Group1Count {

         get { return result.Group1Count; }

@@ -3572,35 +3886,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetGroup1(int index, global::Google.ProtocolBuffers.TestProtos.SizeMessage2.Types.Group1 value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.group1_[index] = value;

         return this;

       }

       public Builder SetGroup1(int index, global::Google.ProtocolBuffers.TestProtos.SizeMessage2.Types.Group1.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.group1_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddGroup1(global::Google.ProtocolBuffers.TestProtos.SizeMessage2.Types.Group1 value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.group1_.Add(value);

         return this;

       }

       public Builder AddGroup1(global::Google.ProtocolBuffers.TestProtos.SizeMessage2.Types.Group1.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.group1_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeGroup1(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.SizeMessage2.Types.Group1> values) {

+        PrepareBuilder();

         base.AddRange(values, result.group1_);

         return this;

       }

       public Builder ClearGroup1() {

+        PrepareBuilder();

         result.group1_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> Field128List {

-        get { return result.field128_; }

+        get { return PrepareBuilder().field128_; }

       }

       public int Field128Count {

         get { return result.Field128Count; }

@@ -3610,19 +3930,23 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField128(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.field128_[index] = value;

         return this;

       }

       public Builder AddField128(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.field128_.Add(value);

         return this;

       }

       public Builder AddRangeField128(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.field128_);

         return this;

       }

       public Builder ClearField128() {

+        PrepareBuilder();

         result.field128_.Clear();

         return this;

       }

@@ -3635,18 +3959,20 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField131(value); }

       }

       public Builder SetField131(long value) {

+        PrepareBuilder();

         result.hasField131 = true;

         result.field131_ = value;

         return this;

       }

       public Builder ClearField131() {

+        PrepareBuilder();

         result.hasField131 = false;

         result.field131_ = 0L;

         return this;

       }

       

       public pbc::IPopsicleList<string> Field127List {

-        get { return result.field127_; }

+        get { return PrepareBuilder().field127_; }

       }

       public int Field127Count {

         get { return result.Field127Count; }

@@ -3656,19 +3982,23 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField127(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.field127_[index] = value;

         return this;

       }

       public Builder AddField127(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.field127_.Add(value);

         return this;

       }

       public Builder AddRangeField127(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.field127_);

         return this;

       }

       public Builder ClearField127() {

+        PrepareBuilder();

         result.field127_.Clear();

         return this;

       }

@@ -3681,18 +4011,20 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField129(value); }

       }

       public Builder SetField129(int value) {

+        PrepareBuilder();

         result.hasField129 = true;

         result.field129_ = value;

         return this;

       }

       public Builder ClearField129() {

+        PrepareBuilder();

         result.hasField129 = false;

         result.field129_ = 0;

         return this;

       }

       

       public pbc::IPopsicleList<long> Field130List {

-        get { return result.field130_; }

+        get { return PrepareBuilder().field130_; }

       }

       public int Field130Count {

         get { return result.Field130Count; }

@@ -3701,18 +4033,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetField130(index);

       }

       public Builder SetField130(int index, long value) {

+        PrepareBuilder();

         result.field130_[index] = value;

         return this;

       }

       public Builder AddField130(long value) {

+        PrepareBuilder();

         result.field130_.Add(value);

         return this;

       }

       public Builder AddRangeField130(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.field130_);

         return this;

       }

       public Builder ClearField130() {

+        PrepareBuilder();

         result.field130_.Clear();

         return this;

       }

@@ -3725,11 +4061,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField205(value); }

       }

       public Builder SetField205(bool value) {

+        PrepareBuilder();

         result.hasField205 = true;

         result.field205_ = value;

         return this;

       }

       public Builder ClearField205() {

+        PrepareBuilder();

         result.hasField205 = false;

         result.field205_ = false;

         return this;

@@ -3743,11 +4081,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField206(value); }

       }

       public Builder SetField206(bool value) {

+        PrepareBuilder();

         result.hasField206 = true;

         result.field206_ = value;

         return this;

       }

       public Builder ClearField206() {

+        PrepareBuilder();

         result.hasField206 = false;

         result.field206_ = false;

         return this;

@@ -3927,7 +4267,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(SizeMessage2GroupedMessage prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -3937,21 +4277,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new SizeMessage2GroupedMessage();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(SizeMessage2GroupedMessage cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      SizeMessage2GroupedMessage result = new SizeMessage2GroupedMessage();

+      bool builderIsReadOnly;

+      SizeMessage2GroupedMessage result;

+      

+      private SizeMessage2GroupedMessage PrepareBuilder() {

+        if (builderIsReadOnly) {

+          SizeMessage2GroupedMessage original = result;

+          result = new SizeMessage2GroupedMessage();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override SizeMessage2GroupedMessage MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new SizeMessage2GroupedMessage();

+        result = DefaultInstance ?? new SizeMessage2GroupedMessage();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -3963,12 +4330,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SizeMessage2GroupedMessage BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        SizeMessage2GroupedMessage returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       

@@ -3980,11 +4346,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField1(value); }

       }

       public Builder SetField1(float value) {

+        PrepareBuilder();

         result.hasField1 = true;

         result.field1_ = value;

         return this;

       }

       public Builder ClearField1() {

+        PrepareBuilder();

         result.hasField1 = false;

         result.field1_ = 0F;

         return this;

@@ -3998,11 +4366,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField2(value); }

       }

       public Builder SetField2(float value) {

+        PrepareBuilder();

         result.hasField2 = true;

         result.field2_ = value;

         return this;

       }

       public Builder ClearField2() {

+        PrepareBuilder();

         result.hasField2 = false;

         result.field2_ = 0F;

         return this;

@@ -4016,11 +4386,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField3(value); }

       }

       public Builder SetField3(float value) {

+        PrepareBuilder();

         result.hasField3 = true;

         result.field3_ = value;

         return this;

       }

       public Builder ClearField3() {

+        PrepareBuilder();

         result.hasField3 = false;

         result.field3_ = 0F;

         return this;

@@ -4034,11 +4406,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField4(value); }

       }

       public Builder SetField4(bool value) {

+        PrepareBuilder();

         result.hasField4 = true;

         result.field4_ = value;

         return this;

       }

       public Builder ClearField4() {

+        PrepareBuilder();

         result.hasField4 = false;

         result.field4_ = false;

         return this;

@@ -4052,11 +4426,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField5(value); }

       }

       public Builder SetField5(bool value) {

+        PrepareBuilder();

         result.hasField5 = true;

         result.field5_ = value;

         return this;

       }

       public Builder ClearField5() {

+        PrepareBuilder();

         result.hasField5 = false;

         result.field5_ = false;

         return this;

@@ -4070,11 +4446,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField6(value); }

       }

       public Builder SetField6(bool value) {

+        PrepareBuilder();

         result.hasField6 = true;

         result.field6_ = value;

         return this;

       }

       public Builder ClearField6() {

+        PrepareBuilder();

         result.hasField6 = false;

         result.field6_ = true;

         return this;

@@ -4088,11 +4466,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField7(value); }

       }

       public Builder SetField7(bool value) {

+        PrepareBuilder();

         result.hasField7 = true;

         result.field7_ = value;

         return this;

       }

       public Builder ClearField7() {

+        PrepareBuilder();

         result.hasField7 = false;

         result.field7_ = false;

         return this;

@@ -4106,11 +4486,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField8(value); }

       }

       public Builder SetField8(float value) {

+        PrepareBuilder();

         result.hasField8 = true;

         result.field8_ = value;

         return this;

       }

       public Builder ClearField8() {

+        PrepareBuilder();

         result.hasField8 = false;

         result.field8_ = 0F;

         return this;

@@ -4124,11 +4506,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField9(value); }

       }

       public Builder SetField9(bool value) {

+        PrepareBuilder();

         result.hasField9 = true;

         result.field9_ = value;

         return this;

       }

       public Builder ClearField9() {

+        PrepareBuilder();

         result.hasField9 = false;

         result.field9_ = false;

         return this;

@@ -4142,11 +4526,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField10(value); }

       }

       public Builder SetField10(float value) {

+        PrepareBuilder();

         result.hasField10 = true;

         result.field10_ = value;

         return this;

       }

       public Builder ClearField10() {

+        PrepareBuilder();

         result.hasField10 = false;

         result.field10_ = 0F;

         return this;

@@ -4160,11 +4546,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField11(value); }

       }

       public Builder SetField11(long value) {

+        PrepareBuilder();

         result.hasField11 = true;

         result.field11_ = value;

         return this;

       }

       public Builder ClearField11() {

+        PrepareBuilder();

         result.hasField11 = false;

         result.field11_ = 0L;

         return this;

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSpeedProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSpeedProtoFile.cs
index 918be643..379a86f9 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSpeedProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSpeedProtoFile.cs
@@ -885,7 +885,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(SpeedMessage1 prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -895,21 +895,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new SpeedMessage1();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(SpeedMessage1 cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      SpeedMessage1 result;

       

-      SpeedMessage1 result = new SpeedMessage1();

+      private SpeedMessage1 PrepareBuilder() {

+        if (builderIsReadOnly) {

+          SpeedMessage1 original = result;

+          result = new SpeedMessage1();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override SpeedMessage1 MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new SpeedMessage1();

+        result = DefaultInstance ?? new SpeedMessage1();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -921,13 +948,12 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SpeedMessage1 BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.field5_.MakeReadOnly();

-        SpeedMessage1 returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -941,6 +967,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(SpeedMessage1 other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.SpeedMessage1.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasField1) {

           Field1 = other.Field1;

         }

@@ -1073,6 +1100,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -1295,11 +1323,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField1(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField1 = true;

         result.field1_ = value;

         return this;

       }

       public Builder ClearField1() {

+        PrepareBuilder();

         result.hasField1 = false;

         result.field1_ = "";

         return this;

@@ -1314,11 +1344,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField9(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField9 = true;

         result.field9_ = value;

         return this;

       }

       public Builder ClearField9() {

+        PrepareBuilder();

         result.hasField9 = false;

         result.field9_ = "";

         return this;

@@ -1333,11 +1365,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField18(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField18 = true;

         result.field18_ = value;

         return this;

       }

       public Builder ClearField18() {

+        PrepareBuilder();

         result.hasField18 = false;

         result.field18_ = "";

         return this;

@@ -1351,11 +1385,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField80(value); }

       }

       public Builder SetField80(bool value) {

+        PrepareBuilder();

         result.hasField80 = true;

         result.field80_ = value;

         return this;

       }

       public Builder ClearField80() {

+        PrepareBuilder();

         result.hasField80 = false;

         result.field80_ = false;

         return this;

@@ -1369,11 +1405,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField81(value); }

       }

       public Builder SetField81(bool value) {

+        PrepareBuilder();

         result.hasField81 = true;

         result.field81_ = value;

         return this;

       }

       public Builder ClearField81() {

+        PrepareBuilder();

         result.hasField81 = false;

         result.field81_ = true;

         return this;

@@ -1387,11 +1425,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField2(value); }

       }

       public Builder SetField2(int value) {

+        PrepareBuilder();

         result.hasField2 = true;

         result.field2_ = value;

         return this;

       }

       public Builder ClearField2() {

+        PrepareBuilder();

         result.hasField2 = false;

         result.field2_ = 0;

         return this;

@@ -1405,11 +1445,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField3(value); }

       }

       public Builder SetField3(int value) {

+        PrepareBuilder();

         result.hasField3 = true;

         result.field3_ = value;

         return this;

       }

       public Builder ClearField3() {

+        PrepareBuilder();

         result.hasField3 = false;

         result.field3_ = 0;

         return this;

@@ -1423,11 +1465,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField280(value); }

       }

       public Builder SetField280(int value) {

+        PrepareBuilder();

         result.hasField280 = true;

         result.field280_ = value;

         return this;

       }

       public Builder ClearField280() {

+        PrepareBuilder();

         result.hasField280 = false;

         result.field280_ = 0;

         return this;

@@ -1441,11 +1485,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField6(value); }

       }

       public Builder SetField6(int value) {

+        PrepareBuilder();

         result.hasField6 = true;

         result.field6_ = value;

         return this;

       }

       public Builder ClearField6() {

+        PrepareBuilder();

         result.hasField6 = false;

         result.field6_ = 0;

         return this;

@@ -1459,11 +1505,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField22(value); }

       }

       public Builder SetField22(long value) {

+        PrepareBuilder();

         result.hasField22 = true;

         result.field22_ = value;

         return this;

       }

       public Builder ClearField22() {

+        PrepareBuilder();

         result.hasField22 = false;

         result.field22_ = 0L;

         return this;

@@ -1478,11 +1526,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField4(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField4 = true;

         result.field4_ = value;

         return this;

       }

       public Builder ClearField4() {

+        PrepareBuilder();

         result.hasField4 = false;

         result.field4_ = "";

         return this;

@@ -1490,7 +1540,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> Field5List {

-        get { return result.field5_; }

+        get { return PrepareBuilder().field5_; }

       }

       public int Field5Count {

         get { return result.Field5Count; }

@@ -1501,20 +1551,24 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetField5(int index, ulong value) {

+        PrepareBuilder();

         result.field5_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddField5(ulong value) {

+        PrepareBuilder();

         result.field5_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeField5(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.field5_);

         return this;

       }

       public Builder ClearField5() {

+        PrepareBuilder();

         result.field5_.Clear();

         return this;

       }

@@ -1527,11 +1581,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField59(value); }

       }

       public Builder SetField59(bool value) {

+        PrepareBuilder();

         result.hasField59 = true;

         result.field59_ = value;

         return this;

       }

       public Builder ClearField59() {

+        PrepareBuilder();

         result.hasField59 = false;

         result.field59_ = false;

         return this;

@@ -1546,11 +1602,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField7(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField7 = true;

         result.field7_ = value;

         return this;

       }

       public Builder ClearField7() {

+        PrepareBuilder();

         result.hasField7 = false;

         result.field7_ = "";

         return this;

@@ -1564,11 +1622,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField16(value); }

       }

       public Builder SetField16(int value) {

+        PrepareBuilder();

         result.hasField16 = true;

         result.field16_ = value;

         return this;

       }

       public Builder ClearField16() {

+        PrepareBuilder();

         result.hasField16 = false;

         result.field16_ = 0;

         return this;

@@ -1582,11 +1642,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField130(value); }

       }

       public Builder SetField130(int value) {

+        PrepareBuilder();

         result.hasField130 = true;

         result.field130_ = value;

         return this;

       }

       public Builder ClearField130() {

+        PrepareBuilder();

         result.hasField130 = false;

         result.field130_ = 0;

         return this;

@@ -1600,11 +1662,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField12(value); }

       }

       public Builder SetField12(bool value) {

+        PrepareBuilder();

         result.hasField12 = true;

         result.field12_ = value;

         return this;

       }

       public Builder ClearField12() {

+        PrepareBuilder();

         result.hasField12 = false;

         result.field12_ = true;

         return this;

@@ -1618,11 +1682,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField17(value); }

       }

       public Builder SetField17(bool value) {

+        PrepareBuilder();

         result.hasField17 = true;

         result.field17_ = value;

         return this;

       }

       public Builder ClearField17() {

+        PrepareBuilder();

         result.hasField17 = false;

         result.field17_ = true;

         return this;

@@ -1636,11 +1702,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField13(value); }

       }

       public Builder SetField13(bool value) {

+        PrepareBuilder();

         result.hasField13 = true;

         result.field13_ = value;

         return this;

       }

       public Builder ClearField13() {

+        PrepareBuilder();

         result.hasField13 = false;

         result.field13_ = true;

         return this;

@@ -1654,11 +1722,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField14(value); }

       }

       public Builder SetField14(bool value) {

+        PrepareBuilder();

         result.hasField14 = true;

         result.field14_ = value;

         return this;

       }

       public Builder ClearField14() {

+        PrepareBuilder();

         result.hasField14 = false;

         result.field14_ = true;

         return this;

@@ -1672,11 +1742,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField104(value); }

       }

       public Builder SetField104(int value) {

+        PrepareBuilder();

         result.hasField104 = true;

         result.field104_ = value;

         return this;

       }

       public Builder ClearField104() {

+        PrepareBuilder();

         result.hasField104 = false;

         result.field104_ = 0;

         return this;

@@ -1690,11 +1762,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField100(value); }

       }

       public Builder SetField100(int value) {

+        PrepareBuilder();

         result.hasField100 = true;

         result.field100_ = value;

         return this;

       }

       public Builder ClearField100() {

+        PrepareBuilder();

         result.hasField100 = false;

         result.field100_ = 0;

         return this;

@@ -1708,11 +1782,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField101(value); }

       }

       public Builder SetField101(int value) {

+        PrepareBuilder();

         result.hasField101 = true;

         result.field101_ = value;

         return this;

       }

       public Builder ClearField101() {

+        PrepareBuilder();

         result.hasField101 = false;

         result.field101_ = 0;

         return this;

@@ -1727,11 +1803,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField102(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField102 = true;

         result.field102_ = value;

         return this;

       }

       public Builder ClearField102() {

+        PrepareBuilder();

         result.hasField102 = false;

         result.field102_ = "";

         return this;

@@ -1746,11 +1824,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField103(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField103 = true;

         result.field103_ = value;

         return this;

       }

       public Builder ClearField103() {

+        PrepareBuilder();

         result.hasField103 = false;

         result.field103_ = "";

         return this;

@@ -1764,11 +1844,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField29(value); }

       }

       public Builder SetField29(int value) {

+        PrepareBuilder();

         result.hasField29 = true;

         result.field29_ = value;

         return this;

       }

       public Builder ClearField29() {

+        PrepareBuilder();

         result.hasField29 = false;

         result.field29_ = 0;

         return this;

@@ -1782,11 +1864,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField30(value); }

       }

       public Builder SetField30(bool value) {

+        PrepareBuilder();

         result.hasField30 = true;

         result.field30_ = value;

         return this;

       }

       public Builder ClearField30() {

+        PrepareBuilder();

         result.hasField30 = false;

         result.field30_ = false;

         return this;

@@ -1800,11 +1884,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField60(value); }

       }

       public Builder SetField60(int value) {

+        PrepareBuilder();

         result.hasField60 = true;

         result.field60_ = value;

         return this;

       }

       public Builder ClearField60() {

+        PrepareBuilder();

         result.hasField60 = false;

         result.field60_ = -1;

         return this;

@@ -1818,11 +1904,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField271(value); }

       }

       public Builder SetField271(int value) {

+        PrepareBuilder();

         result.hasField271 = true;

         result.field271_ = value;

         return this;

       }

       public Builder ClearField271() {

+        PrepareBuilder();

         result.hasField271 = false;

         result.field271_ = -1;

         return this;

@@ -1836,11 +1924,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField272(value); }

       }

       public Builder SetField272(int value) {

+        PrepareBuilder();

         result.hasField272 = true;

         result.field272_ = value;

         return this;

       }

       public Builder ClearField272() {

+        PrepareBuilder();

         result.hasField272 = false;

         result.field272_ = -1;

         return this;

@@ -1854,11 +1944,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField150(value); }

       }

       public Builder SetField150(int value) {

+        PrepareBuilder();

         result.hasField150 = true;

         result.field150_ = value;

         return this;

       }

       public Builder ClearField150() {

+        PrepareBuilder();

         result.hasField150 = false;

         result.field150_ = 0;

         return this;

@@ -1872,11 +1964,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField23(value); }

       }

       public Builder SetField23(int value) {

+        PrepareBuilder();

         result.hasField23 = true;

         result.field23_ = value;

         return this;

       }

       public Builder ClearField23() {

+        PrepareBuilder();

         result.hasField23 = false;

         result.field23_ = 0;

         return this;

@@ -1890,11 +1984,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField24(value); }

       }

       public Builder SetField24(bool value) {

+        PrepareBuilder();

         result.hasField24 = true;

         result.field24_ = value;

         return this;

       }

       public Builder ClearField24() {

+        PrepareBuilder();

         result.hasField24 = false;

         result.field24_ = false;

         return this;

@@ -1908,11 +2004,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField25(value); }

       }

       public Builder SetField25(int value) {

+        PrepareBuilder();

         result.hasField25 = true;

         result.field25_ = value;

         return this;

       }

       public Builder ClearField25() {

+        PrepareBuilder();

         result.hasField25 = false;

         result.field25_ = 0;

         return this;

@@ -1927,18 +2025,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField15(global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField15 = true;

         result.field15_ = value;

         return this;

       }

       public Builder SetField15(global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasField15 = true;

         result.field15_ = builderForValue.Build();

         return this;

       }

       public Builder MergeField15(global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasField15 &&

             result.field15_ != global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage.DefaultInstance) {

             result.field15_ = global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage.CreateBuilder(result.field15_).MergeFrom(value).BuildPartial();

@@ -1949,6 +2050,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearField15() {

+        PrepareBuilder();

         result.hasField15 = false;

         result.field15_ = global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage.DefaultInstance;

         return this;

@@ -1962,11 +2064,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField78(value); }

       }

       public Builder SetField78(bool value) {

+        PrepareBuilder();

         result.hasField78 = true;

         result.field78_ = value;

         return this;

       }

       public Builder ClearField78() {

+        PrepareBuilder();

         result.hasField78 = false;

         result.field78_ = false;

         return this;

@@ -1980,11 +2084,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField67(value); }

       }

       public Builder SetField67(int value) {

+        PrepareBuilder();

         result.hasField67 = true;

         result.field67_ = value;

         return this;

       }

       public Builder ClearField67() {

+        PrepareBuilder();

         result.hasField67 = false;

         result.field67_ = 0;

         return this;

@@ -1998,11 +2104,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField68(value); }

       }

       public Builder SetField68(int value) {

+        PrepareBuilder();

         result.hasField68 = true;

         result.field68_ = value;

         return this;

       }

       public Builder ClearField68() {

+        PrepareBuilder();

         result.hasField68 = false;

         result.field68_ = 0;

         return this;

@@ -2016,11 +2124,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField128(value); }

       }

       public Builder SetField128(int value) {

+        PrepareBuilder();

         result.hasField128 = true;

         result.field128_ = value;

         return this;

       }

       public Builder ClearField128() {

+        PrepareBuilder();

         result.hasField128 = false;

         result.field128_ = 0;

         return this;

@@ -2035,11 +2145,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField129(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField129 = true;

         result.field129_ = value;

         return this;

       }

       public Builder ClearField129() {

+        PrepareBuilder();

         result.hasField129 = false;

         result.field129_ = "xxxxxxxxxxxxxxxxxxxxx";

         return this;

@@ -2053,11 +2165,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField131(value); }

       }

       public Builder SetField131(int value) {

+        PrepareBuilder();

         result.hasField131 = true;

         result.field131_ = value;

         return this;

       }

       public Builder ClearField131() {

+        PrepareBuilder();

         result.hasField131 = false;

         result.field131_ = 0;

         return this;

@@ -2478,7 +2592,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(SpeedMessage1SubMessage prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -2488,21 +2602,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new SpeedMessage1SubMessage();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(SpeedMessage1SubMessage cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      SpeedMessage1SubMessage result = new SpeedMessage1SubMessage();

+      bool builderIsReadOnly;

+      SpeedMessage1SubMessage result;

+      

+      private SpeedMessage1SubMessage PrepareBuilder() {

+        if (builderIsReadOnly) {

+          SpeedMessage1SubMessage original = result;

+          result = new SpeedMessage1SubMessage();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override SpeedMessage1SubMessage MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new SpeedMessage1SubMessage();

+        result = DefaultInstance ?? new SpeedMessage1SubMessage();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -2514,12 +2655,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SpeedMessage1SubMessage BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        SpeedMessage1SubMessage returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -2533,6 +2673,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(SpeedMessage1SubMessage other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasField1) {

           Field1 = other.Field1;

         }

@@ -2602,6 +2743,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -2733,11 +2875,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField1(value); }

       }

       public Builder SetField1(int value) {

+        PrepareBuilder();

         result.hasField1 = true;

         result.field1_ = value;

         return this;

       }

       public Builder ClearField1() {

+        PrepareBuilder();

         result.hasField1 = false;

         result.field1_ = 0;

         return this;

@@ -2751,11 +2895,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField2(value); }

       }

       public Builder SetField2(int value) {

+        PrepareBuilder();

         result.hasField2 = true;

         result.field2_ = value;

         return this;

       }

       public Builder ClearField2() {

+        PrepareBuilder();

         result.hasField2 = false;

         result.field2_ = 0;

         return this;

@@ -2769,11 +2915,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField3(value); }

       }

       public Builder SetField3(int value) {

+        PrepareBuilder();

         result.hasField3 = true;

         result.field3_ = value;

         return this;

       }

       public Builder ClearField3() {

+        PrepareBuilder();

         result.hasField3 = false;

         result.field3_ = 0;

         return this;

@@ -2788,11 +2936,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField15(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField15 = true;

         result.field15_ = value;

         return this;

       }

       public Builder ClearField15() {

+        PrepareBuilder();

         result.hasField15 = false;

         result.field15_ = "";

         return this;

@@ -2806,11 +2956,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField12(value); }

       }

       public Builder SetField12(bool value) {

+        PrepareBuilder();

         result.hasField12 = true;

         result.field12_ = value;

         return this;

       }

       public Builder ClearField12() {

+        PrepareBuilder();

         result.hasField12 = false;

         result.field12_ = true;

         return this;

@@ -2824,11 +2976,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField13(value); }

       }

       public Builder SetField13(long value) {

+        PrepareBuilder();

         result.hasField13 = true;

         result.field13_ = value;

         return this;

       }

       public Builder ClearField13() {

+        PrepareBuilder();

         result.hasField13 = false;

         result.field13_ = 0L;

         return this;

@@ -2842,11 +2996,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField14(value); }

       }

       public Builder SetField14(long value) {

+        PrepareBuilder();

         result.hasField14 = true;

         result.field14_ = value;

         return this;

       }

       public Builder ClearField14() {

+        PrepareBuilder();

         result.hasField14 = false;

         result.field14_ = 0L;

         return this;

@@ -2860,11 +3016,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField16(value); }

       }

       public Builder SetField16(int value) {

+        PrepareBuilder();

         result.hasField16 = true;

         result.field16_ = value;

         return this;

       }

       public Builder ClearField16() {

+        PrepareBuilder();

         result.hasField16 = false;

         result.field16_ = 0;

         return this;

@@ -2878,11 +3036,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField19(value); }

       }

       public Builder SetField19(int value) {

+        PrepareBuilder();

         result.hasField19 = true;

         result.field19_ = value;

         return this;

       }

       public Builder ClearField19() {

+        PrepareBuilder();

         result.hasField19 = false;

         result.field19_ = 2;

         return this;

@@ -2896,11 +3056,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField20(value); }

       }

       public Builder SetField20(bool value) {

+        PrepareBuilder();

         result.hasField20 = true;

         result.field20_ = value;

         return this;

       }

       public Builder ClearField20() {

+        PrepareBuilder();

         result.hasField20 = false;

         result.field20_ = true;

         return this;

@@ -2914,11 +3076,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField28(value); }

       }

       public Builder SetField28(bool value) {

+        PrepareBuilder();

         result.hasField28 = true;

         result.field28_ = value;

         return this;

       }

       public Builder ClearField28() {

+        PrepareBuilder();

         result.hasField28 = false;

         result.field28_ = true;

         return this;

@@ -2934,11 +3098,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetField21(ulong value) {

+        PrepareBuilder();

         result.hasField21 = true;

         result.field21_ = value;

         return this;

       }

       public Builder ClearField21() {

+        PrepareBuilder();

         result.hasField21 = false;

         result.field21_ = 0;

         return this;

@@ -2952,11 +3118,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField22(value); }

       }

       public Builder SetField22(int value) {

+        PrepareBuilder();

         result.hasField22 = true;

         result.field22_ = value;

         return this;

       }

       public Builder ClearField22() {

+        PrepareBuilder();

         result.hasField22 = false;

         result.field22_ = 0;

         return this;

@@ -2970,11 +3138,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField23(value); }

       }

       public Builder SetField23(bool value) {

+        PrepareBuilder();

         result.hasField23 = true;

         result.field23_ = value;

         return this;

       }

       public Builder ClearField23() {

+        PrepareBuilder();

         result.hasField23 = false;

         result.field23_ = false;

         return this;

@@ -2988,11 +3158,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField206(value); }

       }

       public Builder SetField206(bool value) {

+        PrepareBuilder();

         result.hasField206 = true;

         result.field206_ = value;

         return this;

       }

       public Builder ClearField206() {

+        PrepareBuilder();

         result.hasField206 = false;

         result.field206_ = false;

         return this;

@@ -3008,11 +3180,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetField203(uint value) {

+        PrepareBuilder();

         result.hasField203 = true;

         result.field203_ = value;

         return this;

       }

       public Builder ClearField203() {

+        PrepareBuilder();

         result.hasField203 = false;

         result.field203_ = 0;

         return this;

@@ -3026,11 +3200,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField204(value); }

       }

       public Builder SetField204(int value) {

+        PrepareBuilder();

         result.hasField204 = true;

         result.field204_ = value;

         return this;

       }

       public Builder ClearField204() {

+        PrepareBuilder();

         result.hasField204 = false;

         result.field204_ = 0;

         return this;

@@ -3045,11 +3221,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField205(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField205 = true;

         result.field205_ = value;

         return this;

       }

       public Builder ClearField205() {

+        PrepareBuilder();

         result.hasField205 = false;

         result.field205_ = "";

         return this;

@@ -3065,11 +3243,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetField207(ulong value) {

+        PrepareBuilder();

         result.hasField207 = true;

         result.field207_ = value;

         return this;

       }

       public Builder ClearField207() {

+        PrepareBuilder();

         result.hasField207 = false;

         result.field207_ = 0UL;

         return this;

@@ -3085,11 +3265,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetField300(ulong value) {

+        PrepareBuilder();

         result.hasField300 = true;

         result.field300_ = value;

         return this;

       }

       public Builder ClearField300() {

+        PrepareBuilder();

         result.hasField300 = false;

         result.field300_ = 0UL;

         return this;

@@ -3498,7 +3680,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(Group1 prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -3508,21 +3690,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new Group1();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(Group1 cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          Group1 result;

+          

+          private Group1 PrepareBuilder() {

+            if (builderIsReadOnly) {

+              Group1 original = result;

+              result = new Group1();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

           

-          Group1 result = new Group1();

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override Group1 MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new Group1();

+            result = DefaultInstance ?? new Group1();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -3534,15 +3743,14 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Group1 BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

             result.field14_.MakeReadOnly();

             result.field22_.MakeReadOnly();

             result.field73_.MakeReadOnly();

-            Group1 returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -3556,6 +3764,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(Group1 other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.SpeedMessage2.Types.Group1.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasField11) {

               Field11 = other.Field11;

             }

@@ -3613,6 +3822,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -3734,11 +3944,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetField11(value); }

           }

           public Builder SetField11(float value) {

+            PrepareBuilder();

             result.hasField11 = true;

             result.field11_ = value;

             return this;

           }

           public Builder ClearField11() {

+            PrepareBuilder();

             result.hasField11 = false;

             result.field11_ = 0F;

             return this;

@@ -3752,11 +3964,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetField26(value); }

           }

           public Builder SetField26(float value) {

+            PrepareBuilder();

             result.hasField26 = true;

             result.field26_ = value;

             return this;

           }

           public Builder ClearField26() {

+            PrepareBuilder();

             result.hasField26 = false;

             result.field26_ = 0F;

             return this;

@@ -3771,11 +3985,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField12(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasField12 = true;

             result.field12_ = value;

             return this;

           }

           public Builder ClearField12() {

+            PrepareBuilder();

             result.hasField12 = false;

             result.field12_ = "";

             return this;

@@ -3790,18 +4006,20 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField13(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasField13 = true;

             result.field13_ = value;

             return this;

           }

           public Builder ClearField13() {

+            PrepareBuilder();

             result.hasField13 = false;

             result.field13_ = "";

             return this;

           }

           

           public pbc::IPopsicleList<string> Field14List {

-            get { return result.field14_; }

+            get { return PrepareBuilder().field14_; }

           }

           public int Field14Count {

             get { return result.Field14Count; }

@@ -3811,19 +4029,23 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField14(int index, string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.field14_[index] = value;

             return this;

           }

           public Builder AddField14(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.field14_.Add(value);

             return this;

           }

           public Builder AddRangeField14(scg::IEnumerable<string> values) {

+            PrepareBuilder();

             base.AddRange(values, result.field14_);

             return this;

           }

           public Builder ClearField14() {

+            PrepareBuilder();

             result.field14_.Clear();

             return this;

           }

@@ -3838,11 +4060,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           [global::System.CLSCompliant(false)]

           public Builder SetField15(ulong value) {

+            PrepareBuilder();

             result.hasField15 = true;

             result.field15_ = value;

             return this;

           }

           public Builder ClearField15() {

+            PrepareBuilder();

             result.hasField15 = false;

             result.field15_ = 0UL;

             return this;

@@ -3856,11 +4080,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetField5(value); }

           }

           public Builder SetField5(int value) {

+            PrepareBuilder();

             result.hasField5 = true;

             result.field5_ = value;

             return this;

           }

           public Builder ClearField5() {

+            PrepareBuilder();

             result.hasField5 = false;

             result.field5_ = 0;

             return this;

@@ -3875,11 +4101,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField27(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasField27 = true;

             result.field27_ = value;

             return this;

           }

           public Builder ClearField27() {

+            PrepareBuilder();

             result.hasField27 = false;

             result.field27_ = "";

             return this;

@@ -3893,11 +4121,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetField28(value); }

           }

           public Builder SetField28(int value) {

+            PrepareBuilder();

             result.hasField28 = true;

             result.field28_ = value;

             return this;

           }

           public Builder ClearField28() {

+            PrepareBuilder();

             result.hasField28 = false;

             result.field28_ = 0;

             return this;

@@ -3912,11 +4142,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField29(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasField29 = true;

             result.field29_ = value;

             return this;

           }

           public Builder ClearField29() {

+            PrepareBuilder();

             result.hasField29 = false;

             result.field29_ = "";

             return this;

@@ -3931,18 +4163,20 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField16(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasField16 = true;

             result.field16_ = value;

             return this;

           }

           public Builder ClearField16() {

+            PrepareBuilder();

             result.hasField16 = false;

             result.field16_ = "";

             return this;

           }

           

           public pbc::IPopsicleList<string> Field22List {

-            get { return result.field22_; }

+            get { return PrepareBuilder().field22_; }

           }

           public int Field22Count {

             get { return result.Field22Count; }

@@ -3952,25 +4186,29 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField22(int index, string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.field22_[index] = value;

             return this;

           }

           public Builder AddField22(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.field22_.Add(value);

             return this;

           }

           public Builder AddRangeField22(scg::IEnumerable<string> values) {

+            PrepareBuilder();

             base.AddRange(values, result.field22_);

             return this;

           }

           public Builder ClearField22() {

+            PrepareBuilder();

             result.field22_.Clear();

             return this;

           }

           

           public pbc::IPopsicleList<int> Field73List {

-            get { return result.field73_; }

+            get { return PrepareBuilder().field73_; }

           }

           public int Field73Count {

             get { return result.Field73Count; }

@@ -3979,18 +4217,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             return result.GetField73(index);

           }

           public Builder SetField73(int index, int value) {

+            PrepareBuilder();

             result.field73_[index] = value;

             return this;

           }

           public Builder AddField73(int value) {

+            PrepareBuilder();

             result.field73_.Add(value);

             return this;

           }

           public Builder AddRangeField73(scg::IEnumerable<int> values) {

+            PrepareBuilder();

             base.AddRange(values, result.field73_);

             return this;

           }

           public Builder ClearField73() {

+            PrepareBuilder();

             result.field73_.Clear();

             return this;

           }

@@ -4003,11 +4245,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetField20(value); }

           }

           public Builder SetField20(int value) {

+            PrepareBuilder();

             result.hasField20 = true;

             result.field20_ = value;

             return this;

           }

           public Builder ClearField20() {

+            PrepareBuilder();

             result.hasField20 = false;

             result.field20_ = 0;

             return this;

@@ -4022,11 +4266,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField24(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasField24 = true;

             result.field24_ = value;

             return this;

           }

           public Builder ClearField24() {

+            PrepareBuilder();

             result.hasField24 = false;

             result.field24_ = "";

             return this;

@@ -4041,18 +4287,21 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetField31(global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasField31 = true;

             result.field31_ = value;

             return this;

           }

           public Builder SetField31(global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage.Builder builderForValue) {

             pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+            PrepareBuilder();

             result.hasField31 = true;

             result.field31_ = builderForValue.Build();

             return this;

           }

           public Builder MergeField31(global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             if (result.hasField31 &&

                 result.field31_ != global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage.DefaultInstance) {

                 result.field31_ = global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage.CreateBuilder(result.field31_).MergeFrom(value).BuildPartial();

@@ -4063,6 +4312,7 @@ namespace Google.ProtocolBuffers.TestProtos {
             return this;

           }

           public Builder ClearField31() {

+            PrepareBuilder();

             result.hasField31 = false;

             result.field31_ = global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage.DefaultInstance;

             return this;

@@ -4638,7 +4888,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(SpeedMessage2 prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -4648,21 +4898,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new SpeedMessage2();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(SpeedMessage2 cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      SpeedMessage2 result = new SpeedMessage2();

+      bool builderIsReadOnly;

+      SpeedMessage2 result;

+      

+      private SpeedMessage2 PrepareBuilder() {

+        if (builderIsReadOnly) {

+          SpeedMessage2 original = result;

+          result = new SpeedMessage2();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override SpeedMessage2 MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new SpeedMessage2();

+        result = DefaultInstance ?? new SpeedMessage2();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -4674,16 +4951,15 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SpeedMessage2 BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.group1_.MakeReadOnly();

         result.field128_.MakeReadOnly();

         result.field127_.MakeReadOnly();

         result.field130_.MakeReadOnly();

-        SpeedMessage2 returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -4697,6 +4973,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(SpeedMessage2 other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.SpeedMessage2.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasField1) {

           Field1 = other.Field1;

         }

@@ -4796,6 +5073,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -4969,11 +5247,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField1(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField1 = true;

         result.field1_ = value;

         return this;

       }

       public Builder ClearField1() {

+        PrepareBuilder();

         result.hasField1 = false;

         result.field1_ = "";

         return this;

@@ -4987,11 +5267,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField3(value); }

       }

       public Builder SetField3(long value) {

+        PrepareBuilder();

         result.hasField3 = true;

         result.field3_ = value;

         return this;

       }

       public Builder ClearField3() {

+        PrepareBuilder();

         result.hasField3 = false;

         result.field3_ = 0L;

         return this;

@@ -5005,11 +5287,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField4(value); }

       }

       public Builder SetField4(long value) {

+        PrepareBuilder();

         result.hasField4 = true;

         result.field4_ = value;

         return this;

       }

       public Builder ClearField4() {

+        PrepareBuilder();

         result.hasField4 = false;

         result.field4_ = 0L;

         return this;

@@ -5023,11 +5307,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField30(value); }

       }

       public Builder SetField30(long value) {

+        PrepareBuilder();

         result.hasField30 = true;

         result.field30_ = value;

         return this;

       }

       public Builder ClearField30() {

+        PrepareBuilder();

         result.hasField30 = false;

         result.field30_ = 0L;

         return this;

@@ -5041,11 +5327,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField75(value); }

       }

       public Builder SetField75(bool value) {

+        PrepareBuilder();

         result.hasField75 = true;

         result.field75_ = value;

         return this;

       }

       public Builder ClearField75() {

+        PrepareBuilder();

         result.hasField75 = false;

         result.field75_ = false;

         return this;

@@ -5060,11 +5348,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField6(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField6 = true;

         result.field6_ = value;

         return this;

       }

       public Builder ClearField6() {

+        PrepareBuilder();

         result.hasField6 = false;

         result.field6_ = "";

         return this;

@@ -5079,11 +5369,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField2(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasField2 = true;

         result.field2_ = value;

         return this;

       }

       public Builder ClearField2() {

+        PrepareBuilder();

         result.hasField2 = false;

         result.field2_ = pb::ByteString.Empty;

         return this;

@@ -5097,11 +5389,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField21(value); }

       }

       public Builder SetField21(int value) {

+        PrepareBuilder();

         result.hasField21 = true;

         result.field21_ = value;

         return this;

       }

       public Builder ClearField21() {

+        PrepareBuilder();

         result.hasField21 = false;

         result.field21_ = 0;

         return this;

@@ -5115,11 +5409,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField71(value); }

       }

       public Builder SetField71(int value) {

+        PrepareBuilder();

         result.hasField71 = true;

         result.field71_ = value;

         return this;

       }

       public Builder ClearField71() {

+        PrepareBuilder();

         result.hasField71 = false;

         result.field71_ = 0;

         return this;

@@ -5133,11 +5429,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField25(value); }

       }

       public Builder SetField25(float value) {

+        PrepareBuilder();

         result.hasField25 = true;

         result.field25_ = value;

         return this;

       }

       public Builder ClearField25() {

+        PrepareBuilder();

         result.hasField25 = false;

         result.field25_ = 0F;

         return this;

@@ -5151,11 +5449,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField109(value); }

       }

       public Builder SetField109(int value) {

+        PrepareBuilder();

         result.hasField109 = true;

         result.field109_ = value;

         return this;

       }

       public Builder ClearField109() {

+        PrepareBuilder();

         result.hasField109 = false;

         result.field109_ = 0;

         return this;

@@ -5169,11 +5469,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField210(value); }

       }

       public Builder SetField210(int value) {

+        PrepareBuilder();

         result.hasField210 = true;

         result.field210_ = value;

         return this;

       }

       public Builder ClearField210() {

+        PrepareBuilder();

         result.hasField210 = false;

         result.field210_ = 0;

         return this;

@@ -5187,11 +5489,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField211(value); }

       }

       public Builder SetField211(int value) {

+        PrepareBuilder();

         result.hasField211 = true;

         result.field211_ = value;

         return this;

       }

       public Builder ClearField211() {

+        PrepareBuilder();

         result.hasField211 = false;

         result.field211_ = 0;

         return this;

@@ -5205,11 +5509,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField212(value); }

       }

       public Builder SetField212(int value) {

+        PrepareBuilder();

         result.hasField212 = true;

         result.field212_ = value;

         return this;

       }

       public Builder ClearField212() {

+        PrepareBuilder();

         result.hasField212 = false;

         result.field212_ = 0;

         return this;

@@ -5223,11 +5529,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField213(value); }

       }

       public Builder SetField213(int value) {

+        PrepareBuilder();

         result.hasField213 = true;

         result.field213_ = value;

         return this;

       }

       public Builder ClearField213() {

+        PrepareBuilder();

         result.hasField213 = false;

         result.field213_ = 0;

         return this;

@@ -5241,11 +5549,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField216(value); }

       }

       public Builder SetField216(int value) {

+        PrepareBuilder();

         result.hasField216 = true;

         result.field216_ = value;

         return this;

       }

       public Builder ClearField216() {

+        PrepareBuilder();

         result.hasField216 = false;

         result.field216_ = 0;

         return this;

@@ -5259,11 +5569,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField217(value); }

       }

       public Builder SetField217(int value) {

+        PrepareBuilder();

         result.hasField217 = true;

         result.field217_ = value;

         return this;

       }

       public Builder ClearField217() {

+        PrepareBuilder();

         result.hasField217 = false;

         result.field217_ = 0;

         return this;

@@ -5277,11 +5589,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField218(value); }

       }

       public Builder SetField218(int value) {

+        PrepareBuilder();

         result.hasField218 = true;

         result.field218_ = value;

         return this;

       }

       public Builder ClearField218() {

+        PrepareBuilder();

         result.hasField218 = false;

         result.field218_ = 0;

         return this;

@@ -5295,11 +5609,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField220(value); }

       }

       public Builder SetField220(int value) {

+        PrepareBuilder();

         result.hasField220 = true;

         result.field220_ = value;

         return this;

       }

       public Builder ClearField220() {

+        PrepareBuilder();

         result.hasField220 = false;

         result.field220_ = 0;

         return this;

@@ -5313,11 +5629,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField221(value); }

       }

       public Builder SetField221(int value) {

+        PrepareBuilder();

         result.hasField221 = true;

         result.field221_ = value;

         return this;

       }

       public Builder ClearField221() {

+        PrepareBuilder();

         result.hasField221 = false;

         result.field221_ = 0;

         return this;

@@ -5331,11 +5649,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField222(value); }

       }

       public Builder SetField222(float value) {

+        PrepareBuilder();

         result.hasField222 = true;

         result.field222_ = value;

         return this;

       }

       public Builder ClearField222() {

+        PrepareBuilder();

         result.hasField222 = false;

         result.field222_ = 0F;

         return this;

@@ -5349,18 +5669,20 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField63(value); }

       }

       public Builder SetField63(int value) {

+        PrepareBuilder();

         result.hasField63 = true;

         result.field63_ = value;

         return this;

       }

       public Builder ClearField63() {

+        PrepareBuilder();

         result.hasField63 = false;

         result.field63_ = 0;

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.SpeedMessage2.Types.Group1> Group1List {

-        get { return result.group1_; }

+        get { return PrepareBuilder().group1_; }

       }

       public int Group1Count {

         get { return result.Group1Count; }

@@ -5370,35 +5692,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetGroup1(int index, global::Google.ProtocolBuffers.TestProtos.SpeedMessage2.Types.Group1 value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.group1_[index] = value;

         return this;

       }

       public Builder SetGroup1(int index, global::Google.ProtocolBuffers.TestProtos.SpeedMessage2.Types.Group1.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.group1_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddGroup1(global::Google.ProtocolBuffers.TestProtos.SpeedMessage2.Types.Group1 value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.group1_.Add(value);

         return this;

       }

       public Builder AddGroup1(global::Google.ProtocolBuffers.TestProtos.SpeedMessage2.Types.Group1.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.group1_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeGroup1(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.SpeedMessage2.Types.Group1> values) {

+        PrepareBuilder();

         base.AddRange(values, result.group1_);

         return this;

       }

       public Builder ClearGroup1() {

+        PrepareBuilder();

         result.group1_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> Field128List {

-        get { return result.field128_; }

+        get { return PrepareBuilder().field128_; }

       }

       public int Field128Count {

         get { return result.Field128Count; }

@@ -5408,19 +5736,23 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField128(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.field128_[index] = value;

         return this;

       }

       public Builder AddField128(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.field128_.Add(value);

         return this;

       }

       public Builder AddRangeField128(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.field128_);

         return this;

       }

       public Builder ClearField128() {

+        PrepareBuilder();

         result.field128_.Clear();

         return this;

       }

@@ -5433,18 +5765,20 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField131(value); }

       }

       public Builder SetField131(long value) {

+        PrepareBuilder();

         result.hasField131 = true;

         result.field131_ = value;

         return this;

       }

       public Builder ClearField131() {

+        PrepareBuilder();

         result.hasField131 = false;

         result.field131_ = 0L;

         return this;

       }

       

       public pbc::IPopsicleList<string> Field127List {

-        get { return result.field127_; }

+        get { return PrepareBuilder().field127_; }

       }

       public int Field127Count {

         get { return result.Field127Count; }

@@ -5454,19 +5788,23 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetField127(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.field127_[index] = value;

         return this;

       }

       public Builder AddField127(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.field127_.Add(value);

         return this;

       }

       public Builder AddRangeField127(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.field127_);

         return this;

       }

       public Builder ClearField127() {

+        PrepareBuilder();

         result.field127_.Clear();

         return this;

       }

@@ -5479,18 +5817,20 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField129(value); }

       }

       public Builder SetField129(int value) {

+        PrepareBuilder();

         result.hasField129 = true;

         result.field129_ = value;

         return this;

       }

       public Builder ClearField129() {

+        PrepareBuilder();

         result.hasField129 = false;

         result.field129_ = 0;

         return this;

       }

       

       public pbc::IPopsicleList<long> Field130List {

-        get { return result.field130_; }

+        get { return PrepareBuilder().field130_; }

       }

       public int Field130Count {

         get { return result.Field130Count; }

@@ -5499,18 +5839,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetField130(index);

       }

       public Builder SetField130(int index, long value) {

+        PrepareBuilder();

         result.field130_[index] = value;

         return this;

       }

       public Builder AddField130(long value) {

+        PrepareBuilder();

         result.field130_.Add(value);

         return this;

       }

       public Builder AddRangeField130(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.field130_);

         return this;

       }

       public Builder ClearField130() {

+        PrepareBuilder();

         result.field130_.Clear();

         return this;

       }

@@ -5523,11 +5867,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField205(value); }

       }

       public Builder SetField205(bool value) {

+        PrepareBuilder();

         result.hasField205 = true;

         result.field205_ = value;

         return this;

       }

       public Builder ClearField205() {

+        PrepareBuilder();

         result.hasField205 = false;

         result.field205_ = false;

         return this;

@@ -5541,11 +5887,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField206(value); }

       }

       public Builder SetField206(bool value) {

+        PrepareBuilder();

         result.hasField206 = true;

         result.field206_ = value;

         return this;

       }

       public Builder ClearField206() {

+        PrepareBuilder();

         result.hasField206 = false;

         result.field206_ = false;

         return this;

@@ -5818,7 +6166,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(SpeedMessage2GroupedMessage prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -5828,21 +6176,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new SpeedMessage2GroupedMessage();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(SpeedMessage2GroupedMessage cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      SpeedMessage2GroupedMessage result = new SpeedMessage2GroupedMessage();

+      bool builderIsReadOnly;

+      SpeedMessage2GroupedMessage result;

+      

+      private SpeedMessage2GroupedMessage PrepareBuilder() {

+        if (builderIsReadOnly) {

+          SpeedMessage2GroupedMessage original = result;

+          result = new SpeedMessage2GroupedMessage();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override SpeedMessage2GroupedMessage MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new SpeedMessage2GroupedMessage();

+        result = DefaultInstance ?? new SpeedMessage2GroupedMessage();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -5854,12 +6229,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SpeedMessage2GroupedMessage BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        SpeedMessage2GroupedMessage returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -5873,6 +6247,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(SpeedMessage2GroupedMessage other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasField1) {

           Field1 = other.Field1;

         }

@@ -5915,6 +6290,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -6010,11 +6386,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField1(value); }

       }

       public Builder SetField1(float value) {

+        PrepareBuilder();

         result.hasField1 = true;

         result.field1_ = value;

         return this;

       }

       public Builder ClearField1() {

+        PrepareBuilder();

         result.hasField1 = false;

         result.field1_ = 0F;

         return this;

@@ -6028,11 +6406,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField2(value); }

       }

       public Builder SetField2(float value) {

+        PrepareBuilder();

         result.hasField2 = true;

         result.field2_ = value;

         return this;

       }

       public Builder ClearField2() {

+        PrepareBuilder();

         result.hasField2 = false;

         result.field2_ = 0F;

         return this;

@@ -6046,11 +6426,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField3(value); }

       }

       public Builder SetField3(float value) {

+        PrepareBuilder();

         result.hasField3 = true;

         result.field3_ = value;

         return this;

       }

       public Builder ClearField3() {

+        PrepareBuilder();

         result.hasField3 = false;

         result.field3_ = 0F;

         return this;

@@ -6064,11 +6446,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField4(value); }

       }

       public Builder SetField4(bool value) {

+        PrepareBuilder();

         result.hasField4 = true;

         result.field4_ = value;

         return this;

       }

       public Builder ClearField4() {

+        PrepareBuilder();

         result.hasField4 = false;

         result.field4_ = false;

         return this;

@@ -6082,11 +6466,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField5(value); }

       }

       public Builder SetField5(bool value) {

+        PrepareBuilder();

         result.hasField5 = true;

         result.field5_ = value;

         return this;

       }

       public Builder ClearField5() {

+        PrepareBuilder();

         result.hasField5 = false;

         result.field5_ = false;

         return this;

@@ -6100,11 +6486,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField6(value); }

       }

       public Builder SetField6(bool value) {

+        PrepareBuilder();

         result.hasField6 = true;

         result.field6_ = value;

         return this;

       }

       public Builder ClearField6() {

+        PrepareBuilder();

         result.hasField6 = false;

         result.field6_ = true;

         return this;

@@ -6118,11 +6506,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField7(value); }

       }

       public Builder SetField7(bool value) {

+        PrepareBuilder();

         result.hasField7 = true;

         result.field7_ = value;

         return this;

       }

       public Builder ClearField7() {

+        PrepareBuilder();

         result.hasField7 = false;

         result.field7_ = false;

         return this;

@@ -6136,11 +6526,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField8(value); }

       }

       public Builder SetField8(float value) {

+        PrepareBuilder();

         result.hasField8 = true;

         result.field8_ = value;

         return this;

       }

       public Builder ClearField8() {

+        PrepareBuilder();

         result.hasField8 = false;

         result.field8_ = 0F;

         return this;

@@ -6154,11 +6546,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField9(value); }

       }

       public Builder SetField9(bool value) {

+        PrepareBuilder();

         result.hasField9 = true;

         result.field9_ = value;

         return this;

       }

       public Builder ClearField9() {

+        PrepareBuilder();

         result.hasField9 = false;

         result.field9_ = false;

         return this;

@@ -6172,11 +6566,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField10(value); }

       }

       public Builder SetField10(float value) {

+        PrepareBuilder();

         result.hasField10 = true;

         result.field10_ = value;

         return this;

       }

       public Builder ClearField10() {

+        PrepareBuilder();

         result.hasField10 = false;

         result.field10_ = 0F;

         return this;

@@ -6190,11 +6586,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetField11(value); }

       }

       public Builder SetField11(long value) {

+        PrepareBuilder();

         result.hasField11 = true;

         result.field11_ = value;

         return this;

       }

       public Builder ClearField11() {

+        PrepareBuilder();

         result.hasField11 = false;

         result.field11_ = 0L;

         return this;

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestImportLiteProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestImportLiteProtoFile.cs
index a96833be..0df40f42 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestImportLiteProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestImportLiteProtoFile.cs
@@ -150,7 +150,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(ImportMessageLite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -160,21 +160,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new ImportMessageLite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(ImportMessageLite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      ImportMessageLite result;

       

-      ImportMessageLite result = new ImportMessageLite();

+      private ImportMessageLite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          ImportMessageLite original = result;

+          result = new ImportMessageLite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override ImportMessageLite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new ImportMessageLite();

+        result = DefaultInstance ?? new ImportMessageLite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override ImportMessageLite DefaultInstanceForType {

@@ -182,12 +209,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ImportMessageLite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        ImportMessageLite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -201,6 +227,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(ImportMessageLite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.ImportMessageLite.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasD) {

           D = other.D;

         }

@@ -212,6 +239,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -254,11 +282,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetD(value); }

       }

       public Builder SetD(int value) {

+        PrepareBuilder();

         result.hasD = true;

         result.d_ = value;

         return this;

       }

       public Builder ClearD() {

+        PrepareBuilder();

         result.hasD = false;

         result.d_ = 0;

         return this;

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestImportProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestImportProtoFile.cs
index 5749c97f..75e746a8 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestImportProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestImportProtoFile.cs
@@ -169,7 +169,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(ImportMessage prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -179,21 +179,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new ImportMessage();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(ImportMessage cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      ImportMessage result;

       

-      ImportMessage result = new ImportMessage();

+      private ImportMessage PrepareBuilder() {

+        if (builderIsReadOnly) {

+          ImportMessage original = result;

+          result = new ImportMessage();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override ImportMessage MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new ImportMessage();

+        result = DefaultInstance ?? new ImportMessage();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -205,12 +232,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ImportMessage BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        ImportMessage returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -224,6 +250,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(ImportMessage other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.ImportMessage.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasD) {

           D = other.D;

         }

@@ -236,6 +263,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -291,11 +319,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetD(value); }

       }

       public Builder SetD(int value) {

+        PrepareBuilder();

         result.hasD = true;

         result.d_ = value;

         return this;

       }

       public Builder ClearD() {

+        PrepareBuilder();

         result.hasD = false;

         result.d_ = 0;

         return this;

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestMessageSetProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestMessageSetProtoFile.cs
index 8a6bb589..f51b2f42 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestMessageSetProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestMessageSetProtoFile.cs
@@ -189,7 +189,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestMessageSet prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -199,21 +199,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestMessageSet();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestMessageSet cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestMessageSet result = new TestMessageSet();

+      bool builderIsReadOnly;

+      TestMessageSet result;

+      

+      private TestMessageSet PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestMessageSet original = result;

+          result = new TestMessageSet();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestMessageSet MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestMessageSet();

+        result = DefaultInstance ?? new TestMessageSet();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -225,12 +252,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMessageSet BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestMessageSet returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -244,6 +270,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestMessageSet other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestMessageSet.DefaultInstance) return this;

+        PrepareBuilder();

           this.MergeExtensionFields(other);

         this.MergeUnknownFields(other.UnknownFields);

         return this;

@@ -254,6 +281,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -404,7 +432,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestMessageSetContainer prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -414,21 +442,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestMessageSetContainer();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestMessageSetContainer cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestMessageSetContainer result = new TestMessageSetContainer();

+      bool builderIsReadOnly;

+      TestMessageSetContainer result;

+      

+      private TestMessageSetContainer PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestMessageSetContainer original = result;

+          result = new TestMessageSetContainer();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestMessageSetContainer MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestMessageSetContainer();

+        result = DefaultInstance ?? new TestMessageSetContainer();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -440,12 +495,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMessageSetContainer BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestMessageSetContainer returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -459,6 +513,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestMessageSetContainer other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestMessageSetContainer.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasMessageSet) {

           MergeMessageSet(other.MessageSet);

         }

@@ -471,6 +526,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -532,18 +588,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetMessageSet(global::Google.ProtocolBuffers.TestProtos.TestMessageSet value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasMessageSet = true;

         result.messageSet_ = value;

         return this;

       }

       public Builder SetMessageSet(global::Google.ProtocolBuffers.TestProtos.TestMessageSet.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasMessageSet = true;

         result.messageSet_ = builderForValue.Build();

         return this;

       }

       public Builder MergeMessageSet(global::Google.ProtocolBuffers.TestProtos.TestMessageSet value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasMessageSet &&

             result.messageSet_ != global::Google.ProtocolBuffers.TestProtos.TestMessageSet.DefaultInstance) {

             result.messageSet_ = global::Google.ProtocolBuffers.TestProtos.TestMessageSet.CreateBuilder(result.messageSet_).MergeFrom(value).BuildPartial();

@@ -554,6 +613,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearMessageSet() {

+        PrepareBuilder();

         result.hasMessageSet = false;

         result.messageSet_ = global::Google.ProtocolBuffers.TestProtos.TestMessageSet.DefaultInstance;

         return this;

@@ -668,7 +728,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestMessageSetExtension1 prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -678,21 +738,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestMessageSetExtension1();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestMessageSetExtension1 cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestMessageSetExtension1 result = new TestMessageSetExtension1();

+      bool builderIsReadOnly;

+      TestMessageSetExtension1 result;

+      

+      private TestMessageSetExtension1 PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestMessageSetExtension1 original = result;

+          result = new TestMessageSetExtension1();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestMessageSetExtension1 MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestMessageSetExtension1();

+        result = DefaultInstance ?? new TestMessageSetExtension1();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -704,12 +791,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMessageSetExtension1 BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestMessageSetExtension1 returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -723,6 +809,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestMessageSetExtension1 other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestMessageSetExtension1.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasI) {

           I = other.I;

         }

@@ -735,6 +822,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -790,11 +878,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetI(value); }

       }

       public Builder SetI(int value) {

+        PrepareBuilder();

         result.hasI = true;

         result.i_ = value;

         return this;

       }

       public Builder ClearI() {

+        PrepareBuilder();

         result.hasI = false;

         result.i_ = 0;

         return this;

@@ -909,7 +999,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestMessageSetExtension2 prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -919,21 +1009,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestMessageSetExtension2();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestMessageSetExtension2 cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestMessageSetExtension2 result = new TestMessageSetExtension2();

+      bool builderIsReadOnly;

+      TestMessageSetExtension2 result;

+      

+      private TestMessageSetExtension2 PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestMessageSetExtension2 original = result;

+          result = new TestMessageSetExtension2();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestMessageSetExtension2 MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestMessageSetExtension2();

+        result = DefaultInstance ?? new TestMessageSetExtension2();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -945,12 +1062,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMessageSetExtension2 BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestMessageSetExtension2 returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -964,6 +1080,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestMessageSetExtension2 other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestMessageSetExtension2.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasStr) {

           Str = other.Str;

         }

@@ -976,6 +1093,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -1032,11 +1150,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetStr(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasStr = true;

         result.str_ = value;

         return this;

       }

       public Builder ClearStr() {

+        PrepareBuilder();

         result.hasStr = false;

         result.str_ = "";

         return this;

@@ -1199,7 +1319,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(Item prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1209,21 +1329,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new Item();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(Item cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

           

-          Item result = new Item();

+          bool builderIsReadOnly;

+          Item result;

+          

+          private Item PrepareBuilder() {

+            if (builderIsReadOnly) {

+              Item original = result;

+              result = new Item();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override Item MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new Item();

+            result = DefaultInstance ?? new Item();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -1235,12 +1382,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Item BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            Item returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -1254,6 +1400,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(Item other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.RawMessageSet.Types.Item.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasTypeId) {

               TypeId = other.TypeId;

             }

@@ -1269,6 +1416,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -1328,11 +1476,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetTypeId(value); }

           }

           public Builder SetTypeId(int value) {

+            PrepareBuilder();

             result.hasTypeId = true;

             result.typeId_ = value;

             return this;

           }

           public Builder ClearTypeId() {

+            PrepareBuilder();

             result.hasTypeId = false;

             result.typeId_ = 0;

             return this;

@@ -1347,11 +1497,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetMessage(pb::ByteString value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasMessage = true;

             result.message_ = value;

             return this;

           }

           public Builder ClearMessage() {

+            PrepareBuilder();

             result.hasMessage = false;

             result.message_ = pb::ByteString.Empty;

             return this;

@@ -1442,7 +1594,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(RawMessageSet prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1452,21 +1604,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new RawMessageSet();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(RawMessageSet cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      RawMessageSet result = new RawMessageSet();

+      bool builderIsReadOnly;

+      RawMessageSet result;

+      

+      private RawMessageSet PrepareBuilder() {

+        if (builderIsReadOnly) {

+          RawMessageSet original = result;

+          result = new RawMessageSet();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override RawMessageSet MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new RawMessageSet();

+        result = DefaultInstance ?? new RawMessageSet();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1478,13 +1657,12 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override RawMessageSet BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.item_.MakeReadOnly();

-        RawMessageSet returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -1498,6 +1676,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(RawMessageSet other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.RawMessageSet.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.item_.Count != 0) {

           base.AddRange(other.item_, result.item_);

         }

@@ -1510,6 +1689,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -1558,7 +1738,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.RawMessageSet.Types.Item> ItemList {

-        get { return result.item_; }

+        get { return PrepareBuilder().item_; }

       }

       public int ItemCount {

         get { return result.ItemCount; }

@@ -1568,29 +1748,35 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetItem(int index, global::Google.ProtocolBuffers.TestProtos.RawMessageSet.Types.Item value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.item_[index] = value;

         return this;

       }

       public Builder SetItem(int index, global::Google.ProtocolBuffers.TestProtos.RawMessageSet.Types.Item.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.item_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddItem(global::Google.ProtocolBuffers.TestProtos.RawMessageSet.Types.Item value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.item_.Add(value);

         return this;

       }

       public Builder AddItem(global::Google.ProtocolBuffers.TestProtos.RawMessageSet.Types.Item.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.item_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeItem(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.RawMessageSet.Types.Item> values) {

+        PrepareBuilder();

         base.AddRange(values, result.item_);

         return this;

       }

       public Builder ClearItem() {

+        PrepareBuilder();

         result.item_.Clear();

         return this;

       }

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestNoGenericServicesProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestNoGenericServicesProtoFile.cs
index 74d6af60..ca401926 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestNoGenericServicesProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestNoGenericServicesProtoFile.cs
@@ -183,7 +183,7 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestMessage prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -193,21 +193,48 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestMessage();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestMessage cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestMessage result;

       

-      TestMessage result = new TestMessage();

+      private TestMessage PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestMessage original = result;

+          result = new TestMessage();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestMessage MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestMessage();

+        result = DefaultInstance ?? new TestMessage();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -219,12 +246,11 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
       }

       

       public override TestMessage BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestMessage returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -238,6 +264,7 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
       

       public override Builder MergeFrom(TestMessage other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.NoGenericService.TestMessage.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           A = other.A;

         }

@@ -251,6 +278,7 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -306,11 +334,13 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
         set { SetA(value); }

       }

       public Builder SetA(int value) {

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = 0;

         return this;

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestOptimizeForProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestOptimizeForProtoFile.cs
index c7153abc..967d549a 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestOptimizeForProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestOptimizeForProtoFile.cs
@@ -164,7 +164,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestOptimizedForSize prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -174,21 +174,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestOptimizedForSize();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestOptimizedForSize cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestOptimizedForSize result = new TestOptimizedForSize();

+      bool builderIsReadOnly;

+      TestOptimizedForSize result;

+      

+      private TestOptimizedForSize PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestOptimizedForSize original = result;

+          result = new TestOptimizedForSize();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestOptimizedForSize MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestOptimizedForSize();

+        result = DefaultInstance ?? new TestOptimizedForSize();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -200,12 +227,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestOptimizedForSize BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestOptimizedForSize returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       

@@ -217,11 +243,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetI(value); }

       }

       public Builder SetI(int value) {

+        PrepareBuilder();

         result.hasI = true;

         result.i_ = value;

         return this;

       }

       public Builder ClearI() {

+        PrepareBuilder();

         result.hasI = false;

         result.i_ = 0;

         return this;

@@ -236,18 +264,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetMsg(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasMsg = true;

         result.msg_ = value;

         return this;

       }

       public Builder SetMsg(global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasMsg = true;

         result.msg_ = builderForValue.Build();

         return this;

       }

       public Builder MergeMsg(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasMsg &&

             result.msg_ != global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance) {

             result.msg_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.CreateBuilder(result.msg_).MergeFrom(value).BuildPartial();

@@ -258,6 +289,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearMsg() {

+        PrepareBuilder();

         result.hasMsg = false;

         result.msg_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

         return this;

@@ -337,7 +369,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestRequiredOptimizedForSize prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -347,21 +379,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestRequiredOptimizedForSize();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestRequiredOptimizedForSize cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestRequiredOptimizedForSize result = new TestRequiredOptimizedForSize();

+      bool builderIsReadOnly;

+      TestRequiredOptimizedForSize result;

+      

+      private TestRequiredOptimizedForSize PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestRequiredOptimizedForSize original = result;

+          result = new TestRequiredOptimizedForSize();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestRequiredOptimizedForSize MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestRequiredOptimizedForSize();

+        result = DefaultInstance ?? new TestRequiredOptimizedForSize();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -373,12 +432,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRequiredOptimizedForSize BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestRequiredOptimizedForSize returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       

@@ -390,11 +448,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetX(value); }

       }

       public Builder SetX(int value) {

+        PrepareBuilder();

         result.hasX = true;

         result.x_ = value;

         return this;

       }

       public Builder ClearX() {

+        PrepareBuilder();

         result.hasX = false;

         result.x_ = 0;

         return this;

@@ -474,7 +534,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestOptionalOptimizedForSize prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -484,21 +544,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestOptionalOptimizedForSize();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestOptionalOptimizedForSize cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestOptionalOptimizedForSize result = new TestOptionalOptimizedForSize();

+      bool builderIsReadOnly;

+      TestOptionalOptimizedForSize result;

+      

+      private TestOptionalOptimizedForSize PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestOptionalOptimizedForSize original = result;

+          result = new TestOptionalOptimizedForSize();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestOptionalOptimizedForSize MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestOptionalOptimizedForSize();

+        result = DefaultInstance ?? new TestOptionalOptimizedForSize();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -510,12 +597,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestOptionalOptimizedForSize BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestOptionalOptimizedForSize returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       

@@ -528,18 +614,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetO(global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasO = true;

         result.o_ = value;

         return this;

       }

       public Builder SetO(global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasO = true;

         result.o_ = builderForValue.Build();

         return this;

       }

       public Builder MergeO(global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasO &&

             result.o_ != global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize.DefaultInstance) {

             result.o_ = global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize.CreateBuilder(result.o_).MergeFrom(value).BuildPartial();

@@ -550,6 +639,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearO() {

+        PrepareBuilder();

         result.hasO = false;

         result.o_ = global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize.DefaultInstance;

         return this;

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestProtoFile.cs
index d8824d49..aeea673d 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestProtoFile.cs
@@ -1260,7 +1260,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(NestedMessage prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1270,21 +1270,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new NestedMessage();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(NestedMessage cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          NestedMessage result;

+          

+          private NestedMessage PrepareBuilder() {

+            if (builderIsReadOnly) {

+              NestedMessage original = result;

+              result = new NestedMessage();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

           

-          NestedMessage result = new NestedMessage();

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override NestedMessage MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new NestedMessage();

+            result = DefaultInstance ?? new NestedMessage();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -1296,12 +1323,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override NestedMessage BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            NestedMessage returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -1315,6 +1341,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(NestedMessage other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasBb) {

               Bb = other.Bb;

             }

@@ -1327,6 +1354,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -1382,11 +1410,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetBb(value); }

           }

           public Builder SetBb(int value) {

+            PrepareBuilder();

             result.hasBb = true;

             result.bb_ = value;

             return this;

           }

           public Builder ClearBb() {

+            PrepareBuilder();

             result.hasBb = false;

             result.bb_ = 0;

             return this;

@@ -1499,7 +1529,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(OptionalGroup prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1509,21 +1539,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new OptionalGroup();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(OptionalGroup cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          OptionalGroup result;

+          

+          private OptionalGroup PrepareBuilder() {

+            if (builderIsReadOnly) {

+              OptionalGroup original = result;

+              result = new OptionalGroup();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

           

-          OptionalGroup result = new OptionalGroup();

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override OptionalGroup MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new OptionalGroup();

+            result = DefaultInstance ?? new OptionalGroup();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -1535,12 +1592,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override OptionalGroup BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            OptionalGroup returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -1554,6 +1610,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(OptionalGroup other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasA) {

               A = other.A;

             }

@@ -1566,6 +1623,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -1621,11 +1679,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetA(value); }

           }

           public Builder SetA(int value) {

+            PrepareBuilder();

             result.hasA = true;

             result.a_ = value;

             return this;

           }

           public Builder ClearA() {

+            PrepareBuilder();

             result.hasA = false;

             result.a_ = 0;

             return this;

@@ -1738,7 +1798,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(RepeatedGroup prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1748,21 +1808,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new RepeatedGroup();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(RepeatedGroup cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          RepeatedGroup result;

           

-          RepeatedGroup result = new RepeatedGroup();

+          private RepeatedGroup PrepareBuilder() {

+            if (builderIsReadOnly) {

+              RepeatedGroup original = result;

+              result = new RepeatedGroup();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override RepeatedGroup MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new RepeatedGroup();

+            result = DefaultInstance ?? new RepeatedGroup();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -1774,12 +1861,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override RepeatedGroup BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            RepeatedGroup returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -1793,6 +1879,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(RepeatedGroup other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.RepeatedGroup.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasA) {

               A = other.A;

             }

@@ -1805,6 +1892,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -1860,11 +1948,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetA(value); }

           }

           public Builder SetA(int value) {

+            PrepareBuilder();

             result.hasA = true;

             result.a_ = value;

             return this;

           }

           public Builder ClearA() {

+            PrepareBuilder();

             result.hasA = false;

             result.a_ = 0;

             return this;

@@ -3181,7 +3271,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestAllTypes prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -3191,21 +3281,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestAllTypes();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestAllTypes cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestAllTypes result;

+      

+      private TestAllTypes PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestAllTypes original = result;

+          result = new TestAllTypes();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestAllTypes result = new TestAllTypes();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestAllTypes MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestAllTypes();

+        result = DefaultInstance ?? new TestAllTypes();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -3217,8 +3334,8 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestAllTypes BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.repeatedInt32_.MakeReadOnly();

         result.repeatedInt64_.MakeReadOnly();

@@ -3244,9 +3361,8 @@ namespace Google.ProtocolBuffers.TestProtos {
         result.repeatedImportEnum_.MakeReadOnly();

         result.repeatedStringPiece_.MakeReadOnly();

         result.repeatedCord_.MakeReadOnly();

-        TestAllTypes returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -3260,6 +3376,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestAllTypes other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestAllTypes.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasOptionalInt32) {

           OptionalInt32 = other.OptionalInt32;

         }

@@ -3473,6 +3590,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -3907,11 +4025,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalInt32(value); }

       }

       public Builder SetOptionalInt32(int value) {

+        PrepareBuilder();

         result.hasOptionalInt32 = true;

         result.optionalInt32_ = value;

         return this;

       }

       public Builder ClearOptionalInt32() {

+        PrepareBuilder();

         result.hasOptionalInt32 = false;

         result.optionalInt32_ = 0;

         return this;

@@ -3925,11 +4045,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalInt64(value); }

       }

       public Builder SetOptionalInt64(long value) {

+        PrepareBuilder();

         result.hasOptionalInt64 = true;

         result.optionalInt64_ = value;

         return this;

       }

       public Builder ClearOptionalInt64() {

+        PrepareBuilder();

         result.hasOptionalInt64 = false;

         result.optionalInt64_ = 0L;

         return this;

@@ -3945,11 +4067,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetOptionalUint32(uint value) {

+        PrepareBuilder();

         result.hasOptionalUint32 = true;

         result.optionalUint32_ = value;

         return this;

       }

       public Builder ClearOptionalUint32() {

+        PrepareBuilder();

         result.hasOptionalUint32 = false;

         result.optionalUint32_ = 0;

         return this;

@@ -3965,11 +4089,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetOptionalUint64(ulong value) {

+        PrepareBuilder();

         result.hasOptionalUint64 = true;

         result.optionalUint64_ = value;

         return this;

       }

       public Builder ClearOptionalUint64() {

+        PrepareBuilder();

         result.hasOptionalUint64 = false;

         result.optionalUint64_ = 0UL;

         return this;

@@ -3983,11 +4109,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalSint32(value); }

       }

       public Builder SetOptionalSint32(int value) {

+        PrepareBuilder();

         result.hasOptionalSint32 = true;

         result.optionalSint32_ = value;

         return this;

       }

       public Builder ClearOptionalSint32() {

+        PrepareBuilder();

         result.hasOptionalSint32 = false;

         result.optionalSint32_ = 0;

         return this;

@@ -4001,11 +4129,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalSint64(value); }

       }

       public Builder SetOptionalSint64(long value) {

+        PrepareBuilder();

         result.hasOptionalSint64 = true;

         result.optionalSint64_ = value;

         return this;

       }

       public Builder ClearOptionalSint64() {

+        PrepareBuilder();

         result.hasOptionalSint64 = false;

         result.optionalSint64_ = 0;

         return this;

@@ -4021,11 +4151,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetOptionalFixed32(uint value) {

+        PrepareBuilder();

         result.hasOptionalFixed32 = true;

         result.optionalFixed32_ = value;

         return this;

       }

       public Builder ClearOptionalFixed32() {

+        PrepareBuilder();

         result.hasOptionalFixed32 = false;

         result.optionalFixed32_ = 0;

         return this;

@@ -4041,11 +4173,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetOptionalFixed64(ulong value) {

+        PrepareBuilder();

         result.hasOptionalFixed64 = true;

         result.optionalFixed64_ = value;

         return this;

       }

       public Builder ClearOptionalFixed64() {

+        PrepareBuilder();

         result.hasOptionalFixed64 = false;

         result.optionalFixed64_ = 0;

         return this;

@@ -4059,11 +4193,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalSfixed32(value); }

       }

       public Builder SetOptionalSfixed32(int value) {

+        PrepareBuilder();

         result.hasOptionalSfixed32 = true;

         result.optionalSfixed32_ = value;

         return this;

       }

       public Builder ClearOptionalSfixed32() {

+        PrepareBuilder();

         result.hasOptionalSfixed32 = false;

         result.optionalSfixed32_ = 0;

         return this;

@@ -4077,11 +4213,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalSfixed64(value); }

       }

       public Builder SetOptionalSfixed64(long value) {

+        PrepareBuilder();

         result.hasOptionalSfixed64 = true;

         result.optionalSfixed64_ = value;

         return this;

       }

       public Builder ClearOptionalSfixed64() {

+        PrepareBuilder();

         result.hasOptionalSfixed64 = false;

         result.optionalSfixed64_ = 0;

         return this;

@@ -4095,11 +4233,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalFloat(value); }

       }

       public Builder SetOptionalFloat(float value) {

+        PrepareBuilder();

         result.hasOptionalFloat = true;

         result.optionalFloat_ = value;

         return this;

       }

       public Builder ClearOptionalFloat() {

+        PrepareBuilder();

         result.hasOptionalFloat = false;

         result.optionalFloat_ = 0F;

         return this;

@@ -4113,11 +4253,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalDouble(value); }

       }

       public Builder SetOptionalDouble(double value) {

+        PrepareBuilder();

         result.hasOptionalDouble = true;

         result.optionalDouble_ = value;

         return this;

       }

       public Builder ClearOptionalDouble() {

+        PrepareBuilder();

         result.hasOptionalDouble = false;

         result.optionalDouble_ = 0D;

         return this;

@@ -4131,11 +4273,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalBool(value); }

       }

       public Builder SetOptionalBool(bool value) {

+        PrepareBuilder();

         result.hasOptionalBool = true;

         result.optionalBool_ = value;

         return this;

       }

       public Builder ClearOptionalBool() {

+        PrepareBuilder();

         result.hasOptionalBool = false;

         result.optionalBool_ = false;

         return this;

@@ -4150,11 +4294,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalString(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalString = true;

         result.optionalString_ = value;

         return this;

       }

       public Builder ClearOptionalString() {

+        PrepareBuilder();

         result.hasOptionalString = false;

         result.optionalString_ = "";

         return this;

@@ -4169,11 +4315,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalBytes(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalBytes = true;

         result.optionalBytes_ = value;

         return this;

       }

       public Builder ClearOptionalBytes() {

+        PrepareBuilder();

         result.hasOptionalBytes = false;

         result.optionalBytes_ = pb::ByteString.Empty;

         return this;

@@ -4188,18 +4336,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalGroup(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalGroup = true;

         result.optionalGroup_ = value;

         return this;

       }

       public Builder SetOptionalGroup(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalGroup = true;

         result.optionalGroup_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalGroup(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalGroup &&

             result.optionalGroup_ != global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.DefaultInstance) {

             result.optionalGroup_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.CreateBuilder(result.optionalGroup_).MergeFrom(value).BuildPartial();

@@ -4210,6 +4361,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalGroup() {

+        PrepareBuilder();

         result.hasOptionalGroup = false;

         result.optionalGroup_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.DefaultInstance;

         return this;

@@ -4224,18 +4376,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalNestedMessage = true;

         result.optionalNestedMessage_ = value;

         return this;

       }

       public Builder SetOptionalNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalNestedMessage = true;

         result.optionalNestedMessage_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalNestedMessage &&

             result.optionalNestedMessage_ != global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance) {

             result.optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.CreateBuilder(result.optionalNestedMessage_).MergeFrom(value).BuildPartial();

@@ -4246,6 +4401,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalNestedMessage() {

+        PrepareBuilder();

         result.hasOptionalNestedMessage = false;

         result.optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance;

         return this;

@@ -4260,18 +4416,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalForeignMessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalForeignMessage = true;

         result.optionalForeignMessage_ = value;

         return this;

       }

       public Builder SetOptionalForeignMessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalForeignMessage = true;

         result.optionalForeignMessage_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalForeignMessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalForeignMessage &&

             result.optionalForeignMessage_ != global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance) {

             result.optionalForeignMessage_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.CreateBuilder(result.optionalForeignMessage_).MergeFrom(value).BuildPartial();

@@ -4282,6 +4441,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalForeignMessage() {

+        PrepareBuilder();

         result.hasOptionalForeignMessage = false;

         result.optionalForeignMessage_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

         return this;

@@ -4296,18 +4456,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalImportMessage(global::Google.ProtocolBuffers.TestProtos.ImportMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalImportMessage = true;

         result.optionalImportMessage_ = value;

         return this;

       }

       public Builder SetOptionalImportMessage(global::Google.ProtocolBuffers.TestProtos.ImportMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalImportMessage = true;

         result.optionalImportMessage_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalImportMessage(global::Google.ProtocolBuffers.TestProtos.ImportMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalImportMessage &&

             result.optionalImportMessage_ != global::Google.ProtocolBuffers.TestProtos.ImportMessage.DefaultInstance) {

             result.optionalImportMessage_ = global::Google.ProtocolBuffers.TestProtos.ImportMessage.CreateBuilder(result.optionalImportMessage_).MergeFrom(value).BuildPartial();

@@ -4318,6 +4481,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalImportMessage() {

+        PrepareBuilder();

         result.hasOptionalImportMessage = false;

         result.optionalImportMessage_ = global::Google.ProtocolBuffers.TestProtos.ImportMessage.DefaultInstance;

         return this;

@@ -4331,11 +4495,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalNestedEnum(value); }

       }

       public Builder SetOptionalNestedEnum(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum value) {

+        PrepareBuilder();

         result.hasOptionalNestedEnum = true;

         result.optionalNestedEnum_ = value;

         return this;

       }

       public Builder ClearOptionalNestedEnum() {

+        PrepareBuilder();

         result.hasOptionalNestedEnum = false;

         result.optionalNestedEnum_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum.FOO;

         return this;

@@ -4349,11 +4515,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalForeignEnum(value); }

       }

       public Builder SetOptionalForeignEnum(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.hasOptionalForeignEnum = true;

         result.optionalForeignEnum_ = value;

         return this;

       }

       public Builder ClearOptionalForeignEnum() {

+        PrepareBuilder();

         result.hasOptionalForeignEnum = false;

         result.optionalForeignEnum_ = global::Google.ProtocolBuffers.TestProtos.ForeignEnum.FOREIGN_FOO;

         return this;

@@ -4367,11 +4535,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalImportEnum(value); }

       }

       public Builder SetOptionalImportEnum(global::Google.ProtocolBuffers.TestProtos.ImportEnum value) {

+        PrepareBuilder();

         result.hasOptionalImportEnum = true;

         result.optionalImportEnum_ = value;

         return this;

       }

       public Builder ClearOptionalImportEnum() {

+        PrepareBuilder();

         result.hasOptionalImportEnum = false;

         result.optionalImportEnum_ = global::Google.ProtocolBuffers.TestProtos.ImportEnum.IMPORT_FOO;

         return this;

@@ -4386,11 +4556,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalStringPiece(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalStringPiece = true;

         result.optionalStringPiece_ = value;

         return this;

       }

       public Builder ClearOptionalStringPiece() {

+        PrepareBuilder();

         result.hasOptionalStringPiece = false;

         result.optionalStringPiece_ = "";

         return this;

@@ -4405,18 +4577,20 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalCord(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalCord = true;

         result.optionalCord_ = value;

         return this;

       }

       public Builder ClearOptionalCord() {

+        PrepareBuilder();

         result.hasOptionalCord = false;

         result.optionalCord_ = "";

         return this;

       }

       

       public pbc::IPopsicleList<int> RepeatedInt32List {

-        get { return result.repeatedInt32_; }

+        get { return PrepareBuilder().repeatedInt32_; }

       }

       public int RepeatedInt32Count {

         get { return result.RepeatedInt32Count; }

@@ -4425,24 +4599,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedInt32(index);

       }

       public Builder SetRepeatedInt32(int index, int value) {

+        PrepareBuilder();

         result.repeatedInt32_[index] = value;

         return this;

       }

       public Builder AddRepeatedInt32(int value) {

+        PrepareBuilder();

         result.repeatedInt32_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedInt32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedInt32_);

         return this;

       }

       public Builder ClearRepeatedInt32() {

+        PrepareBuilder();

         result.repeatedInt32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> RepeatedInt64List {

-        get { return result.repeatedInt64_; }

+        get { return PrepareBuilder().repeatedInt64_; }

       }

       public int RepeatedInt64Count {

         get { return result.RepeatedInt64Count; }

@@ -4451,25 +4629,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedInt64(index);

       }

       public Builder SetRepeatedInt64(int index, long value) {

+        PrepareBuilder();

         result.repeatedInt64_[index] = value;

         return this;

       }

       public Builder AddRepeatedInt64(long value) {

+        PrepareBuilder();

         result.repeatedInt64_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedInt64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedInt64_);

         return this;

       }

       public Builder ClearRepeatedInt64() {

+        PrepareBuilder();

         result.repeatedInt64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> RepeatedUint32List {

-        get { return result.repeatedUint32_; }

+        get { return PrepareBuilder().repeatedUint32_; }

       }

       public int RepeatedUint32Count {

         get { return result.RepeatedUint32Count; }

@@ -4480,27 +4662,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedUint32(int index, uint value) {

+        PrepareBuilder();

         result.repeatedUint32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedUint32(uint value) {

+        PrepareBuilder();

         result.repeatedUint32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedUint32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedUint32_);

         return this;

       }

       public Builder ClearRepeatedUint32() {

+        PrepareBuilder();

         result.repeatedUint32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> RepeatedUint64List {

-        get { return result.repeatedUint64_; }

+        get { return PrepareBuilder().repeatedUint64_; }

       }

       public int RepeatedUint64Count {

         get { return result.RepeatedUint64Count; }

@@ -4511,26 +4697,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedUint64(int index, ulong value) {

+        PrepareBuilder();

         result.repeatedUint64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedUint64(ulong value) {

+        PrepareBuilder();

         result.repeatedUint64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedUint64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedUint64_);

         return this;

       }

       public Builder ClearRepeatedUint64() {

+        PrepareBuilder();

         result.repeatedUint64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> RepeatedSint32List {

-        get { return result.repeatedSint32_; }

+        get { return PrepareBuilder().repeatedSint32_; }

       }

       public int RepeatedSint32Count {

         get { return result.RepeatedSint32Count; }

@@ -4539,24 +4729,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedSint32(index);

       }

       public Builder SetRepeatedSint32(int index, int value) {

+        PrepareBuilder();

         result.repeatedSint32_[index] = value;

         return this;

       }

       public Builder AddRepeatedSint32(int value) {

+        PrepareBuilder();

         result.repeatedSint32_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedSint32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedSint32_);

         return this;

       }

       public Builder ClearRepeatedSint32() {

+        PrepareBuilder();

         result.repeatedSint32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> RepeatedSint64List {

-        get { return result.repeatedSint64_; }

+        get { return PrepareBuilder().repeatedSint64_; }

       }

       public int RepeatedSint64Count {

         get { return result.RepeatedSint64Count; }

@@ -4565,25 +4759,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedSint64(index);

       }

       public Builder SetRepeatedSint64(int index, long value) {

+        PrepareBuilder();

         result.repeatedSint64_[index] = value;

         return this;

       }

       public Builder AddRepeatedSint64(long value) {

+        PrepareBuilder();

         result.repeatedSint64_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedSint64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedSint64_);

         return this;

       }

       public Builder ClearRepeatedSint64() {

+        PrepareBuilder();

         result.repeatedSint64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> RepeatedFixed32List {

-        get { return result.repeatedFixed32_; }

+        get { return PrepareBuilder().repeatedFixed32_; }

       }

       public int RepeatedFixed32Count {

         get { return result.RepeatedFixed32Count; }

@@ -4594,27 +4792,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedFixed32(int index, uint value) {

+        PrepareBuilder();

         result.repeatedFixed32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedFixed32(uint value) {

+        PrepareBuilder();

         result.repeatedFixed32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedFixed32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedFixed32_);

         return this;

       }

       public Builder ClearRepeatedFixed32() {

+        PrepareBuilder();

         result.repeatedFixed32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> RepeatedFixed64List {

-        get { return result.repeatedFixed64_; }

+        get { return PrepareBuilder().repeatedFixed64_; }

       }

       public int RepeatedFixed64Count {

         get { return result.RepeatedFixed64Count; }

@@ -4625,26 +4827,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedFixed64(int index, ulong value) {

+        PrepareBuilder();

         result.repeatedFixed64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedFixed64(ulong value) {

+        PrepareBuilder();

         result.repeatedFixed64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedFixed64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedFixed64_);

         return this;

       }

       public Builder ClearRepeatedFixed64() {

+        PrepareBuilder();

         result.repeatedFixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> RepeatedSfixed32List {

-        get { return result.repeatedSfixed32_; }

+        get { return PrepareBuilder().repeatedSfixed32_; }

       }

       public int RepeatedSfixed32Count {

         get { return result.RepeatedSfixed32Count; }

@@ -4653,24 +4859,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedSfixed32(index);

       }

       public Builder SetRepeatedSfixed32(int index, int value) {

+        PrepareBuilder();

         result.repeatedSfixed32_[index] = value;

         return this;

       }

       public Builder AddRepeatedSfixed32(int value) {

+        PrepareBuilder();

         result.repeatedSfixed32_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedSfixed32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedSfixed32_);

         return this;

       }

       public Builder ClearRepeatedSfixed32() {

+        PrepareBuilder();

         result.repeatedSfixed32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> RepeatedSfixed64List {

-        get { return result.repeatedSfixed64_; }

+        get { return PrepareBuilder().repeatedSfixed64_; }

       }

       public int RepeatedSfixed64Count {

         get { return result.RepeatedSfixed64Count; }

@@ -4679,24 +4889,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedSfixed64(index);

       }

       public Builder SetRepeatedSfixed64(int index, long value) {

+        PrepareBuilder();

         result.repeatedSfixed64_[index] = value;

         return this;

       }

       public Builder AddRepeatedSfixed64(long value) {

+        PrepareBuilder();

         result.repeatedSfixed64_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedSfixed64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedSfixed64_);

         return this;

       }

       public Builder ClearRepeatedSfixed64() {

+        PrepareBuilder();

         result.repeatedSfixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<float> RepeatedFloatList {

-        get { return result.repeatedFloat_; }

+        get { return PrepareBuilder().repeatedFloat_; }

       }

       public int RepeatedFloatCount {

         get { return result.RepeatedFloatCount; }

@@ -4705,24 +4919,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedFloat(index);

       }

       public Builder SetRepeatedFloat(int index, float value) {

+        PrepareBuilder();

         result.repeatedFloat_[index] = value;

         return this;

       }

       public Builder AddRepeatedFloat(float value) {

+        PrepareBuilder();

         result.repeatedFloat_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedFloat(scg::IEnumerable<float> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedFloat_);

         return this;

       }

       public Builder ClearRepeatedFloat() {

+        PrepareBuilder();

         result.repeatedFloat_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<double> RepeatedDoubleList {

-        get { return result.repeatedDouble_; }

+        get { return PrepareBuilder().repeatedDouble_; }

       }

       public int RepeatedDoubleCount {

         get { return result.RepeatedDoubleCount; }

@@ -4731,24 +4949,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedDouble(index);

       }

       public Builder SetRepeatedDouble(int index, double value) {

+        PrepareBuilder();

         result.repeatedDouble_[index] = value;

         return this;

       }

       public Builder AddRepeatedDouble(double value) {

+        PrepareBuilder();

         result.repeatedDouble_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedDouble(scg::IEnumerable<double> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedDouble_);

         return this;

       }

       public Builder ClearRepeatedDouble() {

+        PrepareBuilder();

         result.repeatedDouble_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<bool> RepeatedBoolList {

-        get { return result.repeatedBool_; }

+        get { return PrepareBuilder().repeatedBool_; }

       }

       public int RepeatedBoolCount {

         get { return result.RepeatedBoolCount; }

@@ -4757,24 +4979,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedBool(index);

       }

       public Builder SetRepeatedBool(int index, bool value) {

+        PrepareBuilder();

         result.repeatedBool_[index] = value;

         return this;

       }

       public Builder AddRepeatedBool(bool value) {

+        PrepareBuilder();

         result.repeatedBool_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedBool(scg::IEnumerable<bool> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedBool_);

         return this;

       }

       public Builder ClearRepeatedBool() {

+        PrepareBuilder();

         result.repeatedBool_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedStringList {

-        get { return result.repeatedString_; }

+        get { return PrepareBuilder().repeatedString_; }

       }

       public int RepeatedStringCount {

         get { return result.RepeatedStringCount; }

@@ -4784,25 +5010,29 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedString(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedString_[index] = value;

         return this;

       }

       public Builder AddRepeatedString(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedString_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedString(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedString_);

         return this;

       }

       public Builder ClearRepeatedString() {

+        PrepareBuilder();

         result.repeatedString_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<pb::ByteString> RepeatedBytesList {

-        get { return result.repeatedBytes_; }

+        get { return PrepareBuilder().repeatedBytes_; }

       }

       public int RepeatedBytesCount {

         get { return result.RepeatedBytesCount; }

@@ -4812,25 +5042,29 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedBytes(int index, pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedBytes_[index] = value;

         return this;

       }

       public Builder AddRepeatedBytes(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedBytes_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedBytes(scg::IEnumerable<pb::ByteString> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedBytes_);

         return this;

       }

       public Builder ClearRepeatedBytes() {

+        PrepareBuilder();

         result.repeatedBytes_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.RepeatedGroup> RepeatedGroupList {

-        get { return result.repeatedGroup_; }

+        get { return PrepareBuilder().repeatedGroup_; }

       }

       public int RepeatedGroupCount {

         get { return result.RepeatedGroupCount; }

@@ -4840,35 +5074,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedGroup(int index, global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.RepeatedGroup value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedGroup_[index] = value;

         return this;

       }

       public Builder SetRepeatedGroup(int index, global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.RepeatedGroup.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedGroup_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedGroup(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.RepeatedGroup value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedGroup_.Add(value);

         return this;

       }

       public Builder AddRepeatedGroup(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.RepeatedGroup.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedGroup_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedGroup(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.RepeatedGroup> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedGroup_);

         return this;

       }

       public Builder ClearRepeatedGroup() {

+        PrepareBuilder();

         result.repeatedGroup_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage> RepeatedNestedMessageList {

-        get { return result.repeatedNestedMessage_; }

+        get { return PrepareBuilder().repeatedNestedMessage_; }

       }

       public int RepeatedNestedMessageCount {

         get { return result.RepeatedNestedMessageCount; }

@@ -4878,35 +5118,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedNestedMessage(int index, global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedNestedMessage_[index] = value;

         return this;

       }

       public Builder SetRepeatedNestedMessage(int index, global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedNestedMessage_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedNestedMessage_.Add(value);

         return this;

       }

       public Builder AddRepeatedNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedNestedMessage_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedNestedMessage(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedNestedMessage_);

         return this;

       }

       public Builder ClearRepeatedNestedMessage() {

+        PrepareBuilder();

         result.repeatedNestedMessage_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignMessage> RepeatedForeignMessageList {

-        get { return result.repeatedForeignMessage_; }

+        get { return PrepareBuilder().repeatedForeignMessage_; }

       }

       public int RepeatedForeignMessageCount {

         get { return result.RepeatedForeignMessageCount; }

@@ -4916,35 +5162,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedForeignMessage(int index, global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedForeignMessage_[index] = value;

         return this;

       }

       public Builder SetRepeatedForeignMessage(int index, global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedForeignMessage_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedForeignMessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedForeignMessage_.Add(value);

         return this;

       }

       public Builder AddRepeatedForeignMessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedForeignMessage_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedForeignMessage(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignMessage> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedForeignMessage_);

         return this;

       }

       public Builder ClearRepeatedForeignMessage() {

+        PrepareBuilder();

         result.repeatedForeignMessage_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ImportMessage> RepeatedImportMessageList {

-        get { return result.repeatedImportMessage_; }

+        get { return PrepareBuilder().repeatedImportMessage_; }

       }

       public int RepeatedImportMessageCount {

         get { return result.RepeatedImportMessageCount; }

@@ -4954,35 +5206,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedImportMessage(int index, global::Google.ProtocolBuffers.TestProtos.ImportMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedImportMessage_[index] = value;

         return this;

       }

       public Builder SetRepeatedImportMessage(int index, global::Google.ProtocolBuffers.TestProtos.ImportMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedImportMessage_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedImportMessage(global::Google.ProtocolBuffers.TestProtos.ImportMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedImportMessage_.Add(value);

         return this;

       }

       public Builder AddRepeatedImportMessage(global::Google.ProtocolBuffers.TestProtos.ImportMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedImportMessage_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedImportMessage(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ImportMessage> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedImportMessage_);

         return this;

       }

       public Builder ClearRepeatedImportMessage() {

+        PrepareBuilder();

         result.repeatedImportMessage_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum> RepeatedNestedEnumList {

-        get { return result.repeatedNestedEnum_; }

+        get { return PrepareBuilder().repeatedNestedEnum_; }

       }

       public int RepeatedNestedEnumCount {

         get { return result.RepeatedNestedEnumCount; }

@@ -4991,24 +5249,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedNestedEnum(index);

       }

       public Builder SetRepeatedNestedEnum(int index, global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum value) {

+        PrepareBuilder();

         result.repeatedNestedEnum_[index] = value;

         return this;

       }

       public Builder AddRepeatedNestedEnum(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum value) {

+        PrepareBuilder();

         result.repeatedNestedEnum_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedNestedEnum(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedNestedEnum_);

         return this;

       }

       public Builder ClearRepeatedNestedEnum() {

+        PrepareBuilder();

         result.repeatedNestedEnum_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> RepeatedForeignEnumList {

-        get { return result.repeatedForeignEnum_; }

+        get { return PrepareBuilder().repeatedForeignEnum_; }

       }

       public int RepeatedForeignEnumCount {

         get { return result.RepeatedForeignEnumCount; }

@@ -5017,24 +5279,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedForeignEnum(index);

       }

       public Builder SetRepeatedForeignEnum(int index, global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.repeatedForeignEnum_[index] = value;

         return this;

       }

       public Builder AddRepeatedForeignEnum(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.repeatedForeignEnum_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedForeignEnum(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedForeignEnum_);

         return this;

       }

       public Builder ClearRepeatedForeignEnum() {

+        PrepareBuilder();

         result.repeatedForeignEnum_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ImportEnum> RepeatedImportEnumList {

-        get { return result.repeatedImportEnum_; }

+        get { return PrepareBuilder().repeatedImportEnum_; }

       }

       public int RepeatedImportEnumCount {

         get { return result.RepeatedImportEnumCount; }

@@ -5043,24 +5309,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedImportEnum(index);

       }

       public Builder SetRepeatedImportEnum(int index, global::Google.ProtocolBuffers.TestProtos.ImportEnum value) {

+        PrepareBuilder();

         result.repeatedImportEnum_[index] = value;

         return this;

       }

       public Builder AddRepeatedImportEnum(global::Google.ProtocolBuffers.TestProtos.ImportEnum value) {

+        PrepareBuilder();

         result.repeatedImportEnum_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedImportEnum(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ImportEnum> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedImportEnum_);

         return this;

       }

       public Builder ClearRepeatedImportEnum() {

+        PrepareBuilder();

         result.repeatedImportEnum_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedStringPieceList {

-        get { return result.repeatedStringPiece_; }

+        get { return PrepareBuilder().repeatedStringPiece_; }

       }

       public int RepeatedStringPieceCount {

         get { return result.RepeatedStringPieceCount; }

@@ -5070,25 +5340,29 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedStringPiece(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedStringPiece_[index] = value;

         return this;

       }

       public Builder AddRepeatedStringPiece(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedStringPiece_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedStringPiece(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedStringPiece_);

         return this;

       }

       public Builder ClearRepeatedStringPiece() {

+        PrepareBuilder();

         result.repeatedStringPiece_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedCordList {

-        get { return result.repeatedCord_; }

+        get { return PrepareBuilder().repeatedCord_; }

       }

       public int RepeatedCordCount {

         get { return result.RepeatedCordCount; }

@@ -5098,19 +5372,23 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedCord(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedCord_[index] = value;

         return this;

       }

       public Builder AddRepeatedCord(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedCord_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedCord(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedCord_);

         return this;

       }

       public Builder ClearRepeatedCord() {

+        PrepareBuilder();

         result.repeatedCord_.Clear();

         return this;

       }

@@ -5123,11 +5401,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultInt32(value); }

       }

       public Builder SetDefaultInt32(int value) {

+        PrepareBuilder();

         result.hasDefaultInt32 = true;

         result.defaultInt32_ = value;

         return this;

       }

       public Builder ClearDefaultInt32() {

+        PrepareBuilder();

         result.hasDefaultInt32 = false;

         result.defaultInt32_ = 41;

         return this;

@@ -5141,11 +5421,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultInt64(value); }

       }

       public Builder SetDefaultInt64(long value) {

+        PrepareBuilder();

         result.hasDefaultInt64 = true;

         result.defaultInt64_ = value;

         return this;

       }

       public Builder ClearDefaultInt64() {

+        PrepareBuilder();

         result.hasDefaultInt64 = false;

         result.defaultInt64_ = 42L;

         return this;

@@ -5161,11 +5443,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetDefaultUint32(uint value) {

+        PrepareBuilder();

         result.hasDefaultUint32 = true;

         result.defaultUint32_ = value;

         return this;

       }

       public Builder ClearDefaultUint32() {

+        PrepareBuilder();

         result.hasDefaultUint32 = false;

         result.defaultUint32_ = 43;

         return this;

@@ -5181,11 +5465,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetDefaultUint64(ulong value) {

+        PrepareBuilder();

         result.hasDefaultUint64 = true;

         result.defaultUint64_ = value;

         return this;

       }

       public Builder ClearDefaultUint64() {

+        PrepareBuilder();

         result.hasDefaultUint64 = false;

         result.defaultUint64_ = 44UL;

         return this;

@@ -5199,11 +5485,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultSint32(value); }

       }

       public Builder SetDefaultSint32(int value) {

+        PrepareBuilder();

         result.hasDefaultSint32 = true;

         result.defaultSint32_ = value;

         return this;

       }

       public Builder ClearDefaultSint32() {

+        PrepareBuilder();

         result.hasDefaultSint32 = false;

         result.defaultSint32_ = -45;

         return this;

@@ -5217,11 +5505,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultSint64(value); }

       }

       public Builder SetDefaultSint64(long value) {

+        PrepareBuilder();

         result.hasDefaultSint64 = true;

         result.defaultSint64_ = value;

         return this;

       }

       public Builder ClearDefaultSint64() {

+        PrepareBuilder();

         result.hasDefaultSint64 = false;

         result.defaultSint64_ = 46;

         return this;

@@ -5237,11 +5527,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetDefaultFixed32(uint value) {

+        PrepareBuilder();

         result.hasDefaultFixed32 = true;

         result.defaultFixed32_ = value;

         return this;

       }

       public Builder ClearDefaultFixed32() {

+        PrepareBuilder();

         result.hasDefaultFixed32 = false;

         result.defaultFixed32_ = 47;

         return this;

@@ -5257,11 +5549,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetDefaultFixed64(ulong value) {

+        PrepareBuilder();

         result.hasDefaultFixed64 = true;

         result.defaultFixed64_ = value;

         return this;

       }

       public Builder ClearDefaultFixed64() {

+        PrepareBuilder();

         result.hasDefaultFixed64 = false;

         result.defaultFixed64_ = 48;

         return this;

@@ -5275,11 +5569,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultSfixed32(value); }

       }

       public Builder SetDefaultSfixed32(int value) {

+        PrepareBuilder();

         result.hasDefaultSfixed32 = true;

         result.defaultSfixed32_ = value;

         return this;

       }

       public Builder ClearDefaultSfixed32() {

+        PrepareBuilder();

         result.hasDefaultSfixed32 = false;

         result.defaultSfixed32_ = 49;

         return this;

@@ -5293,11 +5589,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultSfixed64(value); }

       }

       public Builder SetDefaultSfixed64(long value) {

+        PrepareBuilder();

         result.hasDefaultSfixed64 = true;

         result.defaultSfixed64_ = value;

         return this;

       }

       public Builder ClearDefaultSfixed64() {

+        PrepareBuilder();

         result.hasDefaultSfixed64 = false;

         result.defaultSfixed64_ = -50;

         return this;

@@ -5311,11 +5609,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultFloat(value); }

       }

       public Builder SetDefaultFloat(float value) {

+        PrepareBuilder();

         result.hasDefaultFloat = true;

         result.defaultFloat_ = value;

         return this;

       }

       public Builder ClearDefaultFloat() {

+        PrepareBuilder();

         result.hasDefaultFloat = false;

         result.defaultFloat_ = 51.5F;

         return this;

@@ -5329,11 +5629,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultDouble(value); }

       }

       public Builder SetDefaultDouble(double value) {

+        PrepareBuilder();

         result.hasDefaultDouble = true;

         result.defaultDouble_ = value;

         return this;

       }

       public Builder ClearDefaultDouble() {

+        PrepareBuilder();

         result.hasDefaultDouble = false;

         result.defaultDouble_ = 52000D;

         return this;

@@ -5347,11 +5649,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultBool(value); }

       }

       public Builder SetDefaultBool(bool value) {

+        PrepareBuilder();

         result.hasDefaultBool = true;

         result.defaultBool_ = value;

         return this;

       }

       public Builder ClearDefaultBool() {

+        PrepareBuilder();

         result.hasDefaultBool = false;

         result.defaultBool_ = true;

         return this;

@@ -5366,11 +5670,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetDefaultString(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasDefaultString = true;

         result.defaultString_ = value;

         return this;

       }

       public Builder ClearDefaultString() {

+        PrepareBuilder();

         result.hasDefaultString = false;

         result.defaultString_ = "hello";

         return this;

@@ -5385,11 +5691,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetDefaultBytes(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasDefaultBytes = true;

         result.defaultBytes_ = value;

         return this;

       }

       public Builder ClearDefaultBytes() {

+        PrepareBuilder();

         result.hasDefaultBytes = false;

         result.defaultBytes_ = (pb::ByteString) global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Descriptor.Fields[62].DefaultValue;

         return this;

@@ -5403,11 +5711,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultNestedEnum(value); }

       }

       public Builder SetDefaultNestedEnum(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum value) {

+        PrepareBuilder();

         result.hasDefaultNestedEnum = true;

         result.defaultNestedEnum_ = value;

         return this;

       }

       public Builder ClearDefaultNestedEnum() {

+        PrepareBuilder();

         result.hasDefaultNestedEnum = false;

         result.defaultNestedEnum_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum.BAR;

         return this;

@@ -5421,11 +5731,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultForeignEnum(value); }

       }

       public Builder SetDefaultForeignEnum(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.hasDefaultForeignEnum = true;

         result.defaultForeignEnum_ = value;

         return this;

       }

       public Builder ClearDefaultForeignEnum() {

+        PrepareBuilder();

         result.hasDefaultForeignEnum = false;

         result.defaultForeignEnum_ = global::Google.ProtocolBuffers.TestProtos.ForeignEnum.FOREIGN_BAR;

         return this;

@@ -5439,11 +5751,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultImportEnum(value); }

       }

       public Builder SetDefaultImportEnum(global::Google.ProtocolBuffers.TestProtos.ImportEnum value) {

+        PrepareBuilder();

         result.hasDefaultImportEnum = true;

         result.defaultImportEnum_ = value;

         return this;

       }

       public Builder ClearDefaultImportEnum() {

+        PrepareBuilder();

         result.hasDefaultImportEnum = false;

         result.defaultImportEnum_ = global::Google.ProtocolBuffers.TestProtos.ImportEnum.IMPORT_BAR;

         return this;

@@ -5458,11 +5772,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetDefaultStringPiece(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasDefaultStringPiece = true;

         result.defaultStringPiece_ = value;

         return this;

       }

       public Builder ClearDefaultStringPiece() {

+        PrepareBuilder();

         result.hasDefaultStringPiece = false;

         result.defaultStringPiece_ = "abc";

         return this;

@@ -5477,11 +5793,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetDefaultCord(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasDefaultCord = true;

         result.defaultCord_ = value;

         return this;

       }

       public Builder ClearDefaultCord() {

+        PrepareBuilder();

         result.hasDefaultCord = false;

         result.defaultCord_ = "123";

         return this;

@@ -5594,7 +5912,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestDeprecatedFields prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -5604,21 +5922,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestDeprecatedFields();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestDeprecatedFields cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestDeprecatedFields result;

       

-      TestDeprecatedFields result = new TestDeprecatedFields();

+      private TestDeprecatedFields PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestDeprecatedFields original = result;

+          result = new TestDeprecatedFields();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestDeprecatedFields MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestDeprecatedFields();

+        result = DefaultInstance ?? new TestDeprecatedFields();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -5630,12 +5975,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestDeprecatedFields BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestDeprecatedFields returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -5649,6 +5993,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestDeprecatedFields other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestDeprecatedFields.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasDeprecatedInt32) {

           DeprecatedInt32 = other.DeprecatedInt32;

         }

@@ -5661,6 +6006,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -5716,11 +6062,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDeprecatedInt32(value); }

       }

       public Builder SetDeprecatedInt32(int value) {

+        PrepareBuilder();

         result.hasDeprecatedInt32 = true;

         result.deprecatedInt32_ = value;

         return this;

       }

       public Builder ClearDeprecatedInt32() {

+        PrepareBuilder();

         result.hasDeprecatedInt32 = false;

         result.deprecatedInt32_ = 0;

         return this;

@@ -5833,7 +6181,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(ForeignMessage prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -5843,21 +6191,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new ForeignMessage();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(ForeignMessage cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      ForeignMessage result = new ForeignMessage();

+      bool builderIsReadOnly;

+      ForeignMessage result;

+      

+      private ForeignMessage PrepareBuilder() {

+        if (builderIsReadOnly) {

+          ForeignMessage original = result;

+          result = new ForeignMessage();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override ForeignMessage MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new ForeignMessage();

+        result = DefaultInstance ?? new ForeignMessage();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -5869,12 +6244,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ForeignMessage BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        ForeignMessage returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -5888,6 +6262,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(ForeignMessage other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasC) {

           C = other.C;

         }

@@ -5900,6 +6275,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -5955,11 +6331,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetC(value); }

       }

       public Builder SetC(int value) {

+        PrepareBuilder();

         result.hasC = true;

         result.c_ = value;

         return this;

       }

       public Builder ClearC() {

+        PrepareBuilder();

         result.hasC = false;

         result.c_ = 0;

         return this;

@@ -6060,7 +6438,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestAllExtensions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -6070,21 +6448,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestAllExtensions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestAllExtensions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestAllExtensions result = new TestAllExtensions();

+      bool builderIsReadOnly;

+      TestAllExtensions result;

+      

+      private TestAllExtensions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestAllExtensions original = result;

+          result = new TestAllExtensions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestAllExtensions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestAllExtensions();

+        result = DefaultInstance ?? new TestAllExtensions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -6096,12 +6501,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestAllExtensions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestAllExtensions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -6115,6 +6519,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestAllExtensions other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestAllExtensions.DefaultInstance) return this;

+        PrepareBuilder();

           this.MergeExtensionFields(other);

         this.MergeUnknownFields(other.UnknownFields);

         return this;

@@ -6125,6 +6530,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -6275,7 +6681,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(OptionalGroup_extension prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -6285,21 +6691,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new OptionalGroup_extension();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(OptionalGroup_extension cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      OptionalGroup_extension result = new OptionalGroup_extension();

+      bool builderIsReadOnly;

+      OptionalGroup_extension result;

+      

+      private OptionalGroup_extension PrepareBuilder() {

+        if (builderIsReadOnly) {

+          OptionalGroup_extension original = result;

+          result = new OptionalGroup_extension();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override OptionalGroup_extension MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new OptionalGroup_extension();

+        result = DefaultInstance ?? new OptionalGroup_extension();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -6311,12 +6744,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OptionalGroup_extension BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        OptionalGroup_extension returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -6330,6 +6762,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(OptionalGroup_extension other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.OptionalGroup_extension.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           A = other.A;

         }

@@ -6342,6 +6775,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -6397,11 +6831,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetA(value); }

       }

       public Builder SetA(int value) {

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = 0;

         return this;

@@ -6514,7 +6950,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(RepeatedGroup_extension prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -6524,21 +6960,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new RepeatedGroup_extension();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(RepeatedGroup_extension cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      RepeatedGroup_extension result;

+      

+      private RepeatedGroup_extension PrepareBuilder() {

+        if (builderIsReadOnly) {

+          RepeatedGroup_extension original = result;

+          result = new RepeatedGroup_extension();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      RepeatedGroup_extension result = new RepeatedGroup_extension();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override RepeatedGroup_extension MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new RepeatedGroup_extension();

+        result = DefaultInstance ?? new RepeatedGroup_extension();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -6550,12 +7013,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override RepeatedGroup_extension BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        RepeatedGroup_extension returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -6569,6 +7031,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(RepeatedGroup_extension other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.RepeatedGroup_extension.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           A = other.A;

         }

@@ -6581,6 +7044,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -6636,11 +7100,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetA(value); }

       }

       public Builder SetA(int value) {

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = 0;

         return this;

@@ -6739,7 +7205,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestNestedExtension prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -6749,21 +7215,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestNestedExtension();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestNestedExtension cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestNestedExtension result;

+      

+      private TestNestedExtension PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestNestedExtension original = result;

+          result = new TestNestedExtension();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestNestedExtension result = new TestNestedExtension();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestNestedExtension MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestNestedExtension();

+        result = DefaultInstance ?? new TestNestedExtension();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -6775,12 +7268,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestNestedExtension BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestNestedExtension returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -6794,6 +7286,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestNestedExtension other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestNestedExtension.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -6803,6 +7296,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -7472,7 +7966,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestRequired prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -7482,21 +7976,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestRequired();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestRequired cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestRequired result = new TestRequired();

+      bool builderIsReadOnly;

+      TestRequired result;

+      

+      private TestRequired PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestRequired original = result;

+          result = new TestRequired();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestRequired MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestRequired();

+        result = DefaultInstance ?? new TestRequired();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -7508,12 +8029,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRequired BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestRequired returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -7527,6 +8047,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestRequired other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestRequired.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           A = other.A;

         }

@@ -7635,6 +8156,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -7818,11 +8340,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetA(value); }

       }

       public Builder SetA(int value) {

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = 0;

         return this;

@@ -7836,11 +8360,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy2(value); }

       }

       public Builder SetDummy2(int value) {

+        PrepareBuilder();

         result.hasDummy2 = true;

         result.dummy2_ = value;

         return this;

       }

       public Builder ClearDummy2() {

+        PrepareBuilder();

         result.hasDummy2 = false;

         result.dummy2_ = 0;

         return this;

@@ -7854,11 +8380,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetB(value); }

       }

       public Builder SetB(int value) {

+        PrepareBuilder();

         result.hasB = true;

         result.b_ = value;

         return this;

       }

       public Builder ClearB() {

+        PrepareBuilder();

         result.hasB = false;

         result.b_ = 0;

         return this;

@@ -7872,11 +8400,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy4(value); }

       }

       public Builder SetDummy4(int value) {

+        PrepareBuilder();

         result.hasDummy4 = true;

         result.dummy4_ = value;

         return this;

       }

       public Builder ClearDummy4() {

+        PrepareBuilder();

         result.hasDummy4 = false;

         result.dummy4_ = 0;

         return this;

@@ -7890,11 +8420,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy5(value); }

       }

       public Builder SetDummy5(int value) {

+        PrepareBuilder();

         result.hasDummy5 = true;

         result.dummy5_ = value;

         return this;

       }

       public Builder ClearDummy5() {

+        PrepareBuilder();

         result.hasDummy5 = false;

         result.dummy5_ = 0;

         return this;

@@ -7908,11 +8440,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy6(value); }

       }

       public Builder SetDummy6(int value) {

+        PrepareBuilder();

         result.hasDummy6 = true;

         result.dummy6_ = value;

         return this;

       }

       public Builder ClearDummy6() {

+        PrepareBuilder();

         result.hasDummy6 = false;

         result.dummy6_ = 0;

         return this;

@@ -7926,11 +8460,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy7(value); }

       }

       public Builder SetDummy7(int value) {

+        PrepareBuilder();

         result.hasDummy7 = true;

         result.dummy7_ = value;

         return this;

       }

       public Builder ClearDummy7() {

+        PrepareBuilder();

         result.hasDummy7 = false;

         result.dummy7_ = 0;

         return this;

@@ -7944,11 +8480,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy8(value); }

       }

       public Builder SetDummy8(int value) {

+        PrepareBuilder();

         result.hasDummy8 = true;

         result.dummy8_ = value;

         return this;

       }

       public Builder ClearDummy8() {

+        PrepareBuilder();

         result.hasDummy8 = false;

         result.dummy8_ = 0;

         return this;

@@ -7962,11 +8500,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy9(value); }

       }

       public Builder SetDummy9(int value) {

+        PrepareBuilder();

         result.hasDummy9 = true;

         result.dummy9_ = value;

         return this;

       }

       public Builder ClearDummy9() {

+        PrepareBuilder();

         result.hasDummy9 = false;

         result.dummy9_ = 0;

         return this;

@@ -7980,11 +8520,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy10(value); }

       }

       public Builder SetDummy10(int value) {

+        PrepareBuilder();

         result.hasDummy10 = true;

         result.dummy10_ = value;

         return this;

       }

       public Builder ClearDummy10() {

+        PrepareBuilder();

         result.hasDummy10 = false;

         result.dummy10_ = 0;

         return this;

@@ -7998,11 +8540,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy11(value); }

       }

       public Builder SetDummy11(int value) {

+        PrepareBuilder();

         result.hasDummy11 = true;

         result.dummy11_ = value;

         return this;

       }

       public Builder ClearDummy11() {

+        PrepareBuilder();

         result.hasDummy11 = false;

         result.dummy11_ = 0;

         return this;

@@ -8016,11 +8560,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy12(value); }

       }

       public Builder SetDummy12(int value) {

+        PrepareBuilder();

         result.hasDummy12 = true;

         result.dummy12_ = value;

         return this;

       }

       public Builder ClearDummy12() {

+        PrepareBuilder();

         result.hasDummy12 = false;

         result.dummy12_ = 0;

         return this;

@@ -8034,11 +8580,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy13(value); }

       }

       public Builder SetDummy13(int value) {

+        PrepareBuilder();

         result.hasDummy13 = true;

         result.dummy13_ = value;

         return this;

       }

       public Builder ClearDummy13() {

+        PrepareBuilder();

         result.hasDummy13 = false;

         result.dummy13_ = 0;

         return this;

@@ -8052,11 +8600,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy14(value); }

       }

       public Builder SetDummy14(int value) {

+        PrepareBuilder();

         result.hasDummy14 = true;

         result.dummy14_ = value;

         return this;

       }

       public Builder ClearDummy14() {

+        PrepareBuilder();

         result.hasDummy14 = false;

         result.dummy14_ = 0;

         return this;

@@ -8070,11 +8620,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy15(value); }

       }

       public Builder SetDummy15(int value) {

+        PrepareBuilder();

         result.hasDummy15 = true;

         result.dummy15_ = value;

         return this;

       }

       public Builder ClearDummy15() {

+        PrepareBuilder();

         result.hasDummy15 = false;

         result.dummy15_ = 0;

         return this;

@@ -8088,11 +8640,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy16(value); }

       }

       public Builder SetDummy16(int value) {

+        PrepareBuilder();

         result.hasDummy16 = true;

         result.dummy16_ = value;

         return this;

       }

       public Builder ClearDummy16() {

+        PrepareBuilder();

         result.hasDummy16 = false;

         result.dummy16_ = 0;

         return this;

@@ -8106,11 +8660,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy17(value); }

       }

       public Builder SetDummy17(int value) {

+        PrepareBuilder();

         result.hasDummy17 = true;

         result.dummy17_ = value;

         return this;

       }

       public Builder ClearDummy17() {

+        PrepareBuilder();

         result.hasDummy17 = false;

         result.dummy17_ = 0;

         return this;

@@ -8124,11 +8680,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy18(value); }

       }

       public Builder SetDummy18(int value) {

+        PrepareBuilder();

         result.hasDummy18 = true;

         result.dummy18_ = value;

         return this;

       }

       public Builder ClearDummy18() {

+        PrepareBuilder();

         result.hasDummy18 = false;

         result.dummy18_ = 0;

         return this;

@@ -8142,11 +8700,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy19(value); }

       }

       public Builder SetDummy19(int value) {

+        PrepareBuilder();

         result.hasDummy19 = true;

         result.dummy19_ = value;

         return this;

       }

       public Builder ClearDummy19() {

+        PrepareBuilder();

         result.hasDummy19 = false;

         result.dummy19_ = 0;

         return this;

@@ -8160,11 +8720,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy20(value); }

       }

       public Builder SetDummy20(int value) {

+        PrepareBuilder();

         result.hasDummy20 = true;

         result.dummy20_ = value;

         return this;

       }

       public Builder ClearDummy20() {

+        PrepareBuilder();

         result.hasDummy20 = false;

         result.dummy20_ = 0;

         return this;

@@ -8178,11 +8740,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy21(value); }

       }

       public Builder SetDummy21(int value) {

+        PrepareBuilder();

         result.hasDummy21 = true;

         result.dummy21_ = value;

         return this;

       }

       public Builder ClearDummy21() {

+        PrepareBuilder();

         result.hasDummy21 = false;

         result.dummy21_ = 0;

         return this;

@@ -8196,11 +8760,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy22(value); }

       }

       public Builder SetDummy22(int value) {

+        PrepareBuilder();

         result.hasDummy22 = true;

         result.dummy22_ = value;

         return this;

       }

       public Builder ClearDummy22() {

+        PrepareBuilder();

         result.hasDummy22 = false;

         result.dummy22_ = 0;

         return this;

@@ -8214,11 +8780,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy23(value); }

       }

       public Builder SetDummy23(int value) {

+        PrepareBuilder();

         result.hasDummy23 = true;

         result.dummy23_ = value;

         return this;

       }

       public Builder ClearDummy23() {

+        PrepareBuilder();

         result.hasDummy23 = false;

         result.dummy23_ = 0;

         return this;

@@ -8232,11 +8800,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy24(value); }

       }

       public Builder SetDummy24(int value) {

+        PrepareBuilder();

         result.hasDummy24 = true;

         result.dummy24_ = value;

         return this;

       }

       public Builder ClearDummy24() {

+        PrepareBuilder();

         result.hasDummy24 = false;

         result.dummy24_ = 0;

         return this;

@@ -8250,11 +8820,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy25(value); }

       }

       public Builder SetDummy25(int value) {

+        PrepareBuilder();

         result.hasDummy25 = true;

         result.dummy25_ = value;

         return this;

       }

       public Builder ClearDummy25() {

+        PrepareBuilder();

         result.hasDummy25 = false;

         result.dummy25_ = 0;

         return this;

@@ -8268,11 +8840,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy26(value); }

       }

       public Builder SetDummy26(int value) {

+        PrepareBuilder();

         result.hasDummy26 = true;

         result.dummy26_ = value;

         return this;

       }

       public Builder ClearDummy26() {

+        PrepareBuilder();

         result.hasDummy26 = false;

         result.dummy26_ = 0;

         return this;

@@ -8286,11 +8860,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy27(value); }

       }

       public Builder SetDummy27(int value) {

+        PrepareBuilder();

         result.hasDummy27 = true;

         result.dummy27_ = value;

         return this;

       }

       public Builder ClearDummy27() {

+        PrepareBuilder();

         result.hasDummy27 = false;

         result.dummy27_ = 0;

         return this;

@@ -8304,11 +8880,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy28(value); }

       }

       public Builder SetDummy28(int value) {

+        PrepareBuilder();

         result.hasDummy28 = true;

         result.dummy28_ = value;

         return this;

       }

       public Builder ClearDummy28() {

+        PrepareBuilder();

         result.hasDummy28 = false;

         result.dummy28_ = 0;

         return this;

@@ -8322,11 +8900,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy29(value); }

       }

       public Builder SetDummy29(int value) {

+        PrepareBuilder();

         result.hasDummy29 = true;

         result.dummy29_ = value;

         return this;

       }

       public Builder ClearDummy29() {

+        PrepareBuilder();

         result.hasDummy29 = false;

         result.dummy29_ = 0;

         return this;

@@ -8340,11 +8920,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy30(value); }

       }

       public Builder SetDummy30(int value) {

+        PrepareBuilder();

         result.hasDummy30 = true;

         result.dummy30_ = value;

         return this;

       }

       public Builder ClearDummy30() {

+        PrepareBuilder();

         result.hasDummy30 = false;

         result.dummy30_ = 0;

         return this;

@@ -8358,11 +8940,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy31(value); }

       }

       public Builder SetDummy31(int value) {

+        PrepareBuilder();

         result.hasDummy31 = true;

         result.dummy31_ = value;

         return this;

       }

       public Builder ClearDummy31() {

+        PrepareBuilder();

         result.hasDummy31 = false;

         result.dummy31_ = 0;

         return this;

@@ -8376,11 +8960,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy32(value); }

       }

       public Builder SetDummy32(int value) {

+        PrepareBuilder();

         result.hasDummy32 = true;

         result.dummy32_ = value;

         return this;

       }

       public Builder ClearDummy32() {

+        PrepareBuilder();

         result.hasDummy32 = false;

         result.dummy32_ = 0;

         return this;

@@ -8394,11 +8980,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetC(value); }

       }

       public Builder SetC(int value) {

+        PrepareBuilder();

         result.hasC = true;

         result.c_ = value;

         return this;

       }

       public Builder ClearC() {

+        PrepareBuilder();

         result.hasC = false;

         result.c_ = 0;

         return this;

@@ -8551,7 +9139,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestRequiredForeign prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -8561,21 +9149,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestRequiredForeign();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestRequiredForeign cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestRequiredForeign result;

       

-      TestRequiredForeign result = new TestRequiredForeign();

+      private TestRequiredForeign PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestRequiredForeign original = result;

+          result = new TestRequiredForeign();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestRequiredForeign MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestRequiredForeign();

+        result = DefaultInstance ?? new TestRequiredForeign();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -8587,13 +9202,12 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRequiredForeign BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.repeatedMessage_.MakeReadOnly();

-        TestRequiredForeign returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -8607,6 +9221,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestRequiredForeign other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestRequiredForeign.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasOptionalMessage) {

           MergeOptionalMessage(other.OptionalMessage);

         }

@@ -8625,6 +9240,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -8694,18 +9310,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalMessage(global::Google.ProtocolBuffers.TestProtos.TestRequired value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalMessage = true;

         result.optionalMessage_ = value;

         return this;

       }

       public Builder SetOptionalMessage(global::Google.ProtocolBuffers.TestProtos.TestRequired.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalMessage = true;

         result.optionalMessage_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalMessage(global::Google.ProtocolBuffers.TestProtos.TestRequired value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalMessage &&

             result.optionalMessage_ != global::Google.ProtocolBuffers.TestProtos.TestRequired.DefaultInstance) {

             result.optionalMessage_ = global::Google.ProtocolBuffers.TestProtos.TestRequired.CreateBuilder(result.optionalMessage_).MergeFrom(value).BuildPartial();

@@ -8716,13 +9335,14 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalMessage() {

+        PrepareBuilder();

         result.hasOptionalMessage = false;

         result.optionalMessage_ = global::Google.ProtocolBuffers.TestProtos.TestRequired.DefaultInstance;

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestRequired> RepeatedMessageList {

-        get { return result.repeatedMessage_; }

+        get { return PrepareBuilder().repeatedMessage_; }

       }

       public int RepeatedMessageCount {

         get { return result.RepeatedMessageCount; }

@@ -8732,29 +9352,35 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedMessage(int index, global::Google.ProtocolBuffers.TestProtos.TestRequired value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedMessage_[index] = value;

         return this;

       }

       public Builder SetRepeatedMessage(int index, global::Google.ProtocolBuffers.TestProtos.TestRequired.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedMessage_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedMessage(global::Google.ProtocolBuffers.TestProtos.TestRequired value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedMessage_.Add(value);

         return this;

       }

       public Builder AddRepeatedMessage(global::Google.ProtocolBuffers.TestProtos.TestRequired.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedMessage_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedMessage(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestRequired> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedMessage_);

         return this;

       }

       public Builder ClearRepeatedMessage() {

+        PrepareBuilder();

         result.repeatedMessage_.Clear();

         return this;

       }

@@ -8767,11 +9393,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy(value); }

       }

       public Builder SetDummy(int value) {

+        PrepareBuilder();

         result.hasDummy = true;

         result.dummy_ = value;

         return this;

       }

       public Builder ClearDummy() {

+        PrepareBuilder();

         result.hasDummy = false;

         result.dummy_ = 0;

         return this;

@@ -8884,7 +9512,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestForeignNested prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -8894,21 +9522,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestForeignNested();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestForeignNested cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestForeignNested result = new TestForeignNested();

+      bool builderIsReadOnly;

+      TestForeignNested result;

+      

+      private TestForeignNested PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestForeignNested original = result;

+          result = new TestForeignNested();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestForeignNested MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestForeignNested();

+        result = DefaultInstance ?? new TestForeignNested();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -8920,12 +9575,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestForeignNested BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestForeignNested returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -8939,6 +9593,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestForeignNested other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestForeignNested.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasForeignNested) {

           MergeForeignNested(other.ForeignNested);

         }

@@ -8951,6 +9606,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -9012,18 +9668,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetForeignNested(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasForeignNested = true;

         result.foreignNested_ = value;

         return this;

       }

       public Builder SetForeignNested(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasForeignNested = true;

         result.foreignNested_ = builderForValue.Build();

         return this;

       }

       public Builder MergeForeignNested(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasForeignNested &&

             result.foreignNested_ != global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance) {

             result.foreignNested_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.CreateBuilder(result.foreignNested_).MergeFrom(value).BuildPartial();

@@ -9034,6 +9693,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearForeignNested() {

+        PrepareBuilder();

         result.hasForeignNested = false;

         result.foreignNested_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance;

         return this;

@@ -9130,7 +9790,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestEmptyMessage prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -9140,21 +9800,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestEmptyMessage();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestEmptyMessage cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestEmptyMessage result;

+      

+      private TestEmptyMessage PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestEmptyMessage original = result;

+          result = new TestEmptyMessage();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestEmptyMessage result = new TestEmptyMessage();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestEmptyMessage MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestEmptyMessage();

+        result = DefaultInstance ?? new TestEmptyMessage();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -9166,12 +9853,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestEmptyMessage BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestEmptyMessage returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -9185,6 +9871,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestEmptyMessage other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestEmptyMessage.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -9194,6 +9881,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -9332,7 +10020,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestEmptyMessageWithExtensions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -9342,21 +10030,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestEmptyMessageWithExtensions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestEmptyMessageWithExtensions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestEmptyMessageWithExtensions result = new TestEmptyMessageWithExtensions();

+      bool builderIsReadOnly;

+      TestEmptyMessageWithExtensions result;

+      

+      private TestEmptyMessageWithExtensions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestEmptyMessageWithExtensions original = result;

+          result = new TestEmptyMessageWithExtensions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestEmptyMessageWithExtensions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestEmptyMessageWithExtensions();

+        result = DefaultInstance ?? new TestEmptyMessageWithExtensions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -9368,12 +10083,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestEmptyMessageWithExtensions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestEmptyMessageWithExtensions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -9387,6 +10101,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestEmptyMessageWithExtensions other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestEmptyMessageWithExtensions.DefaultInstance) return this;

+        PrepareBuilder();

           this.MergeExtensionFields(other);

         this.MergeUnknownFields(other.UnknownFields);

         return this;

@@ -9397,6 +10112,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -9537,7 +10253,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestMultipleExtensionRanges prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -9547,21 +10263,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestMultipleExtensionRanges();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestMultipleExtensionRanges cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestMultipleExtensionRanges result;

+      

+      private TestMultipleExtensionRanges PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestMultipleExtensionRanges original = result;

+          result = new TestMultipleExtensionRanges();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestMultipleExtensionRanges result = new TestMultipleExtensionRanges();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestMultipleExtensionRanges MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestMultipleExtensionRanges();

+        result = DefaultInstance ?? new TestMultipleExtensionRanges();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -9573,12 +10316,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMultipleExtensionRanges BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestMultipleExtensionRanges returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -9592,6 +10334,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestMultipleExtensionRanges other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestMultipleExtensionRanges.DefaultInstance) return this;

+        PrepareBuilder();

           this.MergeExtensionFields(other);

         this.MergeUnknownFields(other.UnknownFields);

         return this;

@@ -9602,6 +10345,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -9768,7 +10512,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestReallyLargeTagNumber prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -9778,21 +10522,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestReallyLargeTagNumber();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestReallyLargeTagNumber cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestReallyLargeTagNumber result;

+      

+      private TestReallyLargeTagNumber PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestReallyLargeTagNumber original = result;

+          result = new TestReallyLargeTagNumber();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestReallyLargeTagNumber result = new TestReallyLargeTagNumber();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestReallyLargeTagNumber MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestReallyLargeTagNumber();

+        result = DefaultInstance ?? new TestReallyLargeTagNumber();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -9804,12 +10575,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestReallyLargeTagNumber BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestReallyLargeTagNumber returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -9823,6 +10593,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestReallyLargeTagNumber other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestReallyLargeTagNumber.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           A = other.A;

         }

@@ -9838,6 +10609,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -9897,11 +10669,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetA(value); }

       }

       public Builder SetA(int value) {

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = 0;

         return this;

@@ -9915,11 +10689,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetBb(value); }

       }

       public Builder SetBb(int value) {

+        PrepareBuilder();

         result.hasBb = true;

         result.bb_ = value;

         return this;

       }

       public Builder ClearBb() {

+        PrepareBuilder();

         result.hasBb = false;

         result.bb_ = 0;

         return this;

@@ -10048,7 +10824,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestRecursiveMessage prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -10058,21 +10834,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestRecursiveMessage();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestRecursiveMessage cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestRecursiveMessage result = new TestRecursiveMessage();

+      bool builderIsReadOnly;

+      TestRecursiveMessage result;

+      

+      private TestRecursiveMessage PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestRecursiveMessage original = result;

+          result = new TestRecursiveMessage();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestRecursiveMessage MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestRecursiveMessage();

+        result = DefaultInstance ?? new TestRecursiveMessage();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -10084,12 +10887,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRecursiveMessage BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestRecursiveMessage returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -10103,6 +10905,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestRecursiveMessage other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           MergeA(other.A);

         }

@@ -10118,6 +10921,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -10183,18 +10987,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetA(global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder SetA(global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = builderForValue.Build();

         return this;

       }

       public Builder MergeA(global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasA &&

             result.a_ != global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.DefaultInstance) {

             result.a_ = global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.CreateBuilder(result.a_).MergeFrom(value).BuildPartial();

@@ -10205,6 +11012,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.DefaultInstance;

         return this;

@@ -10218,11 +11026,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetI(value); }

       }

       public Builder SetI(int value) {

+        PrepareBuilder();

         result.hasI = true;

         result.i_ = value;

         return this;

       }

       public Builder ClearI() {

+        PrepareBuilder();

         result.hasI = false;

         result.i_ = 0;

         return this;

@@ -10335,7 +11145,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestMutualRecursionA prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -10345,21 +11155,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestMutualRecursionA();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestMutualRecursionA cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestMutualRecursionA result;

+      

+      private TestMutualRecursionA PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestMutualRecursionA original = result;

+          result = new TestMutualRecursionA();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestMutualRecursionA result = new TestMutualRecursionA();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestMutualRecursionA MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestMutualRecursionA();

+        result = DefaultInstance ?? new TestMutualRecursionA();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -10371,12 +11208,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMutualRecursionA BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestMutualRecursionA returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -10390,6 +11226,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestMutualRecursionA other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasBb) {

           MergeBb(other.Bb);

         }

@@ -10402,6 +11239,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -10463,18 +11301,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetBb(global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasBb = true;

         result.bb_ = value;

         return this;

       }

       public Builder SetBb(global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasBb = true;

         result.bb_ = builderForValue.Build();

         return this;

       }

       public Builder MergeBb(global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasBb &&

             result.bb_ != global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.DefaultInstance) {

             result.bb_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.CreateBuilder(result.bb_).MergeFrom(value).BuildPartial();

@@ -10485,6 +11326,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearBb() {

+        PrepareBuilder();

         result.hasBb = false;

         result.bb_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.DefaultInstance;

         return this;

@@ -10613,7 +11455,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestMutualRecursionB prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -10623,21 +11465,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestMutualRecursionB();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestMutualRecursionB cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestMutualRecursionB result = new TestMutualRecursionB();

+      bool builderIsReadOnly;

+      TestMutualRecursionB result;

+      

+      private TestMutualRecursionB PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestMutualRecursionB original = result;

+          result = new TestMutualRecursionB();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestMutualRecursionB MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestMutualRecursionB();

+        result = DefaultInstance ?? new TestMutualRecursionB();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -10649,12 +11518,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMutualRecursionB BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestMutualRecursionB returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -10668,6 +11536,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestMutualRecursionB other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           MergeA(other.A);

         }

@@ -10683,6 +11552,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -10748,18 +11618,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetA(global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder SetA(global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = builderForValue.Build();

         return this;

       }

       public Builder MergeA(global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasA &&

             result.a_ != global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.DefaultInstance) {

             result.a_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.CreateBuilder(result.a_).MergeFrom(value).BuildPartial();

@@ -10770,6 +11643,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.DefaultInstance;

         return this;

@@ -10783,11 +11657,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalInt32(value); }

       }

       public Builder SetOptionalInt32(int value) {

+        PrepareBuilder();

         result.hasOptionalInt32 = true;

         result.optionalInt32_ = value;

         return this;

       }

       public Builder ClearOptionalInt32() {

+        PrepareBuilder();

         result.hasOptionalInt32 = false;

         result.optionalInt32_ = 0;

         return this;

@@ -10932,7 +11808,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(Foo prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -10942,21 +11818,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new Foo();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(Foo cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          Foo result;

           

-          Foo result = new Foo();

+          private Foo PrepareBuilder() {

+            if (builderIsReadOnly) {

+              Foo original = result;

+              result = new Foo();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override Foo MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new Foo();

+            result = DefaultInstance ?? new Foo();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -10968,12 +11871,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Foo BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            Foo returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -10987,6 +11889,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(Foo other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasA) {

               A = other.A;

             }

@@ -10999,6 +11902,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -11054,11 +11958,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetA(value); }

           }

           public Builder SetA(int value) {

+            PrepareBuilder();

             result.hasA = true;

             result.a_ = value;

             return this;

           }

           public Builder ClearA() {

+            PrepareBuilder();

             result.hasA = false;

             result.a_ = 0;

             return this;

@@ -11171,7 +12077,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(Bar prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -11181,21 +12087,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new Bar();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(Bar cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

           

-          Bar result = new Bar();

+          bool builderIsReadOnly;

+          Bar result;

+          

+          private Bar PrepareBuilder() {

+            if (builderIsReadOnly) {

+              Bar original = result;

+              result = new Bar();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override Bar MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new Bar();

+            result = DefaultInstance ?? new Bar();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -11207,12 +12140,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Bar BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            Bar returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -11226,6 +12158,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(Bar other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasA) {

               A = other.A;

             }

@@ -11238,6 +12171,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -11293,11 +12227,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetA(value); }

           }

           public Builder SetA(int value) {

+            PrepareBuilder();

             result.hasA = true;

             result.a_ = value;

             return this;

           }

           public Builder ClearA() {

+            PrepareBuilder();

             result.hasA = false;

             result.a_ = 0;

             return this;

@@ -11418,7 +12354,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestDupFieldNumber prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -11428,21 +12364,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestDupFieldNumber();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestDupFieldNumber cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestDupFieldNumber result = new TestDupFieldNumber();

+      bool builderIsReadOnly;

+      TestDupFieldNumber result;

+      

+      private TestDupFieldNumber PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestDupFieldNumber original = result;

+          result = new TestDupFieldNumber();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestDupFieldNumber MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestDupFieldNumber();

+        result = DefaultInstance ?? new TestDupFieldNumber();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -11454,12 +12417,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestDupFieldNumber BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestDupFieldNumber returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -11473,6 +12435,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestDupFieldNumber other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           A = other.A;

         }

@@ -11491,6 +12454,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -11564,11 +12528,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetA(value); }

       }

       public Builder SetA(int value) {

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = 0;

         return this;

@@ -11583,18 +12549,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetFoo(global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasFoo = true;

         result.foo_ = value;

         return this;

       }

       public Builder SetFoo(global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasFoo = true;

         result.foo_ = builderForValue.Build();

         return this;

       }

       public Builder MergeFoo(global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasFoo &&

             result.foo_ != global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.DefaultInstance) {

             result.foo_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.CreateBuilder(result.foo_).MergeFrom(value).BuildPartial();

@@ -11605,6 +12574,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearFoo() {

+        PrepareBuilder();

         result.hasFoo = false;

         result.foo_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.DefaultInstance;

         return this;

@@ -11619,18 +12589,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetBar(global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasBar = true;

         result.bar_ = value;

         return this;

       }

       public Builder SetBar(global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasBar = true;

         result.bar_ = builderForValue.Build();

         return this;

       }

       public Builder MergeBar(global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasBar &&

             result.bar_ != global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.DefaultInstance) {

             result.bar_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.CreateBuilder(result.bar_).MergeFrom(value).BuildPartial();

@@ -11641,6 +12614,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearBar() {

+        PrepareBuilder();

         result.hasBar = false;

         result.bar_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.DefaultInstance;

         return this;

@@ -11810,7 +12784,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(NestedMessage prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -11820,21 +12794,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new NestedMessage();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(NestedMessage cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          NestedMessage result;

+          

+          private NestedMessage PrepareBuilder() {

+            if (builderIsReadOnly) {

+              NestedMessage original = result;

+              result = new NestedMessage();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

           

-          NestedMessage result = new NestedMessage();

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override NestedMessage MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new NestedMessage();

+            result = DefaultInstance ?? new NestedMessage();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -11846,14 +12847,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override NestedMessage BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

             result.nestedmessageRepeatedInt32_.MakeReadOnly();

             result.nestedmessageRepeatedForeignmessage_.MakeReadOnly();

-            NestedMessage returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -11867,6 +12867,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(NestedMessage other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.nestedmessageRepeatedInt32_.Count != 0) {

               base.AddRange(other.nestedmessageRepeatedInt32_, result.nestedmessageRepeatedInt32_);

             }

@@ -11882,6 +12883,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -11935,7 +12937,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           

           public pbc::IPopsicleList<int> NestedmessageRepeatedInt32List {

-            get { return result.nestedmessageRepeatedInt32_; }

+            get { return PrepareBuilder().nestedmessageRepeatedInt32_; }

           }

           public int NestedmessageRepeatedInt32Count {

             get { return result.NestedmessageRepeatedInt32Count; }

@@ -11944,24 +12946,28 @@ namespace Google.ProtocolBuffers.TestProtos {
             return result.GetNestedmessageRepeatedInt32(index);

           }

           public Builder SetNestedmessageRepeatedInt32(int index, int value) {

+            PrepareBuilder();

             result.nestedmessageRepeatedInt32_[index] = value;

             return this;

           }

           public Builder AddNestedmessageRepeatedInt32(int value) {

+            PrepareBuilder();

             result.nestedmessageRepeatedInt32_.Add(value);

             return this;

           }

           public Builder AddRangeNestedmessageRepeatedInt32(scg::IEnumerable<int> values) {

+            PrepareBuilder();

             base.AddRange(values, result.nestedmessageRepeatedInt32_);

             return this;

           }

           public Builder ClearNestedmessageRepeatedInt32() {

+            PrepareBuilder();

             result.nestedmessageRepeatedInt32_.Clear();

             return this;

           }

           

           public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignMessage> NestedmessageRepeatedForeignmessageList {

-            get { return result.nestedmessageRepeatedForeignmessage_; }

+            get { return PrepareBuilder().nestedmessageRepeatedForeignmessage_; }

           }

           public int NestedmessageRepeatedForeignmessageCount {

             get { return result.NestedmessageRepeatedForeignmessageCount; }

@@ -11971,29 +12977,35 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetNestedmessageRepeatedForeignmessage(int index, global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.nestedmessageRepeatedForeignmessage_[index] = value;

             return this;

           }

           public Builder SetNestedmessageRepeatedForeignmessage(int index, global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

             pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+            PrepareBuilder();

             result.nestedmessageRepeatedForeignmessage_[index] = builderForValue.Build();

             return this;

           }

           public Builder AddNestedmessageRepeatedForeignmessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.nestedmessageRepeatedForeignmessage_.Add(value);

             return this;

           }

           public Builder AddNestedmessageRepeatedForeignmessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

             pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+            PrepareBuilder();

             result.nestedmessageRepeatedForeignmessage_.Add(builderForValue.Build());

             return this;

           }

           public Builder AddRangeNestedmessageRepeatedForeignmessage(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignMessage> values) {

+            PrepareBuilder();

             base.AddRange(values, result.nestedmessageRepeatedForeignmessage_);

             return this;

           }

           public Builder ClearNestedmessageRepeatedForeignmessage() {

+            PrepareBuilder();

             result.nestedmessageRepeatedForeignmessage_.Clear();

             return this;

           }

@@ -12081,7 +13093,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestNestedMessageHasBits prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -12091,21 +13103,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestNestedMessageHasBits();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestNestedMessageHasBits cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestNestedMessageHasBits result = new TestNestedMessageHasBits();

+      bool builderIsReadOnly;

+      TestNestedMessageHasBits result;

+      

+      private TestNestedMessageHasBits PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestNestedMessageHasBits original = result;

+          result = new TestNestedMessageHasBits();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestNestedMessageHasBits MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestNestedMessageHasBits();

+        result = DefaultInstance ?? new TestNestedMessageHasBits();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -12117,12 +13156,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestNestedMessageHasBits BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestNestedMessageHasBits returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -12136,6 +13174,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestNestedMessageHasBits other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasOptionalNestedMessage) {

           MergeOptionalNestedMessage(other.OptionalNestedMessage);

         }

@@ -12148,6 +13187,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -12209,18 +13249,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalNestedMessage = true;

         result.optionalNestedMessage_ = value;

         return this;

       }

       public Builder SetOptionalNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalNestedMessage = true;

         result.optionalNestedMessage_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalNestedMessage &&

             result.optionalNestedMessage_ != global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.DefaultInstance) {

             result.optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.CreateBuilder(result.optionalNestedMessage_).MergeFrom(value).BuildPartial();

@@ -12231,6 +13274,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalNestedMessage() {

+        PrepareBuilder();

         result.hasOptionalNestedMessage = false;

         result.optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.DefaultInstance;

         return this;

@@ -12558,7 +13602,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestCamelCaseFieldNames prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -12568,21 +13612,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestCamelCaseFieldNames();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestCamelCaseFieldNames cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestCamelCaseFieldNames result;

+      

+      private TestCamelCaseFieldNames PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestCamelCaseFieldNames original = result;

+          result = new TestCamelCaseFieldNames();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestCamelCaseFieldNames result = new TestCamelCaseFieldNames();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestCamelCaseFieldNames MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestCamelCaseFieldNames();

+        result = DefaultInstance ?? new TestCamelCaseFieldNames();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -12594,8 +13665,8 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestCamelCaseFieldNames BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.repeatedPrimitiveField_.MakeReadOnly();

         result.repeatedStringField_.MakeReadOnly();

@@ -12603,9 +13674,8 @@ namespace Google.ProtocolBuffers.TestProtos {
         result.repeatedMessageField_.MakeReadOnly();

         result.repeatedStringPieceField_.MakeReadOnly();

         result.repeatedCordField_.MakeReadOnly();

-        TestCamelCaseFieldNames returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -12619,6 +13689,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestCamelCaseFieldNames other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestCamelCaseFieldNames.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasPrimitiveField) {

           PrimitiveField = other.PrimitiveField;

         }

@@ -12664,6 +13735,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -12787,11 +13859,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetPrimitiveField(value); }

       }

       public Builder SetPrimitiveField(int value) {

+        PrepareBuilder();

         result.hasPrimitiveField = true;

         result.primitiveField_ = value;

         return this;

       }

       public Builder ClearPrimitiveField() {

+        PrepareBuilder();

         result.hasPrimitiveField = false;

         result.primitiveField_ = 0;

         return this;

@@ -12806,11 +13880,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetStringField(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasStringField = true;

         result.stringField_ = value;

         return this;

       }

       public Builder ClearStringField() {

+        PrepareBuilder();

         result.hasStringField = false;

         result.stringField_ = "";

         return this;

@@ -12824,11 +13900,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetEnumField(value); }

       }

       public Builder SetEnumField(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.hasEnumField = true;

         result.enumField_ = value;

         return this;

       }

       public Builder ClearEnumField() {

+        PrepareBuilder();

         result.hasEnumField = false;

         result.enumField_ = global::Google.ProtocolBuffers.TestProtos.ForeignEnum.FOREIGN_FOO;

         return this;

@@ -12843,18 +13921,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetMessageField(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasMessageField = true;

         result.messageField_ = value;

         return this;

       }

       public Builder SetMessageField(global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasMessageField = true;

         result.messageField_ = builderForValue.Build();

         return this;

       }

       public Builder MergeMessageField(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasMessageField &&

             result.messageField_ != global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance) {

             result.messageField_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.CreateBuilder(result.messageField_).MergeFrom(value).BuildPartial();

@@ -12865,6 +13946,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearMessageField() {

+        PrepareBuilder();

         result.hasMessageField = false;

         result.messageField_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

         return this;

@@ -12879,11 +13961,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetStringPieceField(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasStringPieceField = true;

         result.stringPieceField_ = value;

         return this;

       }

       public Builder ClearStringPieceField() {

+        PrepareBuilder();

         result.hasStringPieceField = false;

         result.stringPieceField_ = "";

         return this;

@@ -12898,18 +13982,20 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetCordField(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasCordField = true;

         result.cordField_ = value;

         return this;

       }

       public Builder ClearCordField() {

+        PrepareBuilder();

         result.hasCordField = false;

         result.cordField_ = "";

         return this;

       }

       

       public pbc::IPopsicleList<int> RepeatedPrimitiveFieldList {

-        get { return result.repeatedPrimitiveField_; }

+        get { return PrepareBuilder().repeatedPrimitiveField_; }

       }

       public int RepeatedPrimitiveFieldCount {

         get { return result.RepeatedPrimitiveFieldCount; }

@@ -12918,24 +14004,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedPrimitiveField(index);

       }

       public Builder SetRepeatedPrimitiveField(int index, int value) {

+        PrepareBuilder();

         result.repeatedPrimitiveField_[index] = value;

         return this;

       }

       public Builder AddRepeatedPrimitiveField(int value) {

+        PrepareBuilder();

         result.repeatedPrimitiveField_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedPrimitiveField(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedPrimitiveField_);

         return this;

       }

       public Builder ClearRepeatedPrimitiveField() {

+        PrepareBuilder();

         result.repeatedPrimitiveField_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedStringFieldList {

-        get { return result.repeatedStringField_; }

+        get { return PrepareBuilder().repeatedStringField_; }

       }

       public int RepeatedStringFieldCount {

         get { return result.RepeatedStringFieldCount; }

@@ -12945,25 +14035,29 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedStringField(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedStringField_[index] = value;

         return this;

       }

       public Builder AddRepeatedStringField(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedStringField_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedStringField(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedStringField_);

         return this;

       }

       public Builder ClearRepeatedStringField() {

+        PrepareBuilder();

         result.repeatedStringField_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> RepeatedEnumFieldList {

-        get { return result.repeatedEnumField_; }

+        get { return PrepareBuilder().repeatedEnumField_; }

       }

       public int RepeatedEnumFieldCount {

         get { return result.RepeatedEnumFieldCount; }

@@ -12972,24 +14066,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedEnumField(index);

       }

       public Builder SetRepeatedEnumField(int index, global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.repeatedEnumField_[index] = value;

         return this;

       }

       public Builder AddRepeatedEnumField(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.repeatedEnumField_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedEnumField(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedEnumField_);

         return this;

       }

       public Builder ClearRepeatedEnumField() {

+        PrepareBuilder();

         result.repeatedEnumField_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignMessage> RepeatedMessageFieldList {

-        get { return result.repeatedMessageField_; }

+        get { return PrepareBuilder().repeatedMessageField_; }

       }

       public int RepeatedMessageFieldCount {

         get { return result.RepeatedMessageFieldCount; }

@@ -12999,35 +14097,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedMessageField(int index, global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedMessageField_[index] = value;

         return this;

       }

       public Builder SetRepeatedMessageField(int index, global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedMessageField_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedMessageField(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedMessageField_.Add(value);

         return this;

       }

       public Builder AddRepeatedMessageField(global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedMessageField_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedMessageField(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignMessage> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedMessageField_);

         return this;

       }

       public Builder ClearRepeatedMessageField() {

+        PrepareBuilder();

         result.repeatedMessageField_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedStringPieceFieldList {

-        get { return result.repeatedStringPieceField_; }

+        get { return PrepareBuilder().repeatedStringPieceField_; }

       }

       public int RepeatedStringPieceFieldCount {

         get { return result.RepeatedStringPieceFieldCount; }

@@ -13037,25 +14141,29 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedStringPieceField(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedStringPieceField_[index] = value;

         return this;

       }

       public Builder AddRepeatedStringPieceField(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedStringPieceField_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedStringPieceField(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedStringPieceField_);

         return this;

       }

       public Builder ClearRepeatedStringPieceField() {

+        PrepareBuilder();

         result.repeatedStringPieceField_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedCordFieldList {

-        get { return result.repeatedCordField_; }

+        get { return PrepareBuilder().repeatedCordField_; }

       }

       public int RepeatedCordFieldCount {

         get { return result.RepeatedCordFieldCount; }

@@ -13065,19 +14173,23 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedCordField(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedCordField_[index] = value;

         return this;

       }

       public Builder AddRepeatedCordField(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedCordField_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedCordField(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedCordField_);

         return this;

       }

       public Builder ClearRepeatedCordField() {

+        PrepareBuilder();

         result.repeatedCordField_.Clear();

         return this;

       }

@@ -13226,7 +14338,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestFieldOrderings prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -13236,21 +14348,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestFieldOrderings();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestFieldOrderings cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestFieldOrderings result;

+      

+      private TestFieldOrderings PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestFieldOrderings original = result;

+          result = new TestFieldOrderings();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestFieldOrderings result = new TestFieldOrderings();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestFieldOrderings MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestFieldOrderings();

+        result = DefaultInstance ?? new TestFieldOrderings();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -13262,12 +14401,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestFieldOrderings BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestFieldOrderings returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -13281,6 +14419,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestFieldOrderings other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestFieldOrderings.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasMyString) {

           MyString = other.MyString;

         }

@@ -13300,6 +14439,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -13364,11 +14504,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetMyString(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasMyString = true;

         result.myString_ = value;

         return this;

       }

       public Builder ClearMyString() {

+        PrepareBuilder();

         result.hasMyString = false;

         result.myString_ = "";

         return this;

@@ -13382,11 +14524,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetMyInt(value); }

       }

       public Builder SetMyInt(long value) {

+        PrepareBuilder();

         result.hasMyInt = true;

         result.myInt_ = value;

         return this;

       }

       public Builder ClearMyInt() {

+        PrepareBuilder();

         result.hasMyInt = false;

         result.myInt_ = 0L;

         return this;

@@ -13400,11 +14544,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetMyFloat(value); }

       }

       public Builder SetMyFloat(float value) {

+        PrepareBuilder();

         result.hasMyFloat = true;

         result.myFloat_ = value;

         return this;

       }

       public Builder ClearMyFloat() {

+        PrepareBuilder();

         result.hasMyFloat = false;

         result.myFloat_ = 0F;

         return this;

@@ -13807,7 +14953,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestExtremeDefaultValues prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -13817,21 +14963,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestExtremeDefaultValues();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestExtremeDefaultValues cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestExtremeDefaultValues result;

       

-      TestExtremeDefaultValues result = new TestExtremeDefaultValues();

+      private TestExtremeDefaultValues PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestExtremeDefaultValues original = result;

+          result = new TestExtremeDefaultValues();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestExtremeDefaultValues MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestExtremeDefaultValues();

+        result = DefaultInstance ?? new TestExtremeDefaultValues();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -13843,12 +15016,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestExtremeDefaultValues BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestExtremeDefaultValues returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -13862,6 +15034,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestExtremeDefaultValues other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestExtremeDefaultValues.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasEscapedBytes) {

           EscapedBytes = other.EscapedBytes;

         }

@@ -13928,6 +15101,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -14056,11 +15230,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetEscapedBytes(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasEscapedBytes = true;

         result.escapedBytes_ = value;

         return this;

       }

       public Builder ClearEscapedBytes() {

+        PrepareBuilder();

         result.hasEscapedBytes = false;

         result.escapedBytes_ = (pb::ByteString) global::Google.ProtocolBuffers.TestProtos.TestExtremeDefaultValues.Descriptor.Fields[0].DefaultValue;

         return this;

@@ -14076,11 +15252,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetLargeUint32(uint value) {

+        PrepareBuilder();

         result.hasLargeUint32 = true;

         result.largeUint32_ = value;

         return this;

       }

       public Builder ClearLargeUint32() {

+        PrepareBuilder();

         result.hasLargeUint32 = false;

         result.largeUint32_ = 4294967295;

         return this;

@@ -14096,11 +15274,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetLargeUint64(ulong value) {

+        PrepareBuilder();

         result.hasLargeUint64 = true;

         result.largeUint64_ = value;

         return this;

       }

       public Builder ClearLargeUint64() {

+        PrepareBuilder();

         result.hasLargeUint64 = false;

         result.largeUint64_ = 18446744073709551615UL;

         return this;

@@ -14114,11 +15294,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetSmallInt32(value); }

       }

       public Builder SetSmallInt32(int value) {

+        PrepareBuilder();

         result.hasSmallInt32 = true;

         result.smallInt32_ = value;

         return this;

       }

       public Builder ClearSmallInt32() {

+        PrepareBuilder();

         result.hasSmallInt32 = false;

         result.smallInt32_ = -2147483647;

         return this;

@@ -14132,11 +15314,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetSmallInt64(value); }

       }

       public Builder SetSmallInt64(long value) {

+        PrepareBuilder();

         result.hasSmallInt64 = true;

         result.smallInt64_ = value;

         return this;

       }

       public Builder ClearSmallInt64() {

+        PrepareBuilder();

         result.hasSmallInt64 = false;

         result.smallInt64_ = -9223372036854775807L;

         return this;

@@ -14151,11 +15335,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetUtf8String(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasUtf8String = true;

         result.utf8String_ = value;

         return this;

       }

       public Builder ClearUtf8String() {

+        PrepareBuilder();

         result.hasUtf8String = false;

         result.utf8String_ = (string) global::Google.ProtocolBuffers.TestProtos.TestExtremeDefaultValues.Descriptor.Fields[5].DefaultValue;

         return this;

@@ -14169,11 +15355,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetZeroFloat(value); }

       }

       public Builder SetZeroFloat(float value) {

+        PrepareBuilder();

         result.hasZeroFloat = true;

         result.zeroFloat_ = value;

         return this;

       }

       public Builder ClearZeroFloat() {

+        PrepareBuilder();

         result.hasZeroFloat = false;

         result.zeroFloat_ = 0F;

         return this;

@@ -14187,11 +15375,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOneFloat(value); }

       }

       public Builder SetOneFloat(float value) {

+        PrepareBuilder();

         result.hasOneFloat = true;

         result.oneFloat_ = value;

         return this;

       }

       public Builder ClearOneFloat() {

+        PrepareBuilder();

         result.hasOneFloat = false;

         result.oneFloat_ = 1F;

         return this;

@@ -14205,11 +15395,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetSmallFloat(value); }

       }

       public Builder SetSmallFloat(float value) {

+        PrepareBuilder();

         result.hasSmallFloat = true;

         result.smallFloat_ = value;

         return this;

       }

       public Builder ClearSmallFloat() {

+        PrepareBuilder();

         result.hasSmallFloat = false;

         result.smallFloat_ = 1.5F;

         return this;

@@ -14223,11 +15415,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetNegativeOneFloat(value); }

       }

       public Builder SetNegativeOneFloat(float value) {

+        PrepareBuilder();

         result.hasNegativeOneFloat = true;

         result.negativeOneFloat_ = value;

         return this;

       }

       public Builder ClearNegativeOneFloat() {

+        PrepareBuilder();

         result.hasNegativeOneFloat = false;

         result.negativeOneFloat_ = -1F;

         return this;

@@ -14241,11 +15435,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetNegativeFloat(value); }

       }

       public Builder SetNegativeFloat(float value) {

+        PrepareBuilder();

         result.hasNegativeFloat = true;

         result.negativeFloat_ = value;

         return this;

       }

       public Builder ClearNegativeFloat() {

+        PrepareBuilder();

         result.hasNegativeFloat = false;

         result.negativeFloat_ = -1.5F;

         return this;

@@ -14259,11 +15455,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetLargeFloat(value); }

       }

       public Builder SetLargeFloat(float value) {

+        PrepareBuilder();

         result.hasLargeFloat = true;

         result.largeFloat_ = value;

         return this;

       }

       public Builder ClearLargeFloat() {

+        PrepareBuilder();

         result.hasLargeFloat = false;

         result.largeFloat_ = 2E+08F;

         return this;

@@ -14277,11 +15475,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetSmallNegativeFloat(value); }

       }

       public Builder SetSmallNegativeFloat(float value) {

+        PrepareBuilder();

         result.hasSmallNegativeFloat = true;

         result.smallNegativeFloat_ = value;

         return this;

       }

       public Builder ClearSmallNegativeFloat() {

+        PrepareBuilder();

         result.hasSmallNegativeFloat = false;

         result.smallNegativeFloat_ = -8E-28F;

         return this;

@@ -14295,11 +15495,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetInfDouble(value); }

       }

       public Builder SetInfDouble(double value) {

+        PrepareBuilder();

         result.hasInfDouble = true;

         result.infDouble_ = value;

         return this;

       }

       public Builder ClearInfDouble() {

+        PrepareBuilder();

         result.hasInfDouble = false;

         result.infDouble_ = double.PositiveInfinity;

         return this;

@@ -14313,11 +15515,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetNegInfDouble(value); }

       }

       public Builder SetNegInfDouble(double value) {

+        PrepareBuilder();

         result.hasNegInfDouble = true;

         result.negInfDouble_ = value;

         return this;

       }

       public Builder ClearNegInfDouble() {

+        PrepareBuilder();

         result.hasNegInfDouble = false;

         result.negInfDouble_ = double.NegativeInfinity;

         return this;

@@ -14331,11 +15535,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetNanDouble(value); }

       }

       public Builder SetNanDouble(double value) {

+        PrepareBuilder();

         result.hasNanDouble = true;

         result.nanDouble_ = value;

         return this;

       }

       public Builder ClearNanDouble() {

+        PrepareBuilder();

         result.hasNanDouble = false;

         result.nanDouble_ = double.NaN;

         return this;

@@ -14349,11 +15555,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetInfFloat(value); }

       }

       public Builder SetInfFloat(float value) {

+        PrepareBuilder();

         result.hasInfFloat = true;

         result.infFloat_ = value;

         return this;

       }

       public Builder ClearInfFloat() {

+        PrepareBuilder();

         result.hasInfFloat = false;

         result.infFloat_ = float.PositiveInfinity;

         return this;

@@ -14367,11 +15575,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetNegInfFloat(value); }

       }

       public Builder SetNegInfFloat(float value) {

+        PrepareBuilder();

         result.hasNegInfFloat = true;

         result.negInfFloat_ = value;

         return this;

       }

       public Builder ClearNegInfFloat() {

+        PrepareBuilder();

         result.hasNegInfFloat = false;

         result.negInfFloat_ = float.NegativeInfinity;

         return this;

@@ -14385,11 +15595,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetNanFloat(value); }

       }

       public Builder SetNanFloat(float value) {

+        PrepareBuilder();

         result.hasNanFloat = true;

         result.nanFloat_ = value;

         return this;

       }

       public Builder ClearNanFloat() {

+        PrepareBuilder();

         result.hasNanFloat = false;

         result.nanFloat_ = float.NaN;

         return this;

@@ -14502,7 +15714,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(OneString prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -14512,21 +15724,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new OneString();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(OneString cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      OneString result = new OneString();

+      bool builderIsReadOnly;

+      OneString result;

+      

+      private OneString PrepareBuilder() {

+        if (builderIsReadOnly) {

+          OneString original = result;

+          result = new OneString();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override OneString MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new OneString();

+        result = DefaultInstance ?? new OneString();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -14538,12 +15777,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OneString BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        OneString returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -14557,6 +15795,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(OneString other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.OneString.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasData) {

           Data = other.Data;

         }

@@ -14569,6 +15808,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -14625,11 +15865,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetData(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasData = true;

         result.data_ = value;

         return this;

       }

       public Builder ClearData() {

+        PrepareBuilder();

         result.hasData = false;

         result.data_ = "";

         return this;

@@ -14742,7 +15984,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(OneBytes prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -14752,21 +15994,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new OneBytes();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(OneBytes cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      OneBytes result = new OneBytes();

+      bool builderIsReadOnly;

+      OneBytes result;

+      

+      private OneBytes PrepareBuilder() {

+        if (builderIsReadOnly) {

+          OneBytes original = result;

+          result = new OneBytes();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override OneBytes MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new OneBytes();

+        result = DefaultInstance ?? new OneBytes();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -14778,12 +16047,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OneBytes BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        OneBytes returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -14797,6 +16065,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(OneBytes other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.OneBytes.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasData) {

           Data = other.Data;

         }

@@ -14809,6 +16078,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -14865,11 +16135,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetData(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasData = true;

         result.data_ = value;

         return this;

       }

       public Builder ClearData() {

+        PrepareBuilder();

         result.hasData = false;

         result.data_ = pb::ByteString.Empty;

         return this;

@@ -15339,7 +16611,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestPackedTypes prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -15349,21 +16621,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestPackedTypes();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestPackedTypes cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestPackedTypes result = new TestPackedTypes();

+      bool builderIsReadOnly;

+      TestPackedTypes result;

+      

+      private TestPackedTypes PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestPackedTypes original = result;

+          result = new TestPackedTypes();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestPackedTypes MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestPackedTypes();

+        result = DefaultInstance ?? new TestPackedTypes();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -15375,8 +16674,8 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestPackedTypes BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.packedInt32_.MakeReadOnly();

         result.packedInt64_.MakeReadOnly();

@@ -15392,9 +16691,8 @@ namespace Google.ProtocolBuffers.TestProtos {
         result.packedDouble_.MakeReadOnly();

         result.packedBool_.MakeReadOnly();

         result.packedEnum_.MakeReadOnly();

-        TestPackedTypes returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -15408,6 +16706,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestPackedTypes other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestPackedTypes.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.packedInt32_.Count != 0) {

           base.AddRange(other.packedInt32_, result.packedInt32_);

         }

@@ -15459,6 +16758,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -15582,7 +16882,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       

       public pbc::IPopsicleList<int> PackedInt32List {

-        get { return result.packedInt32_; }

+        get { return PrepareBuilder().packedInt32_; }

       }

       public int PackedInt32Count {

         get { return result.PackedInt32Count; }

@@ -15591,24 +16891,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedInt32(index);

       }

       public Builder SetPackedInt32(int index, int value) {

+        PrepareBuilder();

         result.packedInt32_[index] = value;

         return this;

       }

       public Builder AddPackedInt32(int value) {

+        PrepareBuilder();

         result.packedInt32_.Add(value);

         return this;

       }

       public Builder AddRangePackedInt32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedInt32_);

         return this;

       }

       public Builder ClearPackedInt32() {

+        PrepareBuilder();

         result.packedInt32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> PackedInt64List {

-        get { return result.packedInt64_; }

+        get { return PrepareBuilder().packedInt64_; }

       }

       public int PackedInt64Count {

         get { return result.PackedInt64Count; }

@@ -15617,25 +16921,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedInt64(index);

       }

       public Builder SetPackedInt64(int index, long value) {

+        PrepareBuilder();

         result.packedInt64_[index] = value;

         return this;

       }

       public Builder AddPackedInt64(long value) {

+        PrepareBuilder();

         result.packedInt64_.Add(value);

         return this;

       }

       public Builder AddRangePackedInt64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedInt64_);

         return this;

       }

       public Builder ClearPackedInt64() {

+        PrepareBuilder();

         result.packedInt64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> PackedUint32List {

-        get { return result.packedUint32_; }

+        get { return PrepareBuilder().packedUint32_; }

       }

       public int PackedUint32Count {

         get { return result.PackedUint32Count; }

@@ -15646,27 +16954,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetPackedUint32(int index, uint value) {

+        PrepareBuilder();

         result.packedUint32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddPackedUint32(uint value) {

+        PrepareBuilder();

         result.packedUint32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangePackedUint32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedUint32_);

         return this;

       }

       public Builder ClearPackedUint32() {

+        PrepareBuilder();

         result.packedUint32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> PackedUint64List {

-        get { return result.packedUint64_; }

+        get { return PrepareBuilder().packedUint64_; }

       }

       public int PackedUint64Count {

         get { return result.PackedUint64Count; }

@@ -15677,26 +16989,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetPackedUint64(int index, ulong value) {

+        PrepareBuilder();

         result.packedUint64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddPackedUint64(ulong value) {

+        PrepareBuilder();

         result.packedUint64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangePackedUint64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedUint64_);

         return this;

       }

       public Builder ClearPackedUint64() {

+        PrepareBuilder();

         result.packedUint64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> PackedSint32List {

-        get { return result.packedSint32_; }

+        get { return PrepareBuilder().packedSint32_; }

       }

       public int PackedSint32Count {

         get { return result.PackedSint32Count; }

@@ -15705,24 +17021,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedSint32(index);

       }

       public Builder SetPackedSint32(int index, int value) {

+        PrepareBuilder();

         result.packedSint32_[index] = value;

         return this;

       }

       public Builder AddPackedSint32(int value) {

+        PrepareBuilder();

         result.packedSint32_.Add(value);

         return this;

       }

       public Builder AddRangePackedSint32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedSint32_);

         return this;

       }

       public Builder ClearPackedSint32() {

+        PrepareBuilder();

         result.packedSint32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> PackedSint64List {

-        get { return result.packedSint64_; }

+        get { return PrepareBuilder().packedSint64_; }

       }

       public int PackedSint64Count {

         get { return result.PackedSint64Count; }

@@ -15731,25 +17051,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedSint64(index);

       }

       public Builder SetPackedSint64(int index, long value) {

+        PrepareBuilder();

         result.packedSint64_[index] = value;

         return this;

       }

       public Builder AddPackedSint64(long value) {

+        PrepareBuilder();

         result.packedSint64_.Add(value);

         return this;

       }

       public Builder AddRangePackedSint64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedSint64_);

         return this;

       }

       public Builder ClearPackedSint64() {

+        PrepareBuilder();

         result.packedSint64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> PackedFixed32List {

-        get { return result.packedFixed32_; }

+        get { return PrepareBuilder().packedFixed32_; }

       }

       public int PackedFixed32Count {

         get { return result.PackedFixed32Count; }

@@ -15760,27 +17084,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetPackedFixed32(int index, uint value) {

+        PrepareBuilder();

         result.packedFixed32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddPackedFixed32(uint value) {

+        PrepareBuilder();

         result.packedFixed32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangePackedFixed32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedFixed32_);

         return this;

       }

       public Builder ClearPackedFixed32() {

+        PrepareBuilder();

         result.packedFixed32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> PackedFixed64List {

-        get { return result.packedFixed64_; }

+        get { return PrepareBuilder().packedFixed64_; }

       }

       public int PackedFixed64Count {

         get { return result.PackedFixed64Count; }

@@ -15791,26 +17119,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetPackedFixed64(int index, ulong value) {

+        PrepareBuilder();

         result.packedFixed64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddPackedFixed64(ulong value) {

+        PrepareBuilder();

         result.packedFixed64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangePackedFixed64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedFixed64_);

         return this;

       }

       public Builder ClearPackedFixed64() {

+        PrepareBuilder();

         result.packedFixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> PackedSfixed32List {

-        get { return result.packedSfixed32_; }

+        get { return PrepareBuilder().packedSfixed32_; }

       }

       public int PackedSfixed32Count {

         get { return result.PackedSfixed32Count; }

@@ -15819,24 +17151,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedSfixed32(index);

       }

       public Builder SetPackedSfixed32(int index, int value) {

+        PrepareBuilder();

         result.packedSfixed32_[index] = value;

         return this;

       }

       public Builder AddPackedSfixed32(int value) {

+        PrepareBuilder();

         result.packedSfixed32_.Add(value);

         return this;

       }

       public Builder AddRangePackedSfixed32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedSfixed32_);

         return this;

       }

       public Builder ClearPackedSfixed32() {

+        PrepareBuilder();

         result.packedSfixed32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> PackedSfixed64List {

-        get { return result.packedSfixed64_; }

+        get { return PrepareBuilder().packedSfixed64_; }

       }

       public int PackedSfixed64Count {

         get { return result.PackedSfixed64Count; }

@@ -15845,24 +17181,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedSfixed64(index);

       }

       public Builder SetPackedSfixed64(int index, long value) {

+        PrepareBuilder();

         result.packedSfixed64_[index] = value;

         return this;

       }

       public Builder AddPackedSfixed64(long value) {

+        PrepareBuilder();

         result.packedSfixed64_.Add(value);

         return this;

       }

       public Builder AddRangePackedSfixed64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedSfixed64_);

         return this;

       }

       public Builder ClearPackedSfixed64() {

+        PrepareBuilder();

         result.packedSfixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<float> PackedFloatList {

-        get { return result.packedFloat_; }

+        get { return PrepareBuilder().packedFloat_; }

       }

       public int PackedFloatCount {

         get { return result.PackedFloatCount; }

@@ -15871,24 +17211,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedFloat(index);

       }

       public Builder SetPackedFloat(int index, float value) {

+        PrepareBuilder();

         result.packedFloat_[index] = value;

         return this;

       }

       public Builder AddPackedFloat(float value) {

+        PrepareBuilder();

         result.packedFloat_.Add(value);

         return this;

       }

       public Builder AddRangePackedFloat(scg::IEnumerable<float> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedFloat_);

         return this;

       }

       public Builder ClearPackedFloat() {

+        PrepareBuilder();

         result.packedFloat_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<double> PackedDoubleList {

-        get { return result.packedDouble_; }

+        get { return PrepareBuilder().packedDouble_; }

       }

       public int PackedDoubleCount {

         get { return result.PackedDoubleCount; }

@@ -15897,24 +17241,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedDouble(index);

       }

       public Builder SetPackedDouble(int index, double value) {

+        PrepareBuilder();

         result.packedDouble_[index] = value;

         return this;

       }

       public Builder AddPackedDouble(double value) {

+        PrepareBuilder();

         result.packedDouble_.Add(value);

         return this;

       }

       public Builder AddRangePackedDouble(scg::IEnumerable<double> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedDouble_);

         return this;

       }

       public Builder ClearPackedDouble() {

+        PrepareBuilder();

         result.packedDouble_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<bool> PackedBoolList {

-        get { return result.packedBool_; }

+        get { return PrepareBuilder().packedBool_; }

       }

       public int PackedBoolCount {

         get { return result.PackedBoolCount; }

@@ -15923,24 +17271,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedBool(index);

       }

       public Builder SetPackedBool(int index, bool value) {

+        PrepareBuilder();

         result.packedBool_[index] = value;

         return this;

       }

       public Builder AddPackedBool(bool value) {

+        PrepareBuilder();

         result.packedBool_.Add(value);

         return this;

       }

       public Builder AddRangePackedBool(scg::IEnumerable<bool> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedBool_);

         return this;

       }

       public Builder ClearPackedBool() {

+        PrepareBuilder();

         result.packedBool_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> PackedEnumList {

-        get { return result.packedEnum_; }

+        get { return PrepareBuilder().packedEnum_; }

       }

       public int PackedEnumCount {

         get { return result.PackedEnumCount; }

@@ -15949,18 +17301,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedEnum(index);

       }

       public Builder SetPackedEnum(int index, global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.packedEnum_[index] = value;

         return this;

       }

       public Builder AddPackedEnum(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.packedEnum_.Add(value);

         return this;

       }

       public Builder AddRangePackedEnum(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedEnum_);

         return this;

       }

       public Builder ClearPackedEnum() {

+        PrepareBuilder();

         result.packedEnum_.Clear();

         return this;

       }

@@ -16374,7 +17730,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestUnpackedTypes prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -16384,21 +17740,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestUnpackedTypes();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestUnpackedTypes cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestUnpackedTypes result;

+      

+      private TestUnpackedTypes PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestUnpackedTypes original = result;

+          result = new TestUnpackedTypes();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestUnpackedTypes result = new TestUnpackedTypes();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestUnpackedTypes MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestUnpackedTypes();

+        result = DefaultInstance ?? new TestUnpackedTypes();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -16410,8 +17793,8 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestUnpackedTypes BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.unpackedInt32_.MakeReadOnly();

         result.unpackedInt64_.MakeReadOnly();

@@ -16427,9 +17810,8 @@ namespace Google.ProtocolBuffers.TestProtos {
         result.unpackedDouble_.MakeReadOnly();

         result.unpackedBool_.MakeReadOnly();

         result.unpackedEnum_.MakeReadOnly();

-        TestUnpackedTypes returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -16443,6 +17825,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestUnpackedTypes other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestUnpackedTypes.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.unpackedInt32_.Count != 0) {

           base.AddRange(other.unpackedInt32_, result.unpackedInt32_);

         }

@@ -16494,6 +17877,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -16617,7 +18001,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       

       public pbc::IPopsicleList<int> UnpackedInt32List {

-        get { return result.unpackedInt32_; }

+        get { return PrepareBuilder().unpackedInt32_; }

       }

       public int UnpackedInt32Count {

         get { return result.UnpackedInt32Count; }

@@ -16626,24 +18010,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedInt32(index);

       }

       public Builder SetUnpackedInt32(int index, int value) {

+        PrepareBuilder();

         result.unpackedInt32_[index] = value;

         return this;

       }

       public Builder AddUnpackedInt32(int value) {

+        PrepareBuilder();

         result.unpackedInt32_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedInt32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedInt32_);

         return this;

       }

       public Builder ClearUnpackedInt32() {

+        PrepareBuilder();

         result.unpackedInt32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> UnpackedInt64List {

-        get { return result.unpackedInt64_; }

+        get { return PrepareBuilder().unpackedInt64_; }

       }

       public int UnpackedInt64Count {

         get { return result.UnpackedInt64Count; }

@@ -16652,25 +18040,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedInt64(index);

       }

       public Builder SetUnpackedInt64(int index, long value) {

+        PrepareBuilder();

         result.unpackedInt64_[index] = value;

         return this;

       }

       public Builder AddUnpackedInt64(long value) {

+        PrepareBuilder();

         result.unpackedInt64_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedInt64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedInt64_);

         return this;

       }

       public Builder ClearUnpackedInt64() {

+        PrepareBuilder();

         result.unpackedInt64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> UnpackedUint32List {

-        get { return result.unpackedUint32_; }

+        get { return PrepareBuilder().unpackedUint32_; }

       }

       public int UnpackedUint32Count {

         get { return result.UnpackedUint32Count; }

@@ -16681,27 +18073,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetUnpackedUint32(int index, uint value) {

+        PrepareBuilder();

         result.unpackedUint32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddUnpackedUint32(uint value) {

+        PrepareBuilder();

         result.unpackedUint32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeUnpackedUint32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedUint32_);

         return this;

       }

       public Builder ClearUnpackedUint32() {

+        PrepareBuilder();

         result.unpackedUint32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> UnpackedUint64List {

-        get { return result.unpackedUint64_; }

+        get { return PrepareBuilder().unpackedUint64_; }

       }

       public int UnpackedUint64Count {

         get { return result.UnpackedUint64Count; }

@@ -16712,26 +18108,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetUnpackedUint64(int index, ulong value) {

+        PrepareBuilder();

         result.unpackedUint64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddUnpackedUint64(ulong value) {

+        PrepareBuilder();

         result.unpackedUint64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeUnpackedUint64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedUint64_);

         return this;

       }

       public Builder ClearUnpackedUint64() {

+        PrepareBuilder();

         result.unpackedUint64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> UnpackedSint32List {

-        get { return result.unpackedSint32_; }

+        get { return PrepareBuilder().unpackedSint32_; }

       }

       public int UnpackedSint32Count {

         get { return result.UnpackedSint32Count; }

@@ -16740,24 +18140,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedSint32(index);

       }

       public Builder SetUnpackedSint32(int index, int value) {

+        PrepareBuilder();

         result.unpackedSint32_[index] = value;

         return this;

       }

       public Builder AddUnpackedSint32(int value) {

+        PrepareBuilder();

         result.unpackedSint32_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedSint32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedSint32_);

         return this;

       }

       public Builder ClearUnpackedSint32() {

+        PrepareBuilder();

         result.unpackedSint32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> UnpackedSint64List {

-        get { return result.unpackedSint64_; }

+        get { return PrepareBuilder().unpackedSint64_; }

       }

       public int UnpackedSint64Count {

         get { return result.UnpackedSint64Count; }

@@ -16766,25 +18170,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedSint64(index);

       }

       public Builder SetUnpackedSint64(int index, long value) {

+        PrepareBuilder();

         result.unpackedSint64_[index] = value;

         return this;

       }

       public Builder AddUnpackedSint64(long value) {

+        PrepareBuilder();

         result.unpackedSint64_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedSint64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedSint64_);

         return this;

       }

       public Builder ClearUnpackedSint64() {

+        PrepareBuilder();

         result.unpackedSint64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> UnpackedFixed32List {

-        get { return result.unpackedFixed32_; }

+        get { return PrepareBuilder().unpackedFixed32_; }

       }

       public int UnpackedFixed32Count {

         get { return result.UnpackedFixed32Count; }

@@ -16795,27 +18203,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetUnpackedFixed32(int index, uint value) {

+        PrepareBuilder();

         result.unpackedFixed32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddUnpackedFixed32(uint value) {

+        PrepareBuilder();

         result.unpackedFixed32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeUnpackedFixed32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedFixed32_);

         return this;

       }

       public Builder ClearUnpackedFixed32() {

+        PrepareBuilder();

         result.unpackedFixed32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> UnpackedFixed64List {

-        get { return result.unpackedFixed64_; }

+        get { return PrepareBuilder().unpackedFixed64_; }

       }

       public int UnpackedFixed64Count {

         get { return result.UnpackedFixed64Count; }

@@ -16826,26 +18238,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetUnpackedFixed64(int index, ulong value) {

+        PrepareBuilder();

         result.unpackedFixed64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddUnpackedFixed64(ulong value) {

+        PrepareBuilder();

         result.unpackedFixed64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeUnpackedFixed64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedFixed64_);

         return this;

       }

       public Builder ClearUnpackedFixed64() {

+        PrepareBuilder();

         result.unpackedFixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> UnpackedSfixed32List {

-        get { return result.unpackedSfixed32_; }

+        get { return PrepareBuilder().unpackedSfixed32_; }

       }

       public int UnpackedSfixed32Count {

         get { return result.UnpackedSfixed32Count; }

@@ -16854,24 +18270,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedSfixed32(index);

       }

       public Builder SetUnpackedSfixed32(int index, int value) {

+        PrepareBuilder();

         result.unpackedSfixed32_[index] = value;

         return this;

       }

       public Builder AddUnpackedSfixed32(int value) {

+        PrepareBuilder();

         result.unpackedSfixed32_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedSfixed32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedSfixed32_);

         return this;

       }

       public Builder ClearUnpackedSfixed32() {

+        PrepareBuilder();

         result.unpackedSfixed32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> UnpackedSfixed64List {

-        get { return result.unpackedSfixed64_; }

+        get { return PrepareBuilder().unpackedSfixed64_; }

       }

       public int UnpackedSfixed64Count {

         get { return result.UnpackedSfixed64Count; }

@@ -16880,24 +18300,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedSfixed64(index);

       }

       public Builder SetUnpackedSfixed64(int index, long value) {

+        PrepareBuilder();

         result.unpackedSfixed64_[index] = value;

         return this;

       }

       public Builder AddUnpackedSfixed64(long value) {

+        PrepareBuilder();

         result.unpackedSfixed64_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedSfixed64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedSfixed64_);

         return this;

       }

       public Builder ClearUnpackedSfixed64() {

+        PrepareBuilder();

         result.unpackedSfixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<float> UnpackedFloatList {

-        get { return result.unpackedFloat_; }

+        get { return PrepareBuilder().unpackedFloat_; }

       }

       public int UnpackedFloatCount {

         get { return result.UnpackedFloatCount; }

@@ -16906,24 +18330,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedFloat(index);

       }

       public Builder SetUnpackedFloat(int index, float value) {

+        PrepareBuilder();

         result.unpackedFloat_[index] = value;

         return this;

       }

       public Builder AddUnpackedFloat(float value) {

+        PrepareBuilder();

         result.unpackedFloat_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedFloat(scg::IEnumerable<float> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedFloat_);

         return this;

       }

       public Builder ClearUnpackedFloat() {

+        PrepareBuilder();

         result.unpackedFloat_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<double> UnpackedDoubleList {

-        get { return result.unpackedDouble_; }

+        get { return PrepareBuilder().unpackedDouble_; }

       }

       public int UnpackedDoubleCount {

         get { return result.UnpackedDoubleCount; }

@@ -16932,24 +18360,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedDouble(index);

       }

       public Builder SetUnpackedDouble(int index, double value) {

+        PrepareBuilder();

         result.unpackedDouble_[index] = value;

         return this;

       }

       public Builder AddUnpackedDouble(double value) {

+        PrepareBuilder();

         result.unpackedDouble_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedDouble(scg::IEnumerable<double> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedDouble_);

         return this;

       }

       public Builder ClearUnpackedDouble() {

+        PrepareBuilder();

         result.unpackedDouble_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<bool> UnpackedBoolList {

-        get { return result.unpackedBool_; }

+        get { return PrepareBuilder().unpackedBool_; }

       }

       public int UnpackedBoolCount {

         get { return result.UnpackedBoolCount; }

@@ -16958,24 +18390,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedBool(index);

       }

       public Builder SetUnpackedBool(int index, bool value) {

+        PrepareBuilder();

         result.unpackedBool_[index] = value;

         return this;

       }

       public Builder AddUnpackedBool(bool value) {

+        PrepareBuilder();

         result.unpackedBool_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedBool(scg::IEnumerable<bool> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedBool_);

         return this;

       }

       public Builder ClearUnpackedBool() {

+        PrepareBuilder();

         result.unpackedBool_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> UnpackedEnumList {

-        get { return result.unpackedEnum_; }

+        get { return PrepareBuilder().unpackedEnum_; }

       }

       public int UnpackedEnumCount {

         get { return result.UnpackedEnumCount; }

@@ -16984,18 +18420,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedEnum(index);

       }

       public Builder SetUnpackedEnum(int index, global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.unpackedEnum_[index] = value;

         return this;

       }

       public Builder AddUnpackedEnum(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.unpackedEnum_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedEnum(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedEnum_);

         return this;

       }

       public Builder ClearUnpackedEnum() {

+        PrepareBuilder();

         result.unpackedEnum_.Clear();

         return this;

       }

@@ -17095,7 +18535,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestPackedExtensions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -17105,21 +18545,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestPackedExtensions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestPackedExtensions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestPackedExtensions result;

+      

+      private TestPackedExtensions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestPackedExtensions original = result;

+          result = new TestPackedExtensions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestPackedExtensions result = new TestPackedExtensions();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestPackedExtensions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestPackedExtensions();

+        result = DefaultInstance ?? new TestPackedExtensions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -17131,12 +18598,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestPackedExtensions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestPackedExtensions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -17150,6 +18616,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestPackedExtensions other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestPackedExtensions.DefaultInstance) return this;

+        PrepareBuilder();

           this.MergeExtensionFields(other);

         this.MergeUnknownFields(other.UnknownFields);

         return this;

@@ -17160,6 +18627,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -17298,7 +18766,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestUnpackedExtensions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -17308,21 +18776,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestUnpackedExtensions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestUnpackedExtensions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestUnpackedExtensions result;

+      

+      private TestUnpackedExtensions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestUnpackedExtensions original = result;

+          result = new TestUnpackedExtensions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestUnpackedExtensions result = new TestUnpackedExtensions();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestUnpackedExtensions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestUnpackedExtensions();

+        result = DefaultInstance ?? new TestUnpackedExtensions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -17334,12 +18829,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestUnpackedExtensions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestUnpackedExtensions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -17353,6 +18847,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestUnpackedExtensions other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestUnpackedExtensions.DefaultInstance) return this;

+        PrepareBuilder();

           this.MergeExtensionFields(other);

         this.MergeUnknownFields(other.UnknownFields);

         return this;

@@ -17363,6 +18858,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -17553,7 +19049,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(DynamicMessageType prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -17563,21 +19059,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new DynamicMessageType();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(DynamicMessageType cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          DynamicMessageType result;

           

-          DynamicMessageType result = new DynamicMessageType();

+          private DynamicMessageType PrepareBuilder() {

+            if (builderIsReadOnly) {

+              DynamicMessageType original = result;

+              result = new DynamicMessageType();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override DynamicMessageType MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new DynamicMessageType();

+            result = DefaultInstance ?? new DynamicMessageType();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -17589,12 +19112,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override DynamicMessageType BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            DynamicMessageType returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -17608,6 +19130,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(DynamicMessageType other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasDynamicField) {

               DynamicField = other.DynamicField;

             }

@@ -17620,6 +19143,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -17675,11 +19199,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetDynamicField(value); }

           }

           public Builder SetDynamicField(int value) {

+            PrepareBuilder();

             result.hasDynamicField = true;

             result.dynamicField_ = value;

             return this;

           }

           public Builder ClearDynamicField() {

+            PrepareBuilder();

             result.hasDynamicField = false;

             result.dynamicField_ = 0;

             return this;

@@ -17883,7 +19409,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestDynamicExtensions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -17893,21 +19419,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestDynamicExtensions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestDynamicExtensions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestDynamicExtensions result = new TestDynamicExtensions();

+      bool builderIsReadOnly;

+      TestDynamicExtensions result;

+      

+      private TestDynamicExtensions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestDynamicExtensions original = result;

+          result = new TestDynamicExtensions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestDynamicExtensions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestDynamicExtensions();

+        result = DefaultInstance ?? new TestDynamicExtensions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -17919,14 +19472,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestDynamicExtensions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.repeatedExtension_.MakeReadOnly();

         result.packedExtension_.MakeReadOnly();

-        TestDynamicExtensions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -17940,6 +19492,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestDynamicExtensions other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasScalarExtension) {

           ScalarExtension = other.ScalarExtension;

         }

@@ -17970,6 +19523,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -18078,11 +19632,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetScalarExtension(uint value) {

+        PrepareBuilder();

         result.hasScalarExtension = true;

         result.scalarExtension_ = value;

         return this;

       }

       public Builder ClearScalarExtension() {

+        PrepareBuilder();

         result.hasScalarExtension = false;

         result.scalarExtension_ = 0;

         return this;

@@ -18096,11 +19652,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetEnumExtension(value); }

       }

       public Builder SetEnumExtension(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.hasEnumExtension = true;

         result.enumExtension_ = value;

         return this;

       }

       public Builder ClearEnumExtension() {

+        PrepareBuilder();

         result.hasEnumExtension = false;

         result.enumExtension_ = global::Google.ProtocolBuffers.TestProtos.ForeignEnum.FOREIGN_FOO;

         return this;

@@ -18114,11 +19672,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDynamicEnumExtension(value); }

       }

       public Builder SetDynamicEnumExtension(global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicEnumType value) {

+        PrepareBuilder();

         result.hasDynamicEnumExtension = true;

         result.dynamicEnumExtension_ = value;

         return this;

       }

       public Builder ClearDynamicEnumExtension() {

+        PrepareBuilder();

         result.hasDynamicEnumExtension = false;

         result.dynamicEnumExtension_ = global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicEnumType.DYNAMIC_FOO;

         return this;

@@ -18133,18 +19693,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetMessageExtension(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasMessageExtension = true;

         result.messageExtension_ = value;

         return this;

       }

       public Builder SetMessageExtension(global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasMessageExtension = true;

         result.messageExtension_ = builderForValue.Build();

         return this;

       }

       public Builder MergeMessageExtension(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasMessageExtension &&

             result.messageExtension_ != global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance) {

             result.messageExtension_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.CreateBuilder(result.messageExtension_).MergeFrom(value).BuildPartial();

@@ -18155,6 +19718,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearMessageExtension() {

+        PrepareBuilder();

         result.hasMessageExtension = false;

         result.messageExtension_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

         return this;

@@ -18169,18 +19733,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetDynamicMessageExtension(global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasDynamicMessageExtension = true;

         result.dynamicMessageExtension_ = value;

         return this;

       }

       public Builder SetDynamicMessageExtension(global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasDynamicMessageExtension = true;

         result.dynamicMessageExtension_ = builderForValue.Build();

         return this;

       }

       public Builder MergeDynamicMessageExtension(global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasDynamicMessageExtension &&

             result.dynamicMessageExtension_ != global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.DefaultInstance) {

             result.dynamicMessageExtension_ = global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.CreateBuilder(result.dynamicMessageExtension_).MergeFrom(value).BuildPartial();

@@ -18191,13 +19758,14 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearDynamicMessageExtension() {

+        PrepareBuilder();

         result.hasDynamicMessageExtension = false;

         result.dynamicMessageExtension_ = global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.DefaultInstance;

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedExtensionList {

-        get { return result.repeatedExtension_; }

+        get { return PrepareBuilder().repeatedExtension_; }

       }

       public int RepeatedExtensionCount {

         get { return result.RepeatedExtensionCount; }

@@ -18207,25 +19775,29 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedExtension(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedExtension_[index] = value;

         return this;

       }

       public Builder AddRepeatedExtension(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedExtension_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedExtension(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedExtension_);

         return this;

       }

       public Builder ClearRepeatedExtension() {

+        PrepareBuilder();

         result.repeatedExtension_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> PackedExtensionList {

-        get { return result.packedExtension_; }

+        get { return PrepareBuilder().packedExtension_; }

       }

       public int PackedExtensionCount {

         get { return result.PackedExtensionCount; }

@@ -18234,18 +19806,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedExtension(index);

       }

       public Builder SetPackedExtension(int index, int value) {

+        PrepareBuilder();

         result.packedExtension_[index] = value;

         return this;

       }

       public Builder AddPackedExtension(int value) {

+        PrepareBuilder();

         result.packedExtension_.Add(value);

         return this;

       }

       public Builder AddRangePackedExtension(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedExtension_);

         return this;

       }

       public Builder ClearPackedExtension() {

+        PrepareBuilder();

         result.packedExtension_.Clear();

         return this;

       }

@@ -18479,7 +20055,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestRepeatedScalarDifferentTagSizes prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -18489,21 +20065,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestRepeatedScalarDifferentTagSizes();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestRepeatedScalarDifferentTagSizes cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestRepeatedScalarDifferentTagSizes result;

+      

+      private TestRepeatedScalarDifferentTagSizes PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestRepeatedScalarDifferentTagSizes original = result;

+          result = new TestRepeatedScalarDifferentTagSizes();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestRepeatedScalarDifferentTagSizes result = new TestRepeatedScalarDifferentTagSizes();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestRepeatedScalarDifferentTagSizes MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestRepeatedScalarDifferentTagSizes();

+        result = DefaultInstance ?? new TestRepeatedScalarDifferentTagSizes();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -18515,8 +20118,8 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRepeatedScalarDifferentTagSizes BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.repeatedFixed32_.MakeReadOnly();

         result.repeatedInt32_.MakeReadOnly();

@@ -18524,9 +20127,8 @@ namespace Google.ProtocolBuffers.TestProtos {
         result.repeatedInt64_.MakeReadOnly();

         result.repeatedFloat_.MakeReadOnly();

         result.repeatedUint64_.MakeReadOnly();

-        TestRepeatedScalarDifferentTagSizes returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -18540,6 +20142,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestRepeatedScalarDifferentTagSizes other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestRepeatedScalarDifferentTagSizes.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.repeatedFixed32_.Count != 0) {

           base.AddRange(other.repeatedFixed32_, result.repeatedFixed32_);

         }

@@ -18567,6 +20170,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -18642,7 +20246,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> RepeatedFixed32List {

-        get { return result.repeatedFixed32_; }

+        get { return PrepareBuilder().repeatedFixed32_; }

       }

       public int RepeatedFixed32Count {

         get { return result.RepeatedFixed32Count; }

@@ -18653,26 +20257,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedFixed32(int index, uint value) {

+        PrepareBuilder();

         result.repeatedFixed32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedFixed32(uint value) {

+        PrepareBuilder();

         result.repeatedFixed32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedFixed32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedFixed32_);

         return this;

       }

       public Builder ClearRepeatedFixed32() {

+        PrepareBuilder();

         result.repeatedFixed32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> RepeatedInt32List {

-        get { return result.repeatedInt32_; }

+        get { return PrepareBuilder().repeatedInt32_; }

       }

       public int RepeatedInt32Count {

         get { return result.RepeatedInt32Count; }

@@ -18681,25 +20289,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedInt32(index);

       }

       public Builder SetRepeatedInt32(int index, int value) {

+        PrepareBuilder();

         result.repeatedInt32_[index] = value;

         return this;

       }

       public Builder AddRepeatedInt32(int value) {

+        PrepareBuilder();

         result.repeatedInt32_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedInt32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedInt32_);

         return this;

       }

       public Builder ClearRepeatedInt32() {

+        PrepareBuilder();

         result.repeatedInt32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> RepeatedFixed64List {

-        get { return result.repeatedFixed64_; }

+        get { return PrepareBuilder().repeatedFixed64_; }

       }

       public int RepeatedFixed64Count {

         get { return result.RepeatedFixed64Count; }

@@ -18710,26 +20322,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedFixed64(int index, ulong value) {

+        PrepareBuilder();

         result.repeatedFixed64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedFixed64(ulong value) {

+        PrepareBuilder();

         result.repeatedFixed64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedFixed64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedFixed64_);

         return this;

       }

       public Builder ClearRepeatedFixed64() {

+        PrepareBuilder();

         result.repeatedFixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> RepeatedInt64List {

-        get { return result.repeatedInt64_; }

+        get { return PrepareBuilder().repeatedInt64_; }

       }

       public int RepeatedInt64Count {

         get { return result.RepeatedInt64Count; }

@@ -18738,24 +20354,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedInt64(index);

       }

       public Builder SetRepeatedInt64(int index, long value) {

+        PrepareBuilder();

         result.repeatedInt64_[index] = value;

         return this;

       }

       public Builder AddRepeatedInt64(long value) {

+        PrepareBuilder();

         result.repeatedInt64_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedInt64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedInt64_);

         return this;

       }

       public Builder ClearRepeatedInt64() {

+        PrepareBuilder();

         result.repeatedInt64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<float> RepeatedFloatList {

-        get { return result.repeatedFloat_; }

+        get { return PrepareBuilder().repeatedFloat_; }

       }

       public int RepeatedFloatCount {

         get { return result.RepeatedFloatCount; }

@@ -18764,25 +20384,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedFloat(index);

       }

       public Builder SetRepeatedFloat(int index, float value) {

+        PrepareBuilder();

         result.repeatedFloat_[index] = value;

         return this;

       }

       public Builder AddRepeatedFloat(float value) {

+        PrepareBuilder();

         result.repeatedFloat_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedFloat(scg::IEnumerable<float> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedFloat_);

         return this;

       }

       public Builder ClearRepeatedFloat() {

+        PrepareBuilder();

         result.repeatedFloat_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> RepeatedUint64List {

-        get { return result.repeatedUint64_; }

+        get { return PrepareBuilder().repeatedUint64_; }

       }

       public int RepeatedUint64Count {

         get { return result.RepeatedUint64Count; }

@@ -18793,20 +20417,24 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedUint64(int index, ulong value) {

+        PrepareBuilder();

         result.repeatedUint64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedUint64(ulong value) {

+        PrepareBuilder();

         result.repeatedUint64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedUint64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedUint64_);

         return this;

       }

       public Builder ClearRepeatedUint64() {

+        PrepareBuilder();

         result.repeatedUint64_.Clear();

         return this;

       }

@@ -18902,7 +20530,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(FooRequest prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -18912,21 +20540,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new FooRequest();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(FooRequest cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      FooRequest result = new FooRequest();

+      bool builderIsReadOnly;

+      FooRequest result;

+      

+      private FooRequest PrepareBuilder() {

+        if (builderIsReadOnly) {

+          FooRequest original = result;

+          result = new FooRequest();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override FooRequest MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new FooRequest();

+        result = DefaultInstance ?? new FooRequest();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -18938,12 +20593,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override FooRequest BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        FooRequest returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -18957,6 +20611,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(FooRequest other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.FooRequest.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -18966,6 +20621,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -19100,7 +20756,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(FooResponse prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -19110,21 +20766,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new FooResponse();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(FooResponse cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      FooResponse result;

+      

+      private FooResponse PrepareBuilder() {

+        if (builderIsReadOnly) {

+          FooResponse original = result;

+          result = new FooResponse();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      FooResponse result = new FooResponse();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override FooResponse MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new FooResponse();

+        result = DefaultInstance ?? new FooResponse();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -19136,12 +20819,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override FooResponse BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        FooResponse returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -19155,6 +20837,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(FooResponse other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.FooResponse.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -19164,6 +20847,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -19298,7 +20982,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(BarRequest prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -19308,21 +20992,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new BarRequest();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(BarRequest cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      BarRequest result;

+      

+      private BarRequest PrepareBuilder() {

+        if (builderIsReadOnly) {

+          BarRequest original = result;

+          result = new BarRequest();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      BarRequest result = new BarRequest();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override BarRequest MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new BarRequest();

+        result = DefaultInstance ?? new BarRequest();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -19334,12 +21045,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override BarRequest BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        BarRequest returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -19353,6 +21063,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(BarRequest other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.BarRequest.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -19362,6 +21073,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -19496,7 +21208,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(BarResponse prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -19506,21 +21218,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new BarResponse();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(BarResponse cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      BarResponse result = new BarResponse();

+      bool builderIsReadOnly;

+      BarResponse result;

+      

+      private BarResponse PrepareBuilder() {

+        if (builderIsReadOnly) {

+          BarResponse original = result;

+          result = new BarResponse();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override BarResponse MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new BarResponse();

+        result = DefaultInstance ?? new BarResponse();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -19532,12 +21271,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override BarResponse BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        BarResponse returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -19551,6 +21289,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(BarResponse other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.BarResponse.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -19560,6 +21299,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestRpcInterop.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestRpcInterop.cs
index ae309fa7..3c5e9307 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestRpcInterop.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestRpcInterop.cs
@@ -188,7 +188,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(SearchRequest prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -198,21 +198,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new SearchRequest();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(SearchRequest cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      SearchRequest result;

       

-      SearchRequest result = new SearchRequest();

+      private SearchRequest PrepareBuilder() {

+        if (builderIsReadOnly) {

+          SearchRequest original = result;

+          result = new SearchRequest();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override SearchRequest MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new SearchRequest();

+        result = DefaultInstance ?? new SearchRequest();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -224,13 +251,12 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SearchRequest BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.criteria_.MakeReadOnly();

-        SearchRequest returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -244,6 +270,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(SearchRequest other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.SearchRequest.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.criteria_.Count != 0) {

           base.AddRange(other.criteria_, result.criteria_);

         }

@@ -256,6 +283,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -304,7 +332,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       

       public pbc::IPopsicleList<string> CriteriaList {

-        get { return result.criteria_; }

+        get { return PrepareBuilder().criteria_; }

       }

       public int CriteriaCount {

         get { return result.CriteriaCount; }

@@ -314,19 +342,23 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetCriteria(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.criteria_[index] = value;

         return this;

       }

       public Builder AddCriteria(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.criteria_.Add(value);

         return this;

       }

       public Builder AddRangeCriteria(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.criteria_);

         return this;

       }

       public Builder ClearCriteria() {

+        PrepareBuilder();

         result.criteria_.Clear();

         return this;

       }

@@ -487,7 +519,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(ResultItem prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -497,21 +529,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new ResultItem();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(ResultItem cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          ResultItem result;

           

-          ResultItem result = new ResultItem();

+          private ResultItem PrepareBuilder() {

+            if (builderIsReadOnly) {

+              ResultItem original = result;

+              result = new ResultItem();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override ResultItem MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new ResultItem();

+            result = DefaultInstance ?? new ResultItem();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -523,12 +582,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override ResultItem BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            ResultItem returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -542,6 +600,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(ResultItem other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.SearchResponse.Types.ResultItem.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasUrl) {

               Url = other.Url;

             }

@@ -557,6 +616,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -617,11 +677,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetUrl(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasUrl = true;

             result.url_ = value;

             return this;

           }

           public Builder ClearUrl() {

+            PrepareBuilder();

             result.hasUrl = false;

             result.url_ = "";

             return this;

@@ -636,11 +698,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetName(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasName = true;

             result.name_ = value;

             return this;

           }

           public Builder ClearName() {

+            PrepareBuilder();

             result.hasName = false;

             result.name_ = "";

             return this;

@@ -734,7 +798,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(SearchResponse prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -744,21 +808,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new SearchResponse();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(SearchResponse cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      SearchResponse result;

       

-      SearchResponse result = new SearchResponse();

+      private SearchResponse PrepareBuilder() {

+        if (builderIsReadOnly) {

+          SearchResponse original = result;

+          result = new SearchResponse();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override SearchResponse MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new SearchResponse();

+        result = DefaultInstance ?? new SearchResponse();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -770,13 +861,12 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SearchResponse BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.results_.MakeReadOnly();

-        SearchResponse returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -790,6 +880,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(SearchResponse other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.SearchResponse.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.results_.Count != 0) {

           base.AddRange(other.results_, result.results_);

         }

@@ -802,6 +893,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -850,7 +942,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.SearchResponse.Types.ResultItem> ResultsList {

-        get { return result.results_; }

+        get { return PrepareBuilder().results_; }

       }

       public int ResultsCount {

         get { return result.ResultsCount; }

@@ -860,29 +952,35 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetResults(int index, global::Google.ProtocolBuffers.TestProtos.SearchResponse.Types.ResultItem value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.results_[index] = value;

         return this;

       }

       public Builder SetResults(int index, global::Google.ProtocolBuffers.TestProtos.SearchResponse.Types.ResultItem.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.results_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddResults(global::Google.ProtocolBuffers.TestProtos.SearchResponse.Types.ResultItem value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.results_.Add(value);

         return this;

       }

       public Builder AddResults(global::Google.ProtocolBuffers.TestProtos.SearchResponse.Types.ResultItem.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.results_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeResults(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.SearchResponse.Types.ResultItem> values) {

+        PrepareBuilder();

         base.AddRange(values, result.results_);

         return this;

       }

       public Builder ClearResults() {

+        PrepareBuilder();

         result.results_.Clear();

         return this;

       }

@@ -1019,7 +1117,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(RefineSearchRequest prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1029,21 +1127,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new RefineSearchRequest();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(RefineSearchRequest cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      RefineSearchRequest result;

       

-      RefineSearchRequest result = new RefineSearchRequest();

+      private RefineSearchRequest PrepareBuilder() {

+        if (builderIsReadOnly) {

+          RefineSearchRequest original = result;

+          result = new RefineSearchRequest();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override RefineSearchRequest MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new RefineSearchRequest();

+        result = DefaultInstance ?? new RefineSearchRequest();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1055,13 +1180,12 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override RefineSearchRequest BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.criteria_.MakeReadOnly();

-        RefineSearchRequest returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -1075,6 +1199,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(RefineSearchRequest other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.RefineSearchRequest.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.criteria_.Count != 0) {

           base.AddRange(other.criteria_, result.criteria_);

         }

@@ -1090,6 +1215,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -1147,7 +1273,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       

       public pbc::IPopsicleList<string> CriteriaList {

-        get { return result.criteria_; }

+        get { return PrepareBuilder().criteria_; }

       }

       public int CriteriaCount {

         get { return result.CriteriaCount; }

@@ -1157,19 +1283,23 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetCriteria(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.criteria_[index] = value;

         return this;

       }

       public Builder AddCriteria(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.criteria_.Add(value);

         return this;

       }

       public Builder AddRangeCriteria(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.criteria_);

         return this;

       }

       public Builder ClearCriteria() {

+        PrepareBuilder();

         result.criteria_.Clear();

         return this;

       }

@@ -1183,18 +1313,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetPreviousResults(global::Google.ProtocolBuffers.TestProtos.SearchResponse value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasPreviousResults = true;

         result.previousResults_ = value;

         return this;

       }

       public Builder SetPreviousResults(global::Google.ProtocolBuffers.TestProtos.SearchResponse.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasPreviousResults = true;

         result.previousResults_ = builderForValue.Build();

         return this;

       }

       public Builder MergePreviousResults(global::Google.ProtocolBuffers.TestProtos.SearchResponse value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasPreviousResults &&

             result.previousResults_ != global::Google.ProtocolBuffers.TestProtos.SearchResponse.DefaultInstance) {

             result.previousResults_ = global::Google.ProtocolBuffers.TestProtos.SearchResponse.CreateBuilder(result.previousResults_).MergeFrom(value).BuildPartial();

@@ -1205,6 +1338,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearPreviousResults() {

+        PrepareBuilder();

         result.hasPreviousResults = false;

         result.previousResults_ = global::Google.ProtocolBuffers.TestProtos.SearchResponse.DefaultInstance;

         return this;

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestXmlSerializerTestProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestXmlSerializerTestProtoFile.cs
index 3d818a60..c0e53348 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestXmlSerializerTestProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestXmlSerializerTestProtoFile.cs
@@ -261,7 +261,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestXmlChild prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -271,21 +271,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestXmlChild();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestXmlChild cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestXmlChild result = new TestXmlChild();

+      bool builderIsReadOnly;

+      TestXmlChild result;

+      

+      private TestXmlChild PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestXmlChild original = result;

+          result = new TestXmlChild();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestXmlChild MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestXmlChild();

+        result = DefaultInstance ?? new TestXmlChild();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -297,13 +324,12 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestXmlChild BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.options_.MakeReadOnly();

-        TestXmlChild returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -317,6 +343,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestXmlChild other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestXmlChild.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.options_.Count != 0) {

           base.AddRange(other.options_, result.options_);

         }

@@ -332,6 +359,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -394,7 +422,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.EnumOptions> OptionsList {

-        get { return result.options_; }

+        get { return PrepareBuilder().options_; }

       }

       public int OptionsCount {

         get { return result.OptionsCount; }

@@ -403,18 +431,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetOptions(index);

       }

       public Builder SetOptions(int index, global::Google.ProtocolBuffers.TestProtos.EnumOptions value) {

+        PrepareBuilder();

         result.options_[index] = value;

         return this;

       }

       public Builder AddOptions(global::Google.ProtocolBuffers.TestProtos.EnumOptions value) {

+        PrepareBuilder();

         result.options_.Add(value);

         return this;

       }

       public Builder AddRangeOptions(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.EnumOptions> values) {

+        PrepareBuilder();

         base.AddRange(values, result.options_);

         return this;

       }

       public Builder ClearOptions() {

+        PrepareBuilder();

         result.options_.Clear();

         return this;

       }

@@ -428,11 +460,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetBinary(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasBinary = true;

         result.binary_ = value;

         return this;

       }

       public Builder ClearBinary() {

+        PrepareBuilder();

         result.hasBinary = false;

         result.binary_ = pb::ByteString.Empty;

         return this;

@@ -529,7 +563,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestXmlNoFields prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -539,21 +573,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestXmlNoFields();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestXmlNoFields cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestXmlNoFields result = new TestXmlNoFields();

+      bool builderIsReadOnly;

+      TestXmlNoFields result;

+      

+      private TestXmlNoFields PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestXmlNoFields original = result;

+          result = new TestXmlNoFields();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestXmlNoFields MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestXmlNoFields();

+        result = DefaultInstance ?? new TestXmlNoFields();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -565,12 +626,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestXmlNoFields BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestXmlNoFields returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -584,6 +644,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestXmlNoFields other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestXmlNoFields.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -593,6 +654,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -743,7 +805,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestXmlRescursive prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -753,21 +815,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestXmlRescursive();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestXmlRescursive cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestXmlRescursive result = new TestXmlRescursive();

+      bool builderIsReadOnly;

+      TestXmlRescursive result;

+      

+      private TestXmlRescursive PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestXmlRescursive original = result;

+          result = new TestXmlRescursive();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestXmlRescursive MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestXmlRescursive();

+        result = DefaultInstance ?? new TestXmlRescursive();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -779,12 +868,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestXmlRescursive BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestXmlRescursive returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -798,6 +886,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestXmlRescursive other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasChild) {

           MergeChild(other.Child);

         }

@@ -810,6 +899,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -871,18 +961,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetChild(global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasChild = true;

         result.child_ = value;

         return this;

       }

       public Builder SetChild(global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasChild = true;

         result.child_ = builderForValue.Build();

         return this;

       }

       public Builder MergeChild(global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasChild &&

             result.child_ != global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive.DefaultInstance) {

             result.child_ = global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive.CreateBuilder(result.child_).MergeFrom(value).BuildPartial();

@@ -893,6 +986,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearChild() {

+        PrepareBuilder();

         result.hasChild = false;

         result.child_ = global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive.DefaultInstance;

         return this;

@@ -1062,7 +1156,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(Children prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1072,21 +1166,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new Children();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(Children cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

           

-          Children result = new Children();

+          bool builderIsReadOnly;

+          Children result;

+          

+          private Children PrepareBuilder() {

+            if (builderIsReadOnly) {

+              Children original = result;

+              result = new Children();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override Children MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new Children();

+            result = DefaultInstance ?? new Children();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -1098,13 +1219,12 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Children BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

             result.options_.MakeReadOnly();

-            Children returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -1118,6 +1238,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(Children other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestXmlMessage.Types.Children.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.options_.Count != 0) {

               base.AddRange(other.options_, result.options_);

             }

@@ -1133,6 +1254,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -1195,7 +1317,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           

           public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.EnumOptions> OptionsList {

-            get { return result.options_; }

+            get { return PrepareBuilder().options_; }

           }

           public int OptionsCount {

             get { return result.OptionsCount; }

@@ -1204,18 +1326,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             return result.GetOptions(index);

           }

           public Builder SetOptions(int index, global::Google.ProtocolBuffers.TestProtos.EnumOptions value) {

+            PrepareBuilder();

             result.options_[index] = value;

             return this;

           }

           public Builder AddOptions(global::Google.ProtocolBuffers.TestProtos.EnumOptions value) {

+            PrepareBuilder();

             result.options_.Add(value);

             return this;

           }

           public Builder AddRangeOptions(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.EnumOptions> values) {

+            PrepareBuilder();

             base.AddRange(values, result.options_);

             return this;

           }

           public Builder ClearOptions() {

+            PrepareBuilder();

             result.options_.Clear();

             return this;

           }

@@ -1229,11 +1355,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetBinary(pb::ByteString value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasBinary = true;

             result.binary_ = value;

             return this;

           }

           public Builder ClearBinary() {

+            PrepareBuilder();

             result.hasBinary = false;

             result.binary_ = pb::ByteString.Empty;

             return this;

@@ -1438,7 +1566,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestXmlMessage prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1448,21 +1576,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestXmlMessage();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestXmlMessage cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestXmlMessage result = new TestXmlMessage();

+      bool builderIsReadOnly;

+      TestXmlMessage result;

+      

+      private TestXmlMessage PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestXmlMessage original = result;

+          result = new TestXmlMessage();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestXmlMessage MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestXmlMessage();

+        result = DefaultInstance ?? new TestXmlMessage();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1474,15 +1629,14 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestXmlMessage BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.numbers_.MakeReadOnly();

         result.textlines_.MakeReadOnly();

         result.children_.MakeReadOnly();

-        TestXmlMessage returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -1496,6 +1650,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestXmlMessage other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestXmlMessage.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasNumber) {

           Number = other.Number;

         }

@@ -1527,6 +1682,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -1612,18 +1768,20 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetNumber(value); }

       }

       public Builder SetNumber(long value) {

+        PrepareBuilder();

         result.hasNumber = true;

         result.number_ = value;

         return this;

       }

       public Builder ClearNumber() {

+        PrepareBuilder();

         result.hasNumber = false;

         result.number_ = 0L;

         return this;

       }

       

       public pbc::IPopsicleList<int> NumbersList {

-        get { return result.numbers_; }

+        get { return PrepareBuilder().numbers_; }

       }

       public int NumbersCount {

         get { return result.NumbersCount; }

@@ -1632,18 +1790,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetNumbers(index);

       }

       public Builder SetNumbers(int index, int value) {

+        PrepareBuilder();

         result.numbers_[index] = value;

         return this;

       }

       public Builder AddNumbers(int value) {

+        PrepareBuilder();

         result.numbers_.Add(value);

         return this;

       }

       public Builder AddRangeNumbers(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.numbers_);

         return this;

       }

       public Builder ClearNumbers() {

+        PrepareBuilder();

         result.numbers_.Clear();

         return this;

       }

@@ -1657,18 +1819,20 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetText(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasText = true;

         result.text_ = value;

         return this;

       }

       public Builder ClearText() {

+        PrepareBuilder();

         result.hasText = false;

         result.text_ = "";

         return this;

       }

       

       public pbc::IPopsicleList<string> TextlinesList {

-        get { return result.textlines_; }

+        get { return PrepareBuilder().textlines_; }

       }

       public int TextlinesCount {

         get { return result.TextlinesCount; }

@@ -1678,19 +1842,23 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetTextlines(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.textlines_[index] = value;

         return this;

       }

       public Builder AddTextlines(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.textlines_.Add(value);

         return this;

       }

       public Builder AddRangeTextlines(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.textlines_);

         return this;

       }

       public Builder ClearTextlines() {

+        PrepareBuilder();

         result.textlines_.Clear();

         return this;

       }

@@ -1703,11 +1871,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetValid(value); }

       }

       public Builder SetValid(bool value) {

+        PrepareBuilder();

         result.hasValid = true;

         result.valid_ = value;

         return this;

       }

       public Builder ClearValid() {

+        PrepareBuilder();

         result.hasValid = false;

         result.valid_ = false;

         return this;

@@ -1722,18 +1892,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetChild(global::Google.ProtocolBuffers.TestProtos.TestXmlChild value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasChild = true;

         result.child_ = value;

         return this;

       }

       public Builder SetChild(global::Google.ProtocolBuffers.TestProtos.TestXmlChild.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasChild = true;

         result.child_ = builderForValue.Build();

         return this;

       }

       public Builder MergeChild(global::Google.ProtocolBuffers.TestProtos.TestXmlChild value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasChild &&

             result.child_ != global::Google.ProtocolBuffers.TestProtos.TestXmlChild.DefaultInstance) {

             result.child_ = global::Google.ProtocolBuffers.TestProtos.TestXmlChild.CreateBuilder(result.child_).MergeFrom(value).BuildPartial();

@@ -1744,13 +1917,14 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearChild() {

+        PrepareBuilder();

         result.hasChild = false;

         result.child_ = global::Google.ProtocolBuffers.TestProtos.TestXmlChild.DefaultInstance;

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestXmlMessage.Types.Children> ChildrenList {

-        get { return result.children_; }

+        get { return PrepareBuilder().children_; }

       }

       public int ChildrenCount {

         get { return result.ChildrenCount; }

@@ -1760,29 +1934,35 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetChildren(int index, global::Google.ProtocolBuffers.TestProtos.TestXmlMessage.Types.Children value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.children_[index] = value;

         return this;

       }

       public Builder SetChildren(int index, global::Google.ProtocolBuffers.TestProtos.TestXmlMessage.Types.Children.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.children_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddChildren(global::Google.ProtocolBuffers.TestProtos.TestXmlMessage.Types.Children value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.children_.Add(value);

         return this;

       }

       public Builder AddChildren(global::Google.ProtocolBuffers.TestProtos.TestXmlMessage.Types.Children.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.children_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeChildren(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestXmlMessage.Types.Children> values) {

+        PrepareBuilder();

         base.AddRange(values, result.children_);

         return this;

       }

       public Builder ClearChildren() {

+        PrepareBuilder();

         result.children_.Clear();

         return this;

       }

@@ -1895,7 +2075,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestXmlExtension prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1905,21 +2085,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestXmlExtension();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestXmlExtension cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestXmlExtension result = new TestXmlExtension();

+      bool builderIsReadOnly;

+      TestXmlExtension result;

+      

+      private TestXmlExtension PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestXmlExtension original = result;

+          result = new TestXmlExtension();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestXmlExtension MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestXmlExtension();

+        result = DefaultInstance ?? new TestXmlExtension();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1931,12 +2138,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestXmlExtension BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestXmlExtension returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -1950,6 +2156,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestXmlExtension other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestXmlExtension.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasNumber) {

           Number = other.Number;

         }

@@ -1962,6 +2169,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -2017,11 +2225,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetNumber(value); }

       }

       public Builder SetNumber(int value) {

+        PrepareBuilder();

         result.hasNumber = true;

         result.number_ = value;

         return this;

       }

       public Builder ClearNumber() {

+        PrepareBuilder();

         result.hasNumber = false;

         result.number_ = 0;

         return this;

diff --git a/src/ProtocolBuffers/DescriptorProtos/CSharpOptions.cs b/src/ProtocolBuffers/DescriptorProtos/CSharpOptions.cs
index 853b0e7d..0d9109a2 100644
--- a/src/ProtocolBuffers/DescriptorProtos/CSharpOptions.cs
+++ b/src/ProtocolBuffers/DescriptorProtos/CSharpOptions.cs
@@ -415,7 +415,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(CSharpFileOptions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -425,21 +425,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new CSharpFileOptions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(CSharpFileOptions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      CSharpFileOptions result;

       

-      CSharpFileOptions result = new CSharpFileOptions();

+      private CSharpFileOptions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          CSharpFileOptions original = result;

+          result = new CSharpFileOptions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override CSharpFileOptions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new CSharpFileOptions();

+        result = DefaultInstance ?? new CSharpFileOptions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -451,12 +478,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override CSharpFileOptions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        CSharpFileOptions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -470,6 +496,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(CSharpFileOptions other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.CSharpFileOptions.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasNamespace) {

           Namespace = other.Namespace;

         }

@@ -518,6 +545,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -630,11 +658,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetNamespace(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasNamespace = true;

         result.namespace_ = value;

         return this;

       }

       public Builder ClearNamespace() {

+        PrepareBuilder();

         result.hasNamespace = false;

         result.namespace_ = "";

         return this;

@@ -649,11 +679,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetUmbrellaClassname(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasUmbrellaClassname = true;

         result.umbrellaClassname_ = value;

         return this;

       }

       public Builder ClearUmbrellaClassname() {

+        PrepareBuilder();

         result.hasUmbrellaClassname = false;

         result.umbrellaClassname_ = "";

         return this;

@@ -667,11 +699,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetPublicClasses(value); }

       }

       public Builder SetPublicClasses(bool value) {

+        PrepareBuilder();

         result.hasPublicClasses = true;

         result.publicClasses_ = value;

         return this;

       }

       public Builder ClearPublicClasses() {

+        PrepareBuilder();

         result.hasPublicClasses = false;

         result.publicClasses_ = true;

         return this;

@@ -685,11 +719,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetMultipleFiles(value); }

       }

       public Builder SetMultipleFiles(bool value) {

+        PrepareBuilder();

         result.hasMultipleFiles = true;

         result.multipleFiles_ = value;

         return this;

       }

       public Builder ClearMultipleFiles() {

+        PrepareBuilder();

         result.hasMultipleFiles = false;

         result.multipleFiles_ = false;

         return this;

@@ -703,11 +739,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetNestClasses(value); }

       }

       public Builder SetNestClasses(bool value) {

+        PrepareBuilder();

         result.hasNestClasses = true;

         result.nestClasses_ = value;

         return this;

       }

       public Builder ClearNestClasses() {

+        PrepareBuilder();

         result.hasNestClasses = false;

         result.nestClasses_ = false;

         return this;

@@ -721,11 +759,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetCodeContracts(value); }

       }

       public Builder SetCodeContracts(bool value) {

+        PrepareBuilder();

         result.hasCodeContracts = true;

         result.codeContracts_ = value;

         return this;

       }

       public Builder ClearCodeContracts() {

+        PrepareBuilder();

         result.hasCodeContracts = false;

         result.codeContracts_ = false;

         return this;

@@ -739,11 +779,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetExpandNamespaceDirectories(value); }

       }

       public Builder SetExpandNamespaceDirectories(bool value) {

+        PrepareBuilder();

         result.hasExpandNamespaceDirectories = true;

         result.expandNamespaceDirectories_ = value;

         return this;

       }

       public Builder ClearExpandNamespaceDirectories() {

+        PrepareBuilder();

         result.hasExpandNamespaceDirectories = false;

         result.expandNamespaceDirectories_ = false;

         return this;

@@ -757,11 +799,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetClsCompliance(value); }

       }

       public Builder SetClsCompliance(bool value) {

+        PrepareBuilder();

         result.hasClsCompliance = true;

         result.clsCompliance_ = value;

         return this;

       }

       public Builder ClearClsCompliance() {

+        PrepareBuilder();

         result.hasClsCompliance = false;

         result.clsCompliance_ = true;

         return this;

@@ -776,11 +820,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetFileExtension(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasFileExtension = true;

         result.fileExtension_ = value;

         return this;

       }

       public Builder ClearFileExtension() {

+        PrepareBuilder();

         result.hasFileExtension = false;

         result.fileExtension_ = ".cs";

         return this;

@@ -795,11 +841,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetUmbrellaNamespace(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasUmbrellaNamespace = true;

         result.umbrellaNamespace_ = value;

         return this;

       }

       public Builder ClearUmbrellaNamespace() {

+        PrepareBuilder();

         result.hasUmbrellaNamespace = false;

         result.umbrellaNamespace_ = "";

         return this;

@@ -814,11 +862,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetOutputDirectory(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOutputDirectory = true;

         result.outputDirectory_ = value;

         return this;

       }

       public Builder ClearOutputDirectory() {

+        PrepareBuilder();

         result.hasOutputDirectory = false;

         result.outputDirectory_ = ".";

         return this;

@@ -832,11 +882,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetIgnoreGoogleProtobuf(value); }

       }

       public Builder SetIgnoreGoogleProtobuf(bool value) {

+        PrepareBuilder();

         result.hasIgnoreGoogleProtobuf = true;

         result.ignoreGoogleProtobuf_ = value;

         return this;

       }

       public Builder ClearIgnoreGoogleProtobuf() {

+        PrepareBuilder();

         result.hasIgnoreGoogleProtobuf = false;

         result.ignoreGoogleProtobuf_ = false;

         return this;

@@ -850,11 +902,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetServiceGeneratorType(value); }

       }

       public Builder SetServiceGeneratorType(global::Google.ProtocolBuffers.DescriptorProtos.CSharpServiceType value) {

+        PrepareBuilder();

         result.hasServiceGeneratorType = true;

         result.serviceGeneratorType_ = value;

         return this;

       }

       public Builder ClearServiceGeneratorType() {

+        PrepareBuilder();

         result.hasServiceGeneratorType = false;

         result.serviceGeneratorType_ = global::Google.ProtocolBuffers.DescriptorProtos.CSharpServiceType.NONE;

         return this;

@@ -967,7 +1021,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(CSharpFieldOptions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -977,21 +1031,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new CSharpFieldOptions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(CSharpFieldOptions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      CSharpFieldOptions result;

       

-      CSharpFieldOptions result = new CSharpFieldOptions();

+      private CSharpFieldOptions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          CSharpFieldOptions original = result;

+          result = new CSharpFieldOptions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override CSharpFieldOptions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new CSharpFieldOptions();

+        result = DefaultInstance ?? new CSharpFieldOptions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1003,12 +1084,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override CSharpFieldOptions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        CSharpFieldOptions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -1022,6 +1102,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(CSharpFieldOptions other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.CSharpFieldOptions.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasPropertyName) {

           PropertyName = other.PropertyName;

         }

@@ -1034,6 +1115,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -1090,11 +1172,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetPropertyName(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasPropertyName = true;

         result.propertyName_ = value;

         return this;

       }

       public Builder ClearPropertyName() {

+        PrepareBuilder();

         result.hasPropertyName = false;

         result.propertyName_ = "";

         return this;

@@ -1207,7 +1291,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(CSharpServiceOptions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1217,21 +1301,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new CSharpServiceOptions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(CSharpServiceOptions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      CSharpServiceOptions result;

       

-      CSharpServiceOptions result = new CSharpServiceOptions();

+      private CSharpServiceOptions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          CSharpServiceOptions original = result;

+          result = new CSharpServiceOptions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override CSharpServiceOptions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new CSharpServiceOptions();

+        result = DefaultInstance ?? new CSharpServiceOptions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1243,12 +1354,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override CSharpServiceOptions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        CSharpServiceOptions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -1262,6 +1372,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(CSharpServiceOptions other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.CSharpServiceOptions.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasInterfaceId) {

           InterfaceId = other.InterfaceId;

         }

@@ -1274,6 +1385,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -1330,11 +1442,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetInterfaceId(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasInterfaceId = true;

         result.interfaceId_ = value;

         return this;

       }

       public Builder ClearInterfaceId() {

+        PrepareBuilder();

         result.hasInterfaceId = false;

         result.interfaceId_ = "";

         return this;

@@ -1447,7 +1561,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(CSharpMethodOptions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1457,21 +1571,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new CSharpMethodOptions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(CSharpMethodOptions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      CSharpMethodOptions result;

       

-      CSharpMethodOptions result = new CSharpMethodOptions();

+      private CSharpMethodOptions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          CSharpMethodOptions original = result;

+          result = new CSharpMethodOptions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override CSharpMethodOptions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new CSharpMethodOptions();

+        result = DefaultInstance ?? new CSharpMethodOptions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1483,12 +1624,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override CSharpMethodOptions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        CSharpMethodOptions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -1502,6 +1642,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(CSharpMethodOptions other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.CSharpMethodOptions.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasDispatchId) {

           DispatchId = other.DispatchId;

         }

@@ -1514,6 +1655,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -1569,11 +1711,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetDispatchId(value); }

       }

       public Builder SetDispatchId(int value) {

+        PrepareBuilder();

         result.hasDispatchId = true;

         result.dispatchId_ = value;

         return this;

       }

       public Builder ClearDispatchId() {

+        PrepareBuilder();

         result.hasDispatchId = false;

         result.dispatchId_ = 0;

         return this;

diff --git a/src/ProtocolBuffers/DescriptorProtos/DescriptorProtoFile.cs b/src/ProtocolBuffers/DescriptorProtos/DescriptorProtoFile.cs
index 5b48cde1..631ec21f 100644
--- a/src/ProtocolBuffers/DescriptorProtos/DescriptorProtoFile.cs
+++ b/src/ProtocolBuffers/DescriptorProtos/DescriptorProtoFile.cs
@@ -336,7 +336,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(FileDescriptorSet prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -346,21 +346,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new FileDescriptorSet();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(FileDescriptorSet cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      FileDescriptorSet result;

       

-      FileDescriptorSet result = new FileDescriptorSet();

+      private FileDescriptorSet PrepareBuilder() {

+        if (builderIsReadOnly) {

+          FileDescriptorSet original = result;

+          result = new FileDescriptorSet();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override FileDescriptorSet MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new FileDescriptorSet();

+        result = DefaultInstance ?? new FileDescriptorSet();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -372,13 +399,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override FileDescriptorSet BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.file_.MakeReadOnly();

-        FileDescriptorSet returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -392,6 +418,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(FileDescriptorSet other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.FileDescriptorSet.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.file_.Count != 0) {

           base.AddRange(other.file_, result.file_);

         }

@@ -404,6 +431,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -452,7 +480,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.FileDescriptorProto> FileList {

-        get { return result.file_; }

+        get { return PrepareBuilder().file_; }

       }

       public int FileCount {

         get { return result.FileCount; }

@@ -462,29 +490,35 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetFile(int index, global::Google.ProtocolBuffers.DescriptorProtos.FileDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.file_[index] = value;

         return this;

       }

       public Builder SetFile(int index, global::Google.ProtocolBuffers.DescriptorProtos.FileDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.file_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddFile(global::Google.ProtocolBuffers.DescriptorProtos.FileDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.file_.Add(value);

         return this;

       }

       public Builder AddFile(global::Google.ProtocolBuffers.DescriptorProtos.FileDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.file_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeFile(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.FileDescriptorProto> values) {

+        PrepareBuilder();

         base.AddRange(values, result.file_);

         return this;

       }

       public Builder ClearFile() {

+        PrepareBuilder();

         result.file_.Clear();

         return this;

       }

@@ -738,7 +772,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(FileDescriptorProto prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -748,21 +782,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new FileDescriptorProto();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(FileDescriptorProto cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      FileDescriptorProto result = new FileDescriptorProto();

+      bool builderIsReadOnly;

+      FileDescriptorProto result;

+      

+      private FileDescriptorProto PrepareBuilder() {

+        if (builderIsReadOnly) {

+          FileDescriptorProto original = result;

+          result = new FileDescriptorProto();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override FileDescriptorProto MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new FileDescriptorProto();

+        result = DefaultInstance ?? new FileDescriptorProto();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -774,17 +835,16 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override FileDescriptorProto BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.dependency_.MakeReadOnly();

         result.messageType_.MakeReadOnly();

         result.enumType_.MakeReadOnly();

         result.service_.MakeReadOnly();

         result.extension_.MakeReadOnly();

-        FileDescriptorProto returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -798,6 +858,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(FileDescriptorProto other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.FileDescriptorProto.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasName) {

           Name = other.Name;

         }

@@ -831,6 +892,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -920,11 +982,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetName(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasName = true;

         result.name_ = value;

         return this;

       }

       public Builder ClearName() {

+        PrepareBuilder();

         result.hasName = false;

         result.name_ = "";

         return this;

@@ -939,18 +1003,20 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetPackage(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasPackage = true;

         result.package_ = value;

         return this;

       }

       public Builder ClearPackage() {

+        PrepareBuilder();

         result.hasPackage = false;

         result.package_ = "";

         return this;

       }

       

       public pbc::IPopsicleList<string> DependencyList {

-        get { return result.dependency_; }

+        get { return PrepareBuilder().dependency_; }

       }

       public int DependencyCount {

         get { return result.DependencyCount; }

@@ -960,25 +1026,29 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetDependency(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.dependency_[index] = value;

         return this;

       }

       public Builder AddDependency(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.dependency_.Add(value);

         return this;

       }

       public Builder AddRangeDependency(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.dependency_);

         return this;

       }

       public Builder ClearDependency() {

+        PrepareBuilder();

         result.dependency_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto> MessageTypeList {

-        get { return result.messageType_; }

+        get { return PrepareBuilder().messageType_; }

       }

       public int MessageTypeCount {

         get { return result.MessageTypeCount; }

@@ -988,35 +1058,41 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetMessageType(int index, global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.messageType_[index] = value;

         return this;

       }

       public Builder SetMessageType(int index, global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.messageType_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddMessageType(global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.messageType_.Add(value);

         return this;

       }

       public Builder AddMessageType(global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.messageType_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeMessageType(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto> values) {

+        PrepareBuilder();

         base.AddRange(values, result.messageType_);

         return this;

       }

       public Builder ClearMessageType() {

+        PrepareBuilder();

         result.messageType_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.EnumDescriptorProto> EnumTypeList {

-        get { return result.enumType_; }

+        get { return PrepareBuilder().enumType_; }

       }

       public int EnumTypeCount {

         get { return result.EnumTypeCount; }

@@ -1026,35 +1102,41 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetEnumType(int index, global::Google.ProtocolBuffers.DescriptorProtos.EnumDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.enumType_[index] = value;

         return this;

       }

       public Builder SetEnumType(int index, global::Google.ProtocolBuffers.DescriptorProtos.EnumDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.enumType_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddEnumType(global::Google.ProtocolBuffers.DescriptorProtos.EnumDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.enumType_.Add(value);

         return this;

       }

       public Builder AddEnumType(global::Google.ProtocolBuffers.DescriptorProtos.EnumDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.enumType_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeEnumType(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.EnumDescriptorProto> values) {

+        PrepareBuilder();

         base.AddRange(values, result.enumType_);

         return this;

       }

       public Builder ClearEnumType() {

+        PrepareBuilder();

         result.enumType_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.ServiceDescriptorProto> ServiceList {

-        get { return result.service_; }

+        get { return PrepareBuilder().service_; }

       }

       public int ServiceCount {

         get { return result.ServiceCount; }

@@ -1064,35 +1146,41 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetService(int index, global::Google.ProtocolBuffers.DescriptorProtos.ServiceDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.service_[index] = value;

         return this;

       }

       public Builder SetService(int index, global::Google.ProtocolBuffers.DescriptorProtos.ServiceDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.service_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddService(global::Google.ProtocolBuffers.DescriptorProtos.ServiceDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.service_.Add(value);

         return this;

       }

       public Builder AddService(global::Google.ProtocolBuffers.DescriptorProtos.ServiceDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.service_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeService(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.ServiceDescriptorProto> values) {

+        PrepareBuilder();

         base.AddRange(values, result.service_);

         return this;

       }

       public Builder ClearService() {

+        PrepareBuilder();

         result.service_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto> ExtensionList {

-        get { return result.extension_; }

+        get { return PrepareBuilder().extension_; }

       }

       public int ExtensionCount {

         get { return result.ExtensionCount; }

@@ -1102,29 +1190,35 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetExtension(int index, global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.extension_[index] = value;

         return this;

       }

       public Builder SetExtension(int index, global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.extension_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddExtension(global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.extension_.Add(value);

         return this;

       }

       public Builder AddExtension(global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.extension_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeExtension(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto> values) {

+        PrepareBuilder();

         base.AddRange(values, result.extension_);

         return this;

       }

       public Builder ClearExtension() {

+        PrepareBuilder();

         result.extension_.Clear();

         return this;

       }

@@ -1138,18 +1232,21 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetOptions(global::Google.ProtocolBuffers.DescriptorProtos.FileOptions value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptions = true;

         result.options_ = value;

         return this;

       }

       public Builder SetOptions(global::Google.ProtocolBuffers.DescriptorProtos.FileOptions.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptions = true;

         result.options_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptions(global::Google.ProtocolBuffers.DescriptorProtos.FileOptions value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptions &&

             result.options_ != global::Google.ProtocolBuffers.DescriptorProtos.FileOptions.DefaultInstance) {

             result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.FileOptions.CreateBuilder(result.options_).MergeFrom(value).BuildPartial();

@@ -1160,6 +1257,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         return this;

       }

       public Builder ClearOptions() {

+        PrepareBuilder();

         result.hasOptions = false;

         result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.FileOptions.DefaultInstance;

         return this;

@@ -1320,7 +1418,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(ExtensionRange prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1330,21 +1428,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new ExtensionRange();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(ExtensionRange cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

           

-          ExtensionRange result = new ExtensionRange();

+          bool builderIsReadOnly;

+          ExtensionRange result;

+          

+          private ExtensionRange PrepareBuilder() {

+            if (builderIsReadOnly) {

+              ExtensionRange original = result;

+              result = new ExtensionRange();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override ExtensionRange MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new ExtensionRange();

+            result = DefaultInstance ?? new ExtensionRange();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -1356,12 +1481,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
           }

           

           public override ExtensionRange BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            ExtensionRange returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -1375,6 +1499,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
           

           public override Builder MergeFrom(ExtensionRange other) {

             if (other == global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto.Types.ExtensionRange.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasStart) {

               Start = other.Start;

             }

@@ -1390,6 +1515,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -1449,11 +1575,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
             set { SetStart(value); }

           }

           public Builder SetStart(int value) {

+            PrepareBuilder();

             result.hasStart = true;

             result.start_ = value;

             return this;

           }

           public Builder ClearStart() {

+            PrepareBuilder();

             result.hasStart = false;

             result.start_ = 0;

             return this;

@@ -1467,11 +1595,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
             set { SetEnd(value); }

           }

           public Builder SetEnd(int value) {

+            PrepareBuilder();

             result.hasEnd = true;

             result.end_ = value;

             return this;

           }

           public Builder ClearEnd() {

+            PrepareBuilder();

             result.hasEnd = false;

             result.end_ = 0;

             return this;

@@ -1681,7 +1811,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(DescriptorProto prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1691,21 +1821,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new DescriptorProto();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(DescriptorProto cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      DescriptorProto result;

+      

+      private DescriptorProto PrepareBuilder() {

+        if (builderIsReadOnly) {

+          DescriptorProto original = result;

+          result = new DescriptorProto();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      DescriptorProto result = new DescriptorProto();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override DescriptorProto MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new DescriptorProto();

+        result = DefaultInstance ?? new DescriptorProto();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1717,17 +1874,16 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override DescriptorProto BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.field_.MakeReadOnly();

         result.extension_.MakeReadOnly();

         result.nestedType_.MakeReadOnly();

         result.enumType_.MakeReadOnly();

         result.extensionRange_.MakeReadOnly();

-        DescriptorProto returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -1741,6 +1897,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(DescriptorProto other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasName) {

           Name = other.Name;

         }

@@ -1771,6 +1928,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -1856,18 +2014,20 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetName(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasName = true;

         result.name_ = value;

         return this;

       }

       public Builder ClearName() {

+        PrepareBuilder();

         result.hasName = false;

         result.name_ = "";

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto> FieldList {

-        get { return result.field_; }

+        get { return PrepareBuilder().field_; }

       }

       public int FieldCount {

         get { return result.FieldCount; }

@@ -1877,35 +2037,41 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetField(int index, global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.field_[index] = value;

         return this;

       }

       public Builder SetField(int index, global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.field_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddField(global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.field_.Add(value);

         return this;

       }

       public Builder AddField(global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.field_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeField(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto> values) {

+        PrepareBuilder();

         base.AddRange(values, result.field_);

         return this;

       }

       public Builder ClearField() {

+        PrepareBuilder();

         result.field_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto> ExtensionList {

-        get { return result.extension_; }

+        get { return PrepareBuilder().extension_; }

       }

       public int ExtensionCount {

         get { return result.ExtensionCount; }

@@ -1915,35 +2081,41 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetExtension(int index, global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.extension_[index] = value;

         return this;

       }

       public Builder SetExtension(int index, global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.extension_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddExtension(global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.extension_.Add(value);

         return this;

       }

       public Builder AddExtension(global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.extension_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeExtension(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto> values) {

+        PrepareBuilder();

         base.AddRange(values, result.extension_);

         return this;

       }

       public Builder ClearExtension() {

+        PrepareBuilder();

         result.extension_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto> NestedTypeList {

-        get { return result.nestedType_; }

+        get { return PrepareBuilder().nestedType_; }

       }

       public int NestedTypeCount {

         get { return result.NestedTypeCount; }

@@ -1953,35 +2125,41 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetNestedType(int index, global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.nestedType_[index] = value;

         return this;

       }

       public Builder SetNestedType(int index, global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.nestedType_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddNestedType(global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.nestedType_.Add(value);

         return this;

       }

       public Builder AddNestedType(global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.nestedType_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeNestedType(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto> values) {

+        PrepareBuilder();

         base.AddRange(values, result.nestedType_);

         return this;

       }

       public Builder ClearNestedType() {

+        PrepareBuilder();

         result.nestedType_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.EnumDescriptorProto> EnumTypeList {

-        get { return result.enumType_; }

+        get { return PrepareBuilder().enumType_; }

       }

       public int EnumTypeCount {

         get { return result.EnumTypeCount; }

@@ -1991,35 +2169,41 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetEnumType(int index, global::Google.ProtocolBuffers.DescriptorProtos.EnumDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.enumType_[index] = value;

         return this;

       }

       public Builder SetEnumType(int index, global::Google.ProtocolBuffers.DescriptorProtos.EnumDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.enumType_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddEnumType(global::Google.ProtocolBuffers.DescriptorProtos.EnumDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.enumType_.Add(value);

         return this;

       }

       public Builder AddEnumType(global::Google.ProtocolBuffers.DescriptorProtos.EnumDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.enumType_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeEnumType(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.EnumDescriptorProto> values) {

+        PrepareBuilder();

         base.AddRange(values, result.enumType_);

         return this;

       }

       public Builder ClearEnumType() {

+        PrepareBuilder();

         result.enumType_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto.Types.ExtensionRange> ExtensionRangeList {

-        get { return result.extensionRange_; }

+        get { return PrepareBuilder().extensionRange_; }

       }

       public int ExtensionRangeCount {

         get { return result.ExtensionRangeCount; }

@@ -2029,29 +2213,35 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetExtensionRange(int index, global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto.Types.ExtensionRange value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.extensionRange_[index] = value;

         return this;

       }

       public Builder SetExtensionRange(int index, global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto.Types.ExtensionRange.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.extensionRange_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddExtensionRange(global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto.Types.ExtensionRange value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.extensionRange_.Add(value);

         return this;

       }

       public Builder AddExtensionRange(global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto.Types.ExtensionRange.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.extensionRange_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeExtensionRange(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.DescriptorProto.Types.ExtensionRange> values) {

+        PrepareBuilder();

         base.AddRange(values, result.extensionRange_);

         return this;

       }

       public Builder ClearExtensionRange() {

+        PrepareBuilder();

         result.extensionRange_.Clear();

         return this;

       }

@@ -2065,18 +2255,21 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetOptions(global::Google.ProtocolBuffers.DescriptorProtos.MessageOptions value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptions = true;

         result.options_ = value;

         return this;

       }

       public Builder SetOptions(global::Google.ProtocolBuffers.DescriptorProtos.MessageOptions.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptions = true;

         result.options_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptions(global::Google.ProtocolBuffers.DescriptorProtos.MessageOptions value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptions &&

             result.options_ != global::Google.ProtocolBuffers.DescriptorProtos.MessageOptions.DefaultInstance) {

             result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.MessageOptions.CreateBuilder(result.options_).MergeFrom(value).BuildPartial();

@@ -2087,6 +2280,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         return this;

       }

       public Builder ClearOptions() {

+        PrepareBuilder();

         result.hasOptions = false;

         result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.MessageOptions.DefaultInstance;

         return this;

@@ -2353,7 +2547,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(FieldDescriptorProto prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -2363,21 +2557,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new FieldDescriptorProto();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(FieldDescriptorProto cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      FieldDescriptorProto result;

       

-      FieldDescriptorProto result = new FieldDescriptorProto();

+      private FieldDescriptorProto PrepareBuilder() {

+        if (builderIsReadOnly) {

+          FieldDescriptorProto original = result;

+          result = new FieldDescriptorProto();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override FieldDescriptorProto MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new FieldDescriptorProto();

+        result = DefaultInstance ?? new FieldDescriptorProto();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -2389,12 +2610,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override FieldDescriptorProto BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        FieldDescriptorProto returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -2408,6 +2628,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(FieldDescriptorProto other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasName) {

           Name = other.Name;

         }

@@ -2441,6 +2662,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -2546,11 +2768,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetName(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasName = true;

         result.name_ = value;

         return this;

       }

       public Builder ClearName() {

+        PrepareBuilder();

         result.hasName = false;

         result.name_ = "";

         return this;

@@ -2564,11 +2788,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetNumber(value); }

       }

       public Builder SetNumber(int value) {

+        PrepareBuilder();

         result.hasNumber = true;

         result.number_ = value;

         return this;

       }

       public Builder ClearNumber() {

+        PrepareBuilder();

         result.hasNumber = false;

         result.number_ = 0;

         return this;

@@ -2582,11 +2808,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetLabel(value); }

       }

       public Builder SetLabel(global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto.Types.Label value) {

+        PrepareBuilder();

         result.hasLabel = true;

         result.label_ = value;

         return this;

       }

       public Builder ClearLabel() {

+        PrepareBuilder();

         result.hasLabel = false;

         result.label_ = global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto.Types.Label.LABEL_OPTIONAL;

         return this;

@@ -2600,11 +2828,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetType(value); }

       }

       public Builder SetType(global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto.Types.Type value) {

+        PrepareBuilder();

         result.hasType = true;

         result.type_ = value;

         return this;

       }

       public Builder ClearType() {

+        PrepareBuilder();

         result.hasType = false;

         result.type_ = global::Google.ProtocolBuffers.DescriptorProtos.FieldDescriptorProto.Types.Type.TYPE_DOUBLE;

         return this;

@@ -2619,11 +2849,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetTypeName(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasTypeName = true;

         result.typeName_ = value;

         return this;

       }

       public Builder ClearTypeName() {

+        PrepareBuilder();

         result.hasTypeName = false;

         result.typeName_ = "";

         return this;

@@ -2638,11 +2870,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetExtendee(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasExtendee = true;

         result.extendee_ = value;

         return this;

       }

       public Builder ClearExtendee() {

+        PrepareBuilder();

         result.hasExtendee = false;

         result.extendee_ = "";

         return this;

@@ -2657,11 +2891,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetDefaultValue(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasDefaultValue = true;

         result.defaultValue_ = value;

         return this;

       }

       public Builder ClearDefaultValue() {

+        PrepareBuilder();

         result.hasDefaultValue = false;

         result.defaultValue_ = "";

         return this;

@@ -2676,18 +2912,21 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetOptions(global::Google.ProtocolBuffers.DescriptorProtos.FieldOptions value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptions = true;

         result.options_ = value;

         return this;

       }

       public Builder SetOptions(global::Google.ProtocolBuffers.DescriptorProtos.FieldOptions.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptions = true;

         result.options_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptions(global::Google.ProtocolBuffers.DescriptorProtos.FieldOptions value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptions &&

             result.options_ != global::Google.ProtocolBuffers.DescriptorProtos.FieldOptions.DefaultInstance) {

             result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.FieldOptions.CreateBuilder(result.options_).MergeFrom(value).BuildPartial();

@@ -2698,6 +2937,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         return this;

       }

       public Builder ClearOptions() {

+        PrepareBuilder();

         result.hasOptions = false;

         result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.FieldOptions.DefaultInstance;

         return this;

@@ -2850,7 +3090,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(EnumDescriptorProto prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -2860,21 +3100,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new EnumDescriptorProto();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(EnumDescriptorProto cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      EnumDescriptorProto result;

+      

+      private EnumDescriptorProto PrepareBuilder() {

+        if (builderIsReadOnly) {

+          EnumDescriptorProto original = result;

+          result = new EnumDescriptorProto();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      EnumDescriptorProto result = new EnumDescriptorProto();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override EnumDescriptorProto MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new EnumDescriptorProto();

+        result = DefaultInstance ?? new EnumDescriptorProto();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -2886,13 +3153,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override EnumDescriptorProto BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.value_.MakeReadOnly();

-        EnumDescriptorProto returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -2906,6 +3172,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(EnumDescriptorProto other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.EnumDescriptorProto.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasName) {

           Name = other.Name;

         }

@@ -2924,6 +3191,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -2993,18 +3261,20 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetName(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasName = true;

         result.name_ = value;

         return this;

       }

       public Builder ClearName() {

+        PrepareBuilder();

         result.hasName = false;

         result.name_ = "";

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.EnumValueDescriptorProto> ValueList {

-        get { return result.value_; }

+        get { return PrepareBuilder().value_; }

       }

       public int ValueCount {

         get { return result.ValueCount; }

@@ -3014,29 +3284,35 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetValue(int index, global::Google.ProtocolBuffers.DescriptorProtos.EnumValueDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.value_[index] = value;

         return this;

       }

       public Builder SetValue(int index, global::Google.ProtocolBuffers.DescriptorProtos.EnumValueDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.value_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddValue(global::Google.ProtocolBuffers.DescriptorProtos.EnumValueDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.value_.Add(value);

         return this;

       }

       public Builder AddValue(global::Google.ProtocolBuffers.DescriptorProtos.EnumValueDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.value_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeValue(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.EnumValueDescriptorProto> values) {

+        PrepareBuilder();

         base.AddRange(values, result.value_);

         return this;

       }

       public Builder ClearValue() {

+        PrepareBuilder();

         result.value_.Clear();

         return this;

       }

@@ -3050,18 +3326,21 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetOptions(global::Google.ProtocolBuffers.DescriptorProtos.EnumOptions value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptions = true;

         result.options_ = value;

         return this;

       }

       public Builder SetOptions(global::Google.ProtocolBuffers.DescriptorProtos.EnumOptions.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptions = true;

         result.options_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptions(global::Google.ProtocolBuffers.DescriptorProtos.EnumOptions value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptions &&

             result.options_ != global::Google.ProtocolBuffers.DescriptorProtos.EnumOptions.DefaultInstance) {

             result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.EnumOptions.CreateBuilder(result.options_).MergeFrom(value).BuildPartial();

@@ -3072,6 +3351,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         return this;

       }

       public Builder ClearOptions() {

+        PrepareBuilder();

         result.hasOptions = false;

         result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.EnumOptions.DefaultInstance;

         return this;

@@ -3219,7 +3499,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(EnumValueDescriptorProto prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -3229,21 +3509,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new EnumValueDescriptorProto();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(EnumValueDescriptorProto cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      EnumValueDescriptorProto result = new EnumValueDescriptorProto();

+      bool builderIsReadOnly;

+      EnumValueDescriptorProto result;

+      

+      private EnumValueDescriptorProto PrepareBuilder() {

+        if (builderIsReadOnly) {

+          EnumValueDescriptorProto original = result;

+          result = new EnumValueDescriptorProto();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override EnumValueDescriptorProto MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new EnumValueDescriptorProto();

+        result = DefaultInstance ?? new EnumValueDescriptorProto();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -3255,12 +3562,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override EnumValueDescriptorProto BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        EnumValueDescriptorProto returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -3274,6 +3580,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(EnumValueDescriptorProto other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.EnumValueDescriptorProto.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasName) {

           Name = other.Name;

         }

@@ -3292,6 +3599,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -3361,11 +3669,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetName(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasName = true;

         result.name_ = value;

         return this;

       }

       public Builder ClearName() {

+        PrepareBuilder();

         result.hasName = false;

         result.name_ = "";

         return this;

@@ -3379,11 +3689,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetNumber(value); }

       }

       public Builder SetNumber(int value) {

+        PrepareBuilder();

         result.hasNumber = true;

         result.number_ = value;

         return this;

       }

       public Builder ClearNumber() {

+        PrepareBuilder();

         result.hasNumber = false;

         result.number_ = 0;

         return this;

@@ -3398,18 +3710,21 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetOptions(global::Google.ProtocolBuffers.DescriptorProtos.EnumValueOptions value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptions = true;

         result.options_ = value;

         return this;

       }

       public Builder SetOptions(global::Google.ProtocolBuffers.DescriptorProtos.EnumValueOptions.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptions = true;

         result.options_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptions(global::Google.ProtocolBuffers.DescriptorProtos.EnumValueOptions value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptions &&

             result.options_ != global::Google.ProtocolBuffers.DescriptorProtos.EnumValueOptions.DefaultInstance) {

             result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.EnumValueOptions.CreateBuilder(result.options_).MergeFrom(value).BuildPartial();

@@ -3420,6 +3735,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         return this;

       }

       public Builder ClearOptions() {

+        PrepareBuilder();

         result.hasOptions = false;

         result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.EnumValueOptions.DefaultInstance;

         return this;

@@ -3572,7 +3888,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(ServiceDescriptorProto prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -3582,21 +3898,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new ServiceDescriptorProto();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(ServiceDescriptorProto cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      ServiceDescriptorProto result;

+      

+      private ServiceDescriptorProto PrepareBuilder() {

+        if (builderIsReadOnly) {

+          ServiceDescriptorProto original = result;

+          result = new ServiceDescriptorProto();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      ServiceDescriptorProto result = new ServiceDescriptorProto();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override ServiceDescriptorProto MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new ServiceDescriptorProto();

+        result = DefaultInstance ?? new ServiceDescriptorProto();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -3608,13 +3951,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override ServiceDescriptorProto BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.method_.MakeReadOnly();

-        ServiceDescriptorProto returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -3628,6 +3970,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(ServiceDescriptorProto other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.ServiceDescriptorProto.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasName) {

           Name = other.Name;

         }

@@ -3646,6 +3989,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -3715,18 +4059,20 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetName(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasName = true;

         result.name_ = value;

         return this;

       }

       public Builder ClearName() {

+        PrepareBuilder();

         result.hasName = false;

         result.name_ = "";

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.MethodDescriptorProto> MethodList {

-        get { return result.method_; }

+        get { return PrepareBuilder().method_; }

       }

       public int MethodCount {

         get { return result.MethodCount; }

@@ -3736,29 +4082,35 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetMethod(int index, global::Google.ProtocolBuffers.DescriptorProtos.MethodDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.method_[index] = value;

         return this;

       }

       public Builder SetMethod(int index, global::Google.ProtocolBuffers.DescriptorProtos.MethodDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.method_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddMethod(global::Google.ProtocolBuffers.DescriptorProtos.MethodDescriptorProto value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.method_.Add(value);

         return this;

       }

       public Builder AddMethod(global::Google.ProtocolBuffers.DescriptorProtos.MethodDescriptorProto.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.method_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeMethod(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.MethodDescriptorProto> values) {

+        PrepareBuilder();

         base.AddRange(values, result.method_);

         return this;

       }

       public Builder ClearMethod() {

+        PrepareBuilder();

         result.method_.Clear();

         return this;

       }

@@ -3772,18 +4124,21 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetOptions(global::Google.ProtocolBuffers.DescriptorProtos.ServiceOptions value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptions = true;

         result.options_ = value;

         return this;

       }

       public Builder SetOptions(global::Google.ProtocolBuffers.DescriptorProtos.ServiceOptions.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptions = true;

         result.options_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptions(global::Google.ProtocolBuffers.DescriptorProtos.ServiceOptions value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptions &&

             result.options_ != global::Google.ProtocolBuffers.DescriptorProtos.ServiceOptions.DefaultInstance) {

             result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.ServiceOptions.CreateBuilder(result.options_).MergeFrom(value).BuildPartial();

@@ -3794,6 +4149,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         return this;

       }

       public Builder ClearOptions() {

+        PrepareBuilder();

         result.hasOptions = false;

         result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.ServiceOptions.DefaultInstance;

         return this;

@@ -3957,7 +4313,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(MethodDescriptorProto prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -3967,21 +4323,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new MethodDescriptorProto();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(MethodDescriptorProto cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      MethodDescriptorProto result;

+      

+      private MethodDescriptorProto PrepareBuilder() {

+        if (builderIsReadOnly) {

+          MethodDescriptorProto original = result;

+          result = new MethodDescriptorProto();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      MethodDescriptorProto result = new MethodDescriptorProto();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override MethodDescriptorProto MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new MethodDescriptorProto();

+        result = DefaultInstance ?? new MethodDescriptorProto();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -3993,12 +4376,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override MethodDescriptorProto BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        MethodDescriptorProto returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -4012,6 +4394,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(MethodDescriptorProto other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.MethodDescriptorProto.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasName) {

           Name = other.Name;

         }

@@ -4033,6 +4416,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -4106,11 +4490,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetName(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasName = true;

         result.name_ = value;

         return this;

       }

       public Builder ClearName() {

+        PrepareBuilder();

         result.hasName = false;

         result.name_ = "";

         return this;

@@ -4125,11 +4511,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetInputType(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasInputType = true;

         result.inputType_ = value;

         return this;

       }

       public Builder ClearInputType() {

+        PrepareBuilder();

         result.hasInputType = false;

         result.inputType_ = "";

         return this;

@@ -4144,11 +4532,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetOutputType(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOutputType = true;

         result.outputType_ = value;

         return this;

       }

       public Builder ClearOutputType() {

+        PrepareBuilder();

         result.hasOutputType = false;

         result.outputType_ = "";

         return this;

@@ -4163,18 +4553,21 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetOptions(global::Google.ProtocolBuffers.DescriptorProtos.MethodOptions value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptions = true;

         result.options_ = value;

         return this;

       }

       public Builder SetOptions(global::Google.ProtocolBuffers.DescriptorProtos.MethodOptions.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptions = true;

         result.options_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptions(global::Google.ProtocolBuffers.DescriptorProtos.MethodOptions value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptions &&

             result.options_ != global::Google.ProtocolBuffers.DescriptorProtos.MethodOptions.DefaultInstance) {

             result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.MethodOptions.CreateBuilder(result.options_).MergeFrom(value).BuildPartial();

@@ -4185,6 +4578,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         return this;

       }

       public Builder ClearOptions() {

+        PrepareBuilder();

         result.hasOptions = false;

         result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.MethodOptions.DefaultInstance;

         return this;

@@ -4434,7 +4828,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(FileOptions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -4444,21 +4838,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new FileOptions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(FileOptions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      FileOptions result;

       

-      FileOptions result = new FileOptions();

+      private FileOptions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          FileOptions original = result;

+          result = new FileOptions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override FileOptions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new FileOptions();

+        result = DefaultInstance ?? new FileOptions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -4470,13 +4891,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override FileOptions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.uninterpretedOption_.MakeReadOnly();

-        FileOptions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -4490,6 +4910,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(FileOptions other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.FileOptions.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasJavaPackage) {

           JavaPackage = other.JavaPackage;

         }

@@ -4524,6 +4945,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -4616,11 +5038,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetJavaPackage(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasJavaPackage = true;

         result.javaPackage_ = value;

         return this;

       }

       public Builder ClearJavaPackage() {

+        PrepareBuilder();

         result.hasJavaPackage = false;

         result.javaPackage_ = "";

         return this;

@@ -4635,11 +5059,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetJavaOuterClassname(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasJavaOuterClassname = true;

         result.javaOuterClassname_ = value;

         return this;

       }

       public Builder ClearJavaOuterClassname() {

+        PrepareBuilder();

         result.hasJavaOuterClassname = false;

         result.javaOuterClassname_ = "";

         return this;

@@ -4653,11 +5079,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetJavaMultipleFiles(value); }

       }

       public Builder SetJavaMultipleFiles(bool value) {

+        PrepareBuilder();

         result.hasJavaMultipleFiles = true;

         result.javaMultipleFiles_ = value;

         return this;

       }

       public Builder ClearJavaMultipleFiles() {

+        PrepareBuilder();

         result.hasJavaMultipleFiles = false;

         result.javaMultipleFiles_ = false;

         return this;

@@ -4671,11 +5099,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetOptimizeFor(value); }

       }

       public Builder SetOptimizeFor(global::Google.ProtocolBuffers.DescriptorProtos.FileOptions.Types.OptimizeMode value) {

+        PrepareBuilder();

         result.hasOptimizeFor = true;

         result.optimizeFor_ = value;

         return this;

       }

       public Builder ClearOptimizeFor() {

+        PrepareBuilder();

         result.hasOptimizeFor = false;

         result.optimizeFor_ = global::Google.ProtocolBuffers.DescriptorProtos.FileOptions.Types.OptimizeMode.SPEED;

         return this;

@@ -4689,11 +5119,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetCcGenericServices(value); }

       }

       public Builder SetCcGenericServices(bool value) {

+        PrepareBuilder();

         result.hasCcGenericServices = true;

         result.ccGenericServices_ = value;

         return this;

       }

       public Builder ClearCcGenericServices() {

+        PrepareBuilder();

         result.hasCcGenericServices = false;

         result.ccGenericServices_ = true;

         return this;

@@ -4707,11 +5139,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetJavaGenericServices(value); }

       }

       public Builder SetJavaGenericServices(bool value) {

+        PrepareBuilder();

         result.hasJavaGenericServices = true;

         result.javaGenericServices_ = value;

         return this;

       }

       public Builder ClearJavaGenericServices() {

+        PrepareBuilder();

         result.hasJavaGenericServices = false;

         result.javaGenericServices_ = true;

         return this;

@@ -4725,18 +5159,20 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetPyGenericServices(value); }

       }

       public Builder SetPyGenericServices(bool value) {

+        PrepareBuilder();

         result.hasPyGenericServices = true;

         result.pyGenericServices_ = value;

         return this;

       }

       public Builder ClearPyGenericServices() {

+        PrepareBuilder();

         result.hasPyGenericServices = false;

         result.pyGenericServices_ = true;

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption> UninterpretedOptionList {

-        get { return result.uninterpretedOption_; }

+        get { return PrepareBuilder().uninterpretedOption_; }

       }

       public int UninterpretedOptionCount {

         get { return result.UninterpretedOptionCount; }

@@ -4746,29 +5182,35 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetUninterpretedOption(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.uninterpretedOption_[index] = value;

         return this;

       }

       public Builder SetUninterpretedOption(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.uninterpretedOption_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddUninterpretedOption(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.uninterpretedOption_.Add(value);

         return this;

       }

       public Builder AddUninterpretedOption(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.uninterpretedOption_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeUninterpretedOption(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption> values) {

+        PrepareBuilder();

         base.AddRange(values, result.uninterpretedOption_);

         return this;

       }

       public Builder ClearUninterpretedOption() {

+        PrepareBuilder();

         result.uninterpretedOption_.Clear();

         return this;

       }

@@ -4921,7 +5363,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(MessageOptions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -4931,21 +5373,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new MessageOptions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(MessageOptions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      MessageOptions result = new MessageOptions();

+      bool builderIsReadOnly;

+      MessageOptions result;

+      

+      private MessageOptions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          MessageOptions original = result;

+          result = new MessageOptions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override MessageOptions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new MessageOptions();

+        result = DefaultInstance ?? new MessageOptions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -4957,13 +5426,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override MessageOptions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.uninterpretedOption_.MakeReadOnly();

-        MessageOptions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -4977,6 +5445,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(MessageOptions other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.MessageOptions.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasMessageSetWireFormat) {

           MessageSetWireFormat = other.MessageSetWireFormat;

         }

@@ -4996,6 +5465,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -5059,11 +5529,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetMessageSetWireFormat(value); }

       }

       public Builder SetMessageSetWireFormat(bool value) {

+        PrepareBuilder();

         result.hasMessageSetWireFormat = true;

         result.messageSetWireFormat_ = value;

         return this;

       }

       public Builder ClearMessageSetWireFormat() {

+        PrepareBuilder();

         result.hasMessageSetWireFormat = false;

         result.messageSetWireFormat_ = false;

         return this;

@@ -5077,18 +5549,20 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetNoStandardDescriptorAccessor(value); }

       }

       public Builder SetNoStandardDescriptorAccessor(bool value) {

+        PrepareBuilder();

         result.hasNoStandardDescriptorAccessor = true;

         result.noStandardDescriptorAccessor_ = value;

         return this;

       }

       public Builder ClearNoStandardDescriptorAccessor() {

+        PrepareBuilder();

         result.hasNoStandardDescriptorAccessor = false;

         result.noStandardDescriptorAccessor_ = false;

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption> UninterpretedOptionList {

-        get { return result.uninterpretedOption_; }

+        get { return PrepareBuilder().uninterpretedOption_; }

       }

       public int UninterpretedOptionCount {

         get { return result.UninterpretedOptionCount; }

@@ -5098,29 +5572,35 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetUninterpretedOption(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.uninterpretedOption_[index] = value;

         return this;

       }

       public Builder SetUninterpretedOption(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.uninterpretedOption_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddUninterpretedOption(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.uninterpretedOption_.Add(value);

         return this;

       }

       public Builder AddUninterpretedOption(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.uninterpretedOption_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeUninterpretedOption(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption> values) {

+        PrepareBuilder();

         base.AddRange(values, result.uninterpretedOption_);

         return this;

       }

       public Builder ClearUninterpretedOption() {

+        PrepareBuilder();

         result.uninterpretedOption_.Clear();

         return this;

       }

@@ -5321,7 +5801,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(FieldOptions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -5331,21 +5811,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new FieldOptions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(FieldOptions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      FieldOptions result = new FieldOptions();

+      bool builderIsReadOnly;

+      FieldOptions result;

+      

+      private FieldOptions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          FieldOptions original = result;

+          result = new FieldOptions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override FieldOptions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new FieldOptions();

+        result = DefaultInstance ?? new FieldOptions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -5357,13 +5864,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override FieldOptions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.uninterpretedOption_.MakeReadOnly();

-        FieldOptions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -5377,6 +5883,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(FieldOptions other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.FieldOptions.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasCtype) {

           Ctype = other.Ctype;

         }

@@ -5402,6 +5909,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -5481,11 +5989,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetCtype(value); }

       }

       public Builder SetCtype(global::Google.ProtocolBuffers.DescriptorProtos.FieldOptions.Types.CType value) {

+        PrepareBuilder();

         result.hasCtype = true;

         result.ctype_ = value;

         return this;

       }

       public Builder ClearCtype() {

+        PrepareBuilder();

         result.hasCtype = false;

         result.ctype_ = global::Google.ProtocolBuffers.DescriptorProtos.FieldOptions.Types.CType.STRING;

         return this;

@@ -5499,11 +6009,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetPacked(value); }

       }

       public Builder SetPacked(bool value) {

+        PrepareBuilder();

         result.hasPacked = true;

         result.packed_ = value;

         return this;

       }

       public Builder ClearPacked() {

+        PrepareBuilder();

         result.hasPacked = false;

         result.packed_ = false;

         return this;

@@ -5517,11 +6029,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetDeprecated(value); }

       }

       public Builder SetDeprecated(bool value) {

+        PrepareBuilder();

         result.hasDeprecated = true;

         result.deprecated_ = value;

         return this;

       }

       public Builder ClearDeprecated() {

+        PrepareBuilder();

         result.hasDeprecated = false;

         result.deprecated_ = false;

         return this;

@@ -5536,18 +6050,20 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetExperimentalMapKey(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasExperimentalMapKey = true;

         result.experimentalMapKey_ = value;

         return this;

       }

       public Builder ClearExperimentalMapKey() {

+        PrepareBuilder();

         result.hasExperimentalMapKey = false;

         result.experimentalMapKey_ = "";

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption> UninterpretedOptionList {

-        get { return result.uninterpretedOption_; }

+        get { return PrepareBuilder().uninterpretedOption_; }

       }

       public int UninterpretedOptionCount {

         get { return result.UninterpretedOptionCount; }

@@ -5557,29 +6073,35 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetUninterpretedOption(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.uninterpretedOption_[index] = value;

         return this;

       }

       public Builder SetUninterpretedOption(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.uninterpretedOption_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddUninterpretedOption(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.uninterpretedOption_.Add(value);

         return this;

       }

       public Builder AddUninterpretedOption(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.uninterpretedOption_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeUninterpretedOption(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption> values) {

+        PrepareBuilder();

         base.AddRange(values, result.uninterpretedOption_);

         return this;

       }

       public Builder ClearUninterpretedOption() {

+        PrepareBuilder();

         result.uninterpretedOption_.Clear();

         return this;

       }

@@ -5700,7 +6222,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(EnumOptions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -5710,21 +6232,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new EnumOptions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(EnumOptions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      EnumOptions result;

+      

+      private EnumOptions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          EnumOptions original = result;

+          result = new EnumOptions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      EnumOptions result = new EnumOptions();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override EnumOptions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new EnumOptions();

+        result = DefaultInstance ?? new EnumOptions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -5736,13 +6285,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override EnumOptions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.uninterpretedOption_.MakeReadOnly();

-        EnumOptions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -5756,6 +6304,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(EnumOptions other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.EnumOptions.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.uninterpretedOption_.Count != 0) {

           base.AddRange(other.uninterpretedOption_, result.uninterpretedOption_);

         }

@@ -5769,6 +6318,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -5817,7 +6367,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption> UninterpretedOptionList {

-        get { return result.uninterpretedOption_; }

+        get { return PrepareBuilder().uninterpretedOption_; }

       }

       public int UninterpretedOptionCount {

         get { return result.UninterpretedOptionCount; }

@@ -5827,29 +6377,35 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetUninterpretedOption(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.uninterpretedOption_[index] = value;

         return this;

       }

       public Builder SetUninterpretedOption(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.uninterpretedOption_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddUninterpretedOption(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.uninterpretedOption_.Add(value);

         return this;

       }

       public Builder AddUninterpretedOption(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.uninterpretedOption_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeUninterpretedOption(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption> values) {

+        PrepareBuilder();

         base.AddRange(values, result.uninterpretedOption_);

         return this;

       }

       public Builder ClearUninterpretedOption() {

+        PrepareBuilder();

         result.uninterpretedOption_.Clear();

         return this;

       }

@@ -5970,7 +6526,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(EnumValueOptions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -5980,21 +6536,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new EnumValueOptions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(EnumValueOptions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      EnumValueOptions result;

       

-      EnumValueOptions result = new EnumValueOptions();

+      private EnumValueOptions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          EnumValueOptions original = result;

+          result = new EnumValueOptions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override EnumValueOptions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new EnumValueOptions();

+        result = DefaultInstance ?? new EnumValueOptions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -6006,13 +6589,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override EnumValueOptions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.uninterpretedOption_.MakeReadOnly();

-        EnumValueOptions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -6026,6 +6608,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(EnumValueOptions other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.EnumValueOptions.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.uninterpretedOption_.Count != 0) {

           base.AddRange(other.uninterpretedOption_, result.uninterpretedOption_);

         }

@@ -6039,6 +6622,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -6087,7 +6671,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption> UninterpretedOptionList {

-        get { return result.uninterpretedOption_; }

+        get { return PrepareBuilder().uninterpretedOption_; }

       }

       public int UninterpretedOptionCount {

         get { return result.UninterpretedOptionCount; }

@@ -6097,29 +6681,35 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetUninterpretedOption(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.uninterpretedOption_[index] = value;

         return this;

       }

       public Builder SetUninterpretedOption(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.uninterpretedOption_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddUninterpretedOption(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.uninterpretedOption_.Add(value);

         return this;

       }

       public Builder AddUninterpretedOption(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.uninterpretedOption_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeUninterpretedOption(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption> values) {

+        PrepareBuilder();

         base.AddRange(values, result.uninterpretedOption_);

         return this;

       }

       public Builder ClearUninterpretedOption() {

+        PrepareBuilder();

         result.uninterpretedOption_.Clear();

         return this;

       }

@@ -6240,7 +6830,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(ServiceOptions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -6250,21 +6840,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new ServiceOptions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(ServiceOptions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      ServiceOptions result;

+      

+      private ServiceOptions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          ServiceOptions original = result;

+          result = new ServiceOptions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      ServiceOptions result = new ServiceOptions();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override ServiceOptions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new ServiceOptions();

+        result = DefaultInstance ?? new ServiceOptions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -6276,13 +6893,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override ServiceOptions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.uninterpretedOption_.MakeReadOnly();

-        ServiceOptions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -6296,6 +6912,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(ServiceOptions other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.ServiceOptions.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.uninterpretedOption_.Count != 0) {

           base.AddRange(other.uninterpretedOption_, result.uninterpretedOption_);

         }

@@ -6309,6 +6926,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -6357,7 +6975,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption> UninterpretedOptionList {

-        get { return result.uninterpretedOption_; }

+        get { return PrepareBuilder().uninterpretedOption_; }

       }

       public int UninterpretedOptionCount {

         get { return result.UninterpretedOptionCount; }

@@ -6367,29 +6985,35 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetUninterpretedOption(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.uninterpretedOption_[index] = value;

         return this;

       }

       public Builder SetUninterpretedOption(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.uninterpretedOption_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddUninterpretedOption(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.uninterpretedOption_.Add(value);

         return this;

       }

       public Builder AddUninterpretedOption(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.uninterpretedOption_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeUninterpretedOption(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption> values) {

+        PrepareBuilder();

         base.AddRange(values, result.uninterpretedOption_);

         return this;

       }

       public Builder ClearUninterpretedOption() {

+        PrepareBuilder();

         result.uninterpretedOption_.Clear();

         return this;

       }

@@ -6510,7 +7134,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(MethodOptions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -6520,21 +7144,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new MethodOptions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(MethodOptions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      MethodOptions result = new MethodOptions();

+      bool builderIsReadOnly;

+      MethodOptions result;

+      

+      private MethodOptions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          MethodOptions original = result;

+          result = new MethodOptions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override MethodOptions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new MethodOptions();

+        result = DefaultInstance ?? new MethodOptions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -6546,13 +7197,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override MethodOptions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.uninterpretedOption_.MakeReadOnly();

-        MethodOptions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -6566,6 +7216,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(MethodOptions other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.MethodOptions.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.uninterpretedOption_.Count != 0) {

           base.AddRange(other.uninterpretedOption_, result.uninterpretedOption_);

         }

@@ -6579,6 +7230,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -6627,7 +7279,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption> UninterpretedOptionList {

-        get { return result.uninterpretedOption_; }

+        get { return PrepareBuilder().uninterpretedOption_; }

       }

       public int UninterpretedOptionCount {

         get { return result.UninterpretedOptionCount; }

@@ -6637,29 +7289,35 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetUninterpretedOption(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.uninterpretedOption_[index] = value;

         return this;

       }

       public Builder SetUninterpretedOption(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.uninterpretedOption_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddUninterpretedOption(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.uninterpretedOption_.Add(value);

         return this;

       }

       public Builder AddUninterpretedOption(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.uninterpretedOption_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeUninterpretedOption(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption> values) {

+        PrepareBuilder();

         base.AddRange(values, result.uninterpretedOption_);

         return this;

       }

       public Builder ClearUninterpretedOption() {

+        PrepareBuilder();

         result.uninterpretedOption_.Clear();

         return this;

       }

@@ -6821,7 +7479,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(NamePart prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -6831,21 +7489,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new NamePart();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(NamePart cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          NamePart result;

+          

+          private NamePart PrepareBuilder() {

+            if (builderIsReadOnly) {

+              NamePart original = result;

+              result = new NamePart();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

           

-          NamePart result = new NamePart();

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override NamePart MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new NamePart();

+            result = DefaultInstance ?? new NamePart();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -6857,12 +7542,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
           }

           

           public override NamePart BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            NamePart returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -6876,6 +7560,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
           

           public override Builder MergeFrom(NamePart other) {

             if (other == global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Types.NamePart.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasNamePart_) {

               NamePart_ = other.NamePart_;

             }

@@ -6891,6 +7576,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -6951,11 +7637,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
           }

           public Builder SetNamePart_(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasNamePart_ = true;

             result.namePart_ = value;

             return this;

           }

           public Builder ClearNamePart_() {

+            PrepareBuilder();

             result.hasNamePart_ = false;

             result.namePart_ = "";

             return this;

@@ -6969,11 +7657,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
             set { SetIsExtension(value); }

           }

           public Builder SetIsExtension(bool value) {

+            PrepareBuilder();

             result.hasIsExtension = true;

             result.isExtension_ = value;

             return this;

           }

           public Builder ClearIsExtension() {

+            PrepareBuilder();

             result.hasIsExtension = false;

             result.isExtension_ = false;

             return this;

@@ -7148,7 +7838,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(UninterpretedOption prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -7158,21 +7848,48 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new UninterpretedOption();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(UninterpretedOption cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      UninterpretedOption result;

+      

+      private UninterpretedOption PrepareBuilder() {

+        if (builderIsReadOnly) {

+          UninterpretedOption original = result;

+          result = new UninterpretedOption();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      UninterpretedOption result = new UninterpretedOption();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override UninterpretedOption MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new UninterpretedOption();

+        result = DefaultInstance ?? new UninterpretedOption();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -7184,13 +7901,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override UninterpretedOption BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.name_.MakeReadOnly();

-        UninterpretedOption returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -7204,6 +7920,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       public override Builder MergeFrom(UninterpretedOption other) {

         if (other == global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.name_.Count != 0) {

           base.AddRange(other.name_, result.name_);

         }

@@ -7231,6 +7948,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -7299,7 +8017,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Types.NamePart> NameList {

-        get { return result.name_; }

+        get { return PrepareBuilder().name_; }

       }

       public int NameCount {

         get { return result.NameCount; }

@@ -7309,29 +8027,35 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetName(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Types.NamePart value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.name_[index] = value;

         return this;

       }

       public Builder SetName(int index, global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Types.NamePart.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.name_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddName(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Types.NamePart value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.name_.Add(value);

         return this;

       }

       public Builder AddName(global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Types.NamePart.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.name_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeName(scg::IEnumerable<global::Google.ProtocolBuffers.DescriptorProtos.UninterpretedOption.Types.NamePart> values) {

+        PrepareBuilder();

         base.AddRange(values, result.name_);

         return this;

       }

       public Builder ClearName() {

+        PrepareBuilder();

         result.name_.Clear();

         return this;

       }

@@ -7345,11 +8069,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetIdentifierValue(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasIdentifierValue = true;

         result.identifierValue_ = value;

         return this;

       }

       public Builder ClearIdentifierValue() {

+        PrepareBuilder();

         result.hasIdentifierValue = false;

         result.identifierValue_ = "";

         return this;

@@ -7365,11 +8091,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetPositiveIntValue(ulong value) {

+        PrepareBuilder();

         result.hasPositiveIntValue = true;

         result.positiveIntValue_ = value;

         return this;

       }

       public Builder ClearPositiveIntValue() {

+        PrepareBuilder();

         result.hasPositiveIntValue = false;

         result.positiveIntValue_ = 0UL;

         return this;

@@ -7383,11 +8111,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetNegativeIntValue(value); }

       }

       public Builder SetNegativeIntValue(long value) {

+        PrepareBuilder();

         result.hasNegativeIntValue = true;

         result.negativeIntValue_ = value;

         return this;

       }

       public Builder ClearNegativeIntValue() {

+        PrepareBuilder();

         result.hasNegativeIntValue = false;

         result.negativeIntValue_ = 0L;

         return this;

@@ -7401,11 +8131,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         set { SetDoubleValue(value); }

       }

       public Builder SetDoubleValue(double value) {

+        PrepareBuilder();

         result.hasDoubleValue = true;

         result.doubleValue_ = value;

         return this;

       }

       public Builder ClearDoubleValue() {

+        PrepareBuilder();

         result.hasDoubleValue = false;

         result.doubleValue_ = 0D;

         return this;

@@ -7420,11 +8152,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       public Builder SetStringValue(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasStringValue = true;

         result.stringValue_ = value;

         return this;

       }

       public Builder ClearStringValue() {

+        PrepareBuilder();

         result.hasStringValue = false;

         result.stringValue_ = pb::ByteString.Empty;

         return this;

diff --git a/src/ProtocolBuffers/GeneratedBuilder.cs b/src/ProtocolBuffers/GeneratedBuilder.cs
index 94948e49..2472d2b0 100644
--- a/src/ProtocolBuffers/GeneratedBuilder.cs
+++ b/src/ProtocolBuffers/GeneratedBuilder.cs
@@ -58,12 +58,7 @@ namespace Google.ProtocolBuffers
 

         protected internal FieldAccessorTable<TMessage, TBuilder> InternalFieldAccessors

         {

-            get { return MessageBeingBuilt.FieldAccessorsFromBuilder; }

-        }

-

-        public override bool IsInitialized

-        {

-            get { return MessageBeingBuilt.IsInitialized; }

+            get { return DefaultInstanceForType.FieldAccessorsFromBuilder; }

         }

 

         public override IDictionary<FieldDescriptor, object> AllFields

@@ -123,7 +118,7 @@ namespace Google.ProtocolBuffers
 

         public override MessageDescriptor DescriptorForType

         {

-            get { return MessageBeingBuilt.DescriptorForType; }

+            get { return DefaultInstanceForType.DescriptorForType; }

         }

 

         public override int GetRepeatedFieldCount(FieldDescriptor field)

@@ -230,7 +225,7 @@ namespace Google.ProtocolBuffers
         public override TMessage Build()

         {

             // If the message is null, we'll throw a more appropriate exception in BuildPartial.

-            if (MessageBeingBuilt != null && !IsInitialized)

+            if (!IsInitialized)

             {

                 throw new UninitializedMessageException(MessageBeingBuilt);

             }

diff --git a/src/ProtocolBuffers/GeneratedBuilderLite.cs b/src/ProtocolBuffers/GeneratedBuilderLite.cs
index cd2b8395..165edc10 100644
--- a/src/ProtocolBuffers/GeneratedBuilderLite.cs
+++ b/src/ProtocolBuffers/GeneratedBuilderLite.cs
@@ -61,11 +61,6 @@ namespace Google.ProtocolBuffers
 

         public abstract TBuilder MergeFrom(TMessage other);

 

-        public override bool IsInitialized

-        {

-            get { return MessageBeingBuilt.IsInitialized; }

-        }

-

         /// <summary>

         /// Adds all of the specified values to the given collection.

         /// </summary>

@@ -122,7 +117,7 @@ namespace Google.ProtocolBuffers
         public override TMessage Build()

         {

             // If the message is null, we'll throw a more appropriate exception in BuildPartial.

-            if (MessageBeingBuilt != null && !IsInitialized)

+            if (!IsInitialized)

             {

                 throw new UninitializedMessageException(MessageBeingBuilt);

             }

diff --git a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasFullProtoFile.cs b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasFullProtoFile.cs
index a9ee7772..563cea5e 100644
--- a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasFullProtoFile.cs
+++ b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasFullProtoFile.cs
@@ -241,7 +241,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(PhoneNumber prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -251,21 +251,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new PhoneNumber();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(PhoneNumber cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          PhoneNumber result;

           

-          PhoneNumber result = new PhoneNumber();

+          private PhoneNumber PrepareBuilder() {

+            if (builderIsReadOnly) {

+              PhoneNumber original = result;

+              result = new PhoneNumber();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override PhoneNumber MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new PhoneNumber();

+            result = DefaultInstance ?? new PhoneNumber();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -277,12 +304,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override PhoneNumber BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            PhoneNumber returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           

@@ -295,11 +321,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetNumber(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasNumber = true;

             result.number_ = value;

             return this;

           }

           public Builder ClearNumber() {

+            PrepareBuilder();

             result.hasNumber = false;

             result.number_ = "";

             return this;

@@ -313,11 +341,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetType(value); }

           }

           public Builder SetType(global::Google.ProtocolBuffers.TestProtos.TestInteropPerson.Types.PhoneType value) {

+            PrepareBuilder();

             result.hasType = true;

             result.type_ = value;

             return this;

           }

           public Builder ClearType() {

+            PrepareBuilder();

             result.hasType = false;

             result.type_ = global::Google.ProtocolBuffers.TestProtos.TestInteropPerson.Types.PhoneType.HOME;

             return this;

@@ -438,7 +468,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(Addresses prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -448,21 +478,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new Addresses();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(Addresses cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          Addresses result;

+          

+          private Addresses PrepareBuilder() {

+            if (builderIsReadOnly) {

+              Addresses original = result;

+              result = new Addresses();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

           

-          Addresses result = new Addresses();

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override Addresses MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new Addresses();

+            result = DefaultInstance ?? new Addresses();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -474,12 +531,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Addresses BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            Addresses returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           

@@ -492,11 +548,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetAddress(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasAddress = true;

             result.address_ = value;

             return this;

           }

           public Builder ClearAddress() {

+            PrepareBuilder();

             result.hasAddress = false;

             result.address_ = "";

             return this;

@@ -511,11 +569,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetAddress2(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasAddress2 = true;

             result.address2_ = value;

             return this;

           }

           public Builder ClearAddress2() {

+            PrepareBuilder();

             result.hasAddress2 = false;

             result.address2_ = "";

             return this;

@@ -530,11 +590,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetCity(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasCity = true;

             result.city_ = value;

             return this;

           }

           public Builder ClearCity() {

+            PrepareBuilder();

             result.hasCity = false;

             result.city_ = "";

             return this;

@@ -549,11 +611,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetState(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasState = true;

             result.state_ = value;

             return this;

           }

           public Builder ClearState() {

+            PrepareBuilder();

             result.hasState = false;

             result.state_ = "";

             return this;

@@ -569,11 +633,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           [global::System.CLSCompliant(false)]

           public Builder SetZip(uint value) {

+            PrepareBuilder();

             result.hasZip = true;

             result.zip_ = value;

             return this;

           }

           public Builder ClearZip() {

+            PrepareBuilder();

             result.hasZip = false;

             result.zip_ = 0;

             return this;

@@ -687,7 +753,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestInteropPerson prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -697,21 +763,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestInteropPerson();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestInteropPerson cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestInteropPerson result = new TestInteropPerson();

+      bool builderIsReadOnly;

+      TestInteropPerson result;

+      

+      private TestInteropPerson PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestInteropPerson original = result;

+          result = new TestInteropPerson();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestInteropPerson MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestInteropPerson();

+        result = DefaultInstance ?? new TestInteropPerson();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -723,15 +816,14 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestInteropPerson BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.codes_.MakeReadOnly();

         result.phone_.MakeReadOnly();

         result.addresses_.MakeReadOnly();

-        TestInteropPerson returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       

@@ -744,11 +836,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetName(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasName = true;

         result.name_ = value;

         return this;

       }

       public Builder ClearName() {

+        PrepareBuilder();

         result.hasName = false;

         result.name_ = "";

         return this;

@@ -762,11 +856,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetId(value); }

       }

       public Builder SetId(int value) {

+        PrepareBuilder();

         result.hasId = true;

         result.id_ = value;

         return this;

       }

       public Builder ClearId() {

+        PrepareBuilder();

         result.hasId = false;

         result.id_ = 0;

         return this;

@@ -781,18 +877,20 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetEmail(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasEmail = true;

         result.email_ = value;

         return this;

       }

       public Builder ClearEmail() {

+        PrepareBuilder();

         result.hasEmail = false;

         result.email_ = "";

         return this;

       }

       

       public pbc::IPopsicleList<int> CodesList {

-        get { return result.codes_; }

+        get { return PrepareBuilder().codes_; }

       }

       public int CodesCount {

         get { return result.CodesCount; }

@@ -801,24 +899,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetCodes(index);

       }

       public Builder SetCodes(int index, int value) {

+        PrepareBuilder();

         result.codes_[index] = value;

         return this;

       }

       public Builder AddCodes(int value) {

+        PrepareBuilder();

         result.codes_.Add(value);

         return this;

       }

       public Builder AddRangeCodes(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.codes_);

         return this;

       }

       public Builder ClearCodes() {

+        PrepareBuilder();

         result.codes_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestInteropPerson.Types.PhoneNumber> PhoneList {

-        get { return result.phone_; }

+        get { return PrepareBuilder().phone_; }

       }

       public int PhoneCount {

         get { return result.PhoneCount; }

@@ -828,35 +930,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetPhone(int index, global::Google.ProtocolBuffers.TestProtos.TestInteropPerson.Types.PhoneNumber value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.phone_[index] = value;

         return this;

       }

       public Builder SetPhone(int index, global::Google.ProtocolBuffers.TestProtos.TestInteropPerson.Types.PhoneNumber.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.phone_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddPhone(global::Google.ProtocolBuffers.TestProtos.TestInteropPerson.Types.PhoneNumber value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.phone_.Add(value);

         return this;

       }

       public Builder AddPhone(global::Google.ProtocolBuffers.TestProtos.TestInteropPerson.Types.PhoneNumber.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.phone_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangePhone(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestInteropPerson.Types.PhoneNumber> values) {

+        PrepareBuilder();

         base.AddRange(values, result.phone_);

         return this;

       }

       public Builder ClearPhone() {

+        PrepareBuilder();

         result.phone_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestInteropPerson.Types.Addresses> AddressesList {

-        get { return result.addresses_; }

+        get { return PrepareBuilder().addresses_; }

       }

       public int AddressesCount {

         get { return result.AddressesCount; }

@@ -866,29 +974,35 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetAddresses(int index, global::Google.ProtocolBuffers.TestProtos.TestInteropPerson.Types.Addresses value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.addresses_[index] = value;

         return this;

       }

       public Builder SetAddresses(int index, global::Google.ProtocolBuffers.TestProtos.TestInteropPerson.Types.Addresses.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.addresses_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddAddresses(global::Google.ProtocolBuffers.TestProtos.TestInteropPerson.Types.Addresses value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.addresses_.Add(value);

         return this;

       }

       public Builder AddAddresses(global::Google.ProtocolBuffers.TestProtos.TestInteropPerson.Types.Addresses.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.addresses_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeAddresses(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestInteropPerson.Types.Addresses> values) {

+        PrepareBuilder();

         base.AddRange(values, result.addresses_);

         return this;

       }

       public Builder ClearAddresses() {

+        PrepareBuilder();

         result.addresses_.Clear();

         return this;

       }

@@ -967,7 +1081,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestInteropEmployeeId prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -977,21 +1091,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestInteropEmployeeId();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestInteropEmployeeId cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestInteropEmployeeId result;

+      

+      private TestInteropEmployeeId PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestInteropEmployeeId original = result;

+          result = new TestInteropEmployeeId();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestInteropEmployeeId result = new TestInteropEmployeeId();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestInteropEmployeeId MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestInteropEmployeeId();

+        result = DefaultInstance ?? new TestInteropEmployeeId();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1003,12 +1144,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestInteropEmployeeId BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestInteropEmployeeId returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       

@@ -1021,11 +1161,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetNumber(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasNumber = true;

         result.number_ = value;

         return this;

       }

       public Builder ClearNumber() {

+        PrepareBuilder();

         result.hasNumber = false;

         result.number_ = "";

         return this;

@@ -1147,7 +1289,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(SubA prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1157,21 +1299,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new SubA();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(SubA cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          SubA result;

           

-          SubA result = new SubA();

+          private SubA PrepareBuilder() {

+            if (builderIsReadOnly) {

+              SubA original = result;

+              result = new SubA();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override SubA MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new SubA();

+            result = DefaultInstance ?? new SubA();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -1183,13 +1352,12 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override SubA BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

             result.values_.MakeReadOnly();

-            SubA returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           

@@ -1201,18 +1369,20 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetCount(value); }

           }

           public Builder SetCount(int value) {

+            PrepareBuilder();

             result.hasCount = true;

             result.count_ = value;

             return this;

           }

           public Builder ClearCount() {

+            PrepareBuilder();

             result.hasCount = false;

             result.count_ = 0;

             return this;

           }

           

           public pbc::IPopsicleList<string> ValuesList {

-            get { return result.values_; }

+            get { return PrepareBuilder().values_; }

           }

           public int ValuesCount {

             get { return result.ValuesCount; }

@@ -1222,19 +1392,23 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetValues(int index, string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.values_[index] = value;

             return this;

           }

           public Builder AddValues(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.values_.Add(value);

             return this;

           }

           public Builder AddRangeValues(scg::IEnumerable<string> values) {

+            PrepareBuilder();

             base.AddRange(values, result.values_);

             return this;

           }

           public Builder ClearValues() {

+            PrepareBuilder();

             result.values_.Clear();

             return this;

           }

@@ -1321,7 +1495,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestMissingFieldsA prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1331,21 +1505,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestMissingFieldsA();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestMissingFieldsA cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestMissingFieldsA result;

+      

+      private TestMissingFieldsA PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestMissingFieldsA original = result;

+          result = new TestMissingFieldsA();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestMissingFieldsA result = new TestMissingFieldsA();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestMissingFieldsA MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestMissingFieldsA();

+        result = DefaultInstance ?? new TestMissingFieldsA();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1357,12 +1558,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMissingFieldsA BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestMissingFieldsA returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       

@@ -1375,11 +1575,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetName(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasName = true;

         result.name_ = value;

         return this;

       }

       public Builder ClearName() {

+        PrepareBuilder();

         result.hasName = false;

         result.name_ = "";

         return this;

@@ -1393,11 +1595,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetId(value); }

       }

       public Builder SetId(int value) {

+        PrepareBuilder();

         result.hasId = true;

         result.id_ = value;

         return this;

       }

       public Builder ClearId() {

+        PrepareBuilder();

         result.hasId = false;

         result.id_ = 0;

         return this;

@@ -1412,11 +1616,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetEmail(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasEmail = true;

         result.email_ = value;

         return this;

       }

       public Builder ClearEmail() {

+        PrepareBuilder();

         result.hasEmail = false;

         result.email_ = "";

         return this;

@@ -1431,18 +1637,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetTestA(global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasTestA = true;

         result.testA_ = value;

         return this;

       }

       public Builder SetTestA(global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasTestA = true;

         result.testA_ = builderForValue.Build();

         return this;

       }

       public Builder MergeTestA(global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasTestA &&

             result.testA_ != global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA.DefaultInstance) {

             result.testA_ = global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA.CreateBuilder(result.testA_).MergeFrom(value).BuildPartial();

@@ -1453,6 +1662,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearTestA() {

+        PrepareBuilder();

         result.hasTestA = false;

         result.testA_ = global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA.DefaultInstance;

         return this;

@@ -1564,7 +1774,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(SubB prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1574,21 +1784,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new SubB();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(SubB cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          SubB result;

           

-          SubB result = new SubB();

+          private SubB PrepareBuilder() {

+            if (builderIsReadOnly) {

+              SubB original = result;

+              result = new SubB();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override SubB MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new SubB();

+            result = DefaultInstance ?? new SubB();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -1600,18 +1837,17 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override SubB BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

             result.values_.MakeReadOnly();

-            SubB returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           

           public pbc::IPopsicleList<string> ValuesList {

-            get { return result.values_; }

+            get { return PrepareBuilder().values_; }

           }

           public int ValuesCount {

             get { return result.ValuesCount; }

@@ -1621,19 +1857,23 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetValues(int index, string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.values_[index] = value;

             return this;

           }

           public Builder AddValues(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.values_.Add(value);

             return this;

           }

           public Builder AddRangeValues(scg::IEnumerable<string> values) {

+            PrepareBuilder();

             base.AddRange(values, result.values_);

             return this;

           }

           public Builder ClearValues() {

+            PrepareBuilder();

             result.values_.Clear();

             return this;

           }

@@ -1720,7 +1960,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestMissingFieldsB prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1730,21 +1970,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestMissingFieldsB();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestMissingFieldsB cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestMissingFieldsB result;

+      

+      private TestMissingFieldsB PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestMissingFieldsB original = result;

+          result = new TestMissingFieldsB();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestMissingFieldsB result = new TestMissingFieldsB();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestMissingFieldsB MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestMissingFieldsB();

+        result = DefaultInstance ?? new TestMissingFieldsB();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -1756,12 +2023,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMissingFieldsB BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestMissingFieldsB returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       

@@ -1774,11 +2040,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetName(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasName = true;

         result.name_ = value;

         return this;

       }

       public Builder ClearName() {

+        PrepareBuilder();

         result.hasName = false;

         result.name_ = "";

         return this;

@@ -1792,11 +2060,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetId(value); }

       }

       public Builder SetId(int value) {

+        PrepareBuilder();

         result.hasId = true;

         result.id_ = value;

         return this;

       }

       public Builder ClearId() {

+        PrepareBuilder();

         result.hasId = false;

         result.id_ = 0;

         return this;

@@ -1811,11 +2081,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetWebsite(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasWebsite = true;

         result.website_ = value;

         return this;

       }

       public Builder ClearWebsite() {

+        PrepareBuilder();

         result.hasWebsite = false;

         result.website_ = "";

         return this;

@@ -1830,18 +2102,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetTestB(global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasTestB = true;

         result.testB_ = value;

         return this;

       }

       public Builder SetTestB(global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasTestB = true;

         result.testB_ = builderForValue.Build();

         return this;

       }

       public Builder MergeTestB(global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasTestB &&

             result.testB_ != global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB.DefaultInstance) {

             result.testB_ = global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB.CreateBuilder(result.testB_).MergeFrom(value).BuildPartial();

@@ -1852,6 +2127,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearTestB() {

+        PrepareBuilder();

         result.hasTestB = false;

         result.testB_ = global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB.DefaultInstance;

         return this;

diff --git a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasLiteProtoFile.cs b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasLiteProtoFile.cs
index bc214e25..cbbc50db 100644
--- a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasLiteProtoFile.cs
+++ b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasLiteProtoFile.cs
@@ -187,7 +187,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestRequiredLite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -197,21 +197,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestRequiredLite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestRequiredLite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestRequiredLite result;

+      

+      private TestRequiredLite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestRequiredLite original = result;

+          result = new TestRequiredLite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestRequiredLite result = new TestRequiredLite();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestRequiredLite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestRequiredLite();

+        result = DefaultInstance ?? new TestRequiredLite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override TestRequiredLite DefaultInstanceForType {

@@ -219,12 +246,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRequiredLite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestRequiredLite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -238,6 +264,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestRequiredLite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestRequiredLite.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasD) {

           D = other.D;

         }

@@ -252,6 +279,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -302,11 +330,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetD(value); }

       }

       public Builder SetD(int value) {

+        PrepareBuilder();

         result.hasD = true;

         result.d_ = value;

         return this;

       }

       public Builder ClearD() {

+        PrepareBuilder();

         result.hasD = false;

         result.d_ = 0;

         return this;

@@ -320,11 +350,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetEn(value); }

       }

       public Builder SetEn(global::Google.ProtocolBuffers.TestProtos.ExtraEnum value) {

+        PrepareBuilder();

         result.hasEn = true;

         result.en_ = value;

         return this;

       }

       public Builder ClearEn() {

+        PrepareBuilder();

         result.hasEn = false;

         result.en_ = global::Google.ProtocolBuffers.TestProtos.ExtraEnum.DEFAULT;

         return this;

@@ -498,7 +530,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(PhoneNumber prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -508,21 +540,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new PhoneNumber();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(PhoneNumber cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

           

-          PhoneNumber result = new PhoneNumber();

+          bool builderIsReadOnly;

+          PhoneNumber result;

+          

+          private PhoneNumber PrepareBuilder() {

+            if (builderIsReadOnly) {

+              PhoneNumber original = result;

+              result = new PhoneNumber();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override PhoneNumber MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new PhoneNumber();

+            result = DefaultInstance ?? new PhoneNumber();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override PhoneNumber DefaultInstanceForType {

@@ -530,12 +589,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override PhoneNumber BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            PhoneNumber returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessageLite other) {

@@ -549,6 +607,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(PhoneNumber other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.PhoneNumber.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasNumber) {

               Number = other.Number;

             }

@@ -563,6 +622,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             uint tag;

             string field_name;

             while (input.ReadTag(out tag, out field_name)) {

@@ -614,11 +674,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetNumber(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasNumber = true;

             result.number_ = value;

             return this;

           }

           public Builder ClearNumber() {

+            PrepareBuilder();

             result.hasNumber = false;

             result.number_ = "";

             return this;

@@ -632,11 +694,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetType(value); }

           }

           public Builder SetType(global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.PhoneType value) {

+            PrepareBuilder();

             result.hasType = true;

             result.type_ = value;

             return this;

           }

           public Builder ClearType() {

+            PrepareBuilder();

             result.hasType = false;

             result.type_ = global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.PhoneType.HOME;

             return this;

@@ -839,7 +903,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(Addresses prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -849,21 +913,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new Addresses();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(Addresses cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          Addresses result;

           

-          Addresses result = new Addresses();

+          private Addresses PrepareBuilder() {

+            if (builderIsReadOnly) {

+              Addresses original = result;

+              result = new Addresses();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override Addresses MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new Addresses();

+            result = DefaultInstance ?? new Addresses();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override Addresses DefaultInstanceForType {

@@ -871,12 +962,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Addresses BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            Addresses returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessageLite other) {

@@ -890,6 +980,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(Addresses other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.Addresses.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasAddress) {

               Address = other.Address;

             }

@@ -913,6 +1004,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             uint tag;

             string field_name;

             while (input.ReadTag(out tag, out field_name)) {

@@ -972,11 +1064,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetAddress(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasAddress = true;

             result.address_ = value;

             return this;

           }

           public Builder ClearAddress() {

+            PrepareBuilder();

             result.hasAddress = false;

             result.address_ = "";

             return this;

@@ -991,11 +1085,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetAddress2(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasAddress2 = true;

             result.address2_ = value;

             return this;

           }

           public Builder ClearAddress2() {

+            PrepareBuilder();

             result.hasAddress2 = false;

             result.address2_ = "";

             return this;

@@ -1010,11 +1106,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetCity(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasCity = true;

             result.city_ = value;

             return this;

           }

           public Builder ClearCity() {

+            PrepareBuilder();

             result.hasCity = false;

             result.city_ = "";

             return this;

@@ -1029,11 +1127,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetState(string value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.hasState = true;

             result.state_ = value;

             return this;

           }

           public Builder ClearState() {

+            PrepareBuilder();

             result.hasState = false;

             result.state_ = "";

             return this;

@@ -1049,11 +1149,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           [global::System.CLSCompliant(false)]

           public Builder SetZip(uint value) {

+            PrepareBuilder();

             result.hasZip = true;

             result.zip_ = value;

             return this;

           }

           public Builder ClearZip() {

+            PrepareBuilder();

             result.hasZip = false;

             result.zip_ = 0;

             return this;

@@ -1290,7 +1392,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestInteropPersonLite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1300,21 +1402,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestInteropPersonLite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestInteropPersonLite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestInteropPersonLite result = new TestInteropPersonLite();

+      bool builderIsReadOnly;

+      TestInteropPersonLite result;

+      

+      private TestInteropPersonLite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestInteropPersonLite original = result;

+          result = new TestInteropPersonLite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestInteropPersonLite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestInteropPersonLite();

+        result = DefaultInstance ?? new TestInteropPersonLite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override TestInteropPersonLite DefaultInstanceForType {

@@ -1322,15 +1451,14 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestInteropPersonLite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.codes_.MakeReadOnly();

         result.phone_.MakeReadOnly();

         result.addresses_.MakeReadOnly();

-        TestInteropPersonLite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -1344,6 +1472,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestInteropPersonLite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasName) {

           Name = other.Name;

         }

@@ -1371,6 +1500,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -1435,11 +1565,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetName(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasName = true;

         result.name_ = value;

         return this;

       }

       public Builder ClearName() {

+        PrepareBuilder();

         result.hasName = false;

         result.name_ = "";

         return this;

@@ -1453,11 +1585,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetId(value); }

       }

       public Builder SetId(int value) {

+        PrepareBuilder();

         result.hasId = true;

         result.id_ = value;

         return this;

       }

       public Builder ClearId() {

+        PrepareBuilder();

         result.hasId = false;

         result.id_ = 0;

         return this;

@@ -1472,18 +1606,20 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetEmail(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasEmail = true;

         result.email_ = value;

         return this;

       }

       public Builder ClearEmail() {

+        PrepareBuilder();

         result.hasEmail = false;

         result.email_ = "";

         return this;

       }

       

       public pbc::IPopsicleList<int> CodesList {

-        get { return result.codes_; }

+        get { return PrepareBuilder().codes_; }

       }

       public int CodesCount {

         get { return result.CodesCount; }

@@ -1492,24 +1628,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetCodes(index);

       }

       public Builder SetCodes(int index, int value) {

+        PrepareBuilder();

         result.codes_[index] = value;

         return this;

       }

       public Builder AddCodes(int value) {

+        PrepareBuilder();

         result.codes_.Add(value);

         return this;

       }

       public Builder AddRangeCodes(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.codes_);

         return this;

       }

       public Builder ClearCodes() {

+        PrepareBuilder();

         result.codes_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.PhoneNumber> PhoneList {

-        get { return result.phone_; }

+        get { return PrepareBuilder().phone_; }

       }

       public int PhoneCount {

         get { return result.PhoneCount; }

@@ -1519,35 +1659,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetPhone(int index, global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.PhoneNumber value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.phone_[index] = value;

         return this;

       }

       public Builder SetPhone(int index, global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.PhoneNumber.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.phone_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddPhone(global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.PhoneNumber value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.phone_.Add(value);

         return this;

       }

       public Builder AddPhone(global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.PhoneNumber.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.phone_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangePhone(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.PhoneNumber> values) {

+        PrepareBuilder();

         base.AddRange(values, result.phone_);

         return this;

       }

       public Builder ClearPhone() {

+        PrepareBuilder();

         result.phone_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.Addresses> AddressesList {

-        get { return result.addresses_; }

+        get { return PrepareBuilder().addresses_; }

       }

       public int AddressesCount {

         get { return result.AddressesCount; }

@@ -1557,29 +1703,35 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetAddresses(int index, global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.Addresses value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.addresses_[index] = value;

         return this;

       }

       public Builder SetAddresses(int index, global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.Addresses.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.addresses_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddAddresses(global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.Addresses value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.addresses_.Add(value);

         return this;

       }

       public Builder AddAddresses(global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.Addresses.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.addresses_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeAddresses(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestInteropPersonLite.Types.Addresses> values) {

+        PrepareBuilder();

         base.AddRange(values, result.addresses_);

         return this;

       }

       public Builder ClearAddresses() {

+        PrepareBuilder();

         result.addresses_.Clear();

         return this;

       }

@@ -1701,7 +1853,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestInteropEmployeeIdLite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1711,21 +1863,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestInteropEmployeeIdLite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestInteropEmployeeIdLite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestInteropEmployeeIdLite result = new TestInteropEmployeeIdLite();

+      bool builderIsReadOnly;

+      TestInteropEmployeeIdLite result;

+      

+      private TestInteropEmployeeIdLite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestInteropEmployeeIdLite original = result;

+          result = new TestInteropEmployeeIdLite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestInteropEmployeeIdLite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestInteropEmployeeIdLite();

+        result = DefaultInstance ?? new TestInteropEmployeeIdLite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override TestInteropEmployeeIdLite DefaultInstanceForType {

@@ -1733,12 +1912,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestInteropEmployeeIdLite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestInteropEmployeeIdLite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -1752,6 +1930,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestInteropEmployeeIdLite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestInteropEmployeeIdLite.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasNumber) {

           Number = other.Number;

         }

@@ -1763,6 +1942,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -1806,11 +1986,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetNumber(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasNumber = true;

         result.number_ = value;

         return this;

       }

       public Builder ClearNumber() {

+        PrepareBuilder();

         result.hasNumber = false;

         result.number_ = "";

         return this;

diff --git a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportLiteProtoFile.cs b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportLiteProtoFile.cs
index a96833be..0df40f42 100644
--- a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportLiteProtoFile.cs
+++ b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportLiteProtoFile.cs
@@ -150,7 +150,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(ImportMessageLite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -160,21 +160,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new ImportMessageLite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(ImportMessageLite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      ImportMessageLite result;

       

-      ImportMessageLite result = new ImportMessageLite();

+      private ImportMessageLite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          ImportMessageLite original = result;

+          result = new ImportMessageLite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override ImportMessageLite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new ImportMessageLite();

+        result = DefaultInstance ?? new ImportMessageLite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override ImportMessageLite DefaultInstanceForType {

@@ -182,12 +209,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ImportMessageLite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        ImportMessageLite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -201,6 +227,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(ImportMessageLite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.ImportMessageLite.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasD) {

           D = other.D;

         }

@@ -212,6 +239,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -254,11 +282,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetD(value); }

       }

       public Builder SetD(int value) {

+        PrepareBuilder();

         result.hasD = true;

         result.d_ = value;

         return this;

       }

       public Builder ClearD() {

+        PrepareBuilder();

         result.hasD = false;

         result.d_ = 0;

         return this;

diff --git a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportProtoFile.cs b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportProtoFile.cs
index 5749c97f..75e746a8 100644
--- a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportProtoFile.cs
+++ b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportProtoFile.cs
@@ -169,7 +169,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(ImportMessage prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -179,21 +179,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new ImportMessage();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(ImportMessage cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      ImportMessage result;

       

-      ImportMessage result = new ImportMessage();

+      private ImportMessage PrepareBuilder() {

+        if (builderIsReadOnly) {

+          ImportMessage original = result;

+          result = new ImportMessage();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override ImportMessage MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new ImportMessage();

+        result = DefaultInstance ?? new ImportMessage();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -205,12 +232,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ImportMessage BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        ImportMessage returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -224,6 +250,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(ImportMessage other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.ImportMessage.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasD) {

           D = other.D;

         }

@@ -236,6 +263,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -291,11 +319,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetD(value); }

       }

       public Builder SetD(int value) {

+        PrepareBuilder();

         result.hasD = true;

         result.d_ = value;

         return this;

       }

       public Builder ClearD() {

+        PrepareBuilder();

         result.hasD = false;

         result.d_ = 0;

         return this;

diff --git a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteImportNonLiteProtoFile.cs b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteImportNonLiteProtoFile.cs
index 7c2836cd..5a6033a1 100644
--- a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteImportNonLiteProtoFile.cs
+++ b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteImportNonLiteProtoFile.cs
@@ -139,7 +139,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestLiteImportsNonlite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -149,21 +149,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestLiteImportsNonlite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestLiteImportsNonlite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestLiteImportsNonlite result;

       

-      TestLiteImportsNonlite result = new TestLiteImportsNonlite();

+      private TestLiteImportsNonlite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestLiteImportsNonlite original = result;

+          result = new TestLiteImportsNonlite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestLiteImportsNonlite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestLiteImportsNonlite();

+        result = DefaultInstance ?? new TestLiteImportsNonlite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override TestLiteImportsNonlite DefaultInstanceForType {

@@ -171,12 +198,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestLiteImportsNonlite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestLiteImportsNonlite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -190,6 +216,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestLiteImportsNonlite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestLiteImportsNonlite.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasMessage) {

           MergeMessage(other.Message);

         }

@@ -201,6 +228,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -249,18 +277,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypes value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasMessage = true;

         result.message_ = value;

         return this;

       }

       public Builder SetMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasMessage = true;

         result.message_ = builderForValue.Build();

         return this;

       }

       public Builder MergeMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypes value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasMessage &&

             result.message_ != global::Google.ProtocolBuffers.TestProtos.TestAllTypes.DefaultInstance) {

             result.message_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.CreateBuilder(result.message_).MergeFrom(value).BuildPartial();

@@ -271,6 +302,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearMessage() {

+        PrepareBuilder();

         result.hasMessage = false;

         result.message_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.DefaultInstance;

         return this;

diff --git a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteProtoFile.cs b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteProtoFile.cs
index 78bed7e2..2592c833 100644
--- a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteProtoFile.cs
+++ b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteProtoFile.cs
@@ -1367,7 +1367,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(NestedMessage prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1377,21 +1377,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new NestedMessage();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(NestedMessage cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          NestedMessage result;

+          

+          private NestedMessage PrepareBuilder() {

+            if (builderIsReadOnly) {

+              NestedMessage original = result;

+              result = new NestedMessage();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

           

-          NestedMessage result = new NestedMessage();

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override NestedMessage MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new NestedMessage();

+            result = DefaultInstance ?? new NestedMessage();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override NestedMessage DefaultInstanceForType {

@@ -1399,12 +1426,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override NestedMessage BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            NestedMessage returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessageLite other) {

@@ -1418,6 +1444,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(NestedMessage other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasBb) {

               Bb = other.Bb;

             }

@@ -1429,6 +1456,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             uint tag;

             string field_name;

             while (input.ReadTag(out tag, out field_name)) {

@@ -1471,11 +1499,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetBb(value); }

           }

           public Builder SetBb(int value) {

+            PrepareBuilder();

             result.hasBb = true;

             result.bb_ = value;

             return this;

           }

           public Builder ClearBb() {

+            PrepareBuilder();

             result.hasBb = false;

             result.bb_ = 0;

             return this;

@@ -1597,7 +1627,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(OptionalGroup prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1607,21 +1637,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new OptionalGroup();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(OptionalGroup cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          OptionalGroup result;

+          

+          private OptionalGroup PrepareBuilder() {

+            if (builderIsReadOnly) {

+              OptionalGroup original = result;

+              result = new OptionalGroup();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

           

-          OptionalGroup result = new OptionalGroup();

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override OptionalGroup MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new OptionalGroup();

+            result = DefaultInstance ?? new OptionalGroup();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override OptionalGroup DefaultInstanceForType {

@@ -1629,12 +1686,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override OptionalGroup BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            OptionalGroup returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessageLite other) {

@@ -1648,6 +1704,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(OptionalGroup other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.OptionalGroup.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasA) {

               A = other.A;

             }

@@ -1659,6 +1716,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             uint tag;

             string field_name;

             while (input.ReadTag(out tag, out field_name)) {

@@ -1701,11 +1759,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetA(value); }

           }

           public Builder SetA(int value) {

+            PrepareBuilder();

             result.hasA = true;

             result.a_ = value;

             return this;

           }

           public Builder ClearA() {

+            PrepareBuilder();

             result.hasA = false;

             result.a_ = 0;

             return this;

@@ -1827,7 +1887,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(RepeatedGroup prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1837,21 +1897,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new RepeatedGroup();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(RepeatedGroup cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          RepeatedGroup result;

+          

+          private RepeatedGroup PrepareBuilder() {

+            if (builderIsReadOnly) {

+              RepeatedGroup original = result;

+              result = new RepeatedGroup();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

           

-          RepeatedGroup result = new RepeatedGroup();

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override RepeatedGroup MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new RepeatedGroup();

+            result = DefaultInstance ?? new RepeatedGroup();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override RepeatedGroup DefaultInstanceForType {

@@ -1859,12 +1946,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override RepeatedGroup BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            RepeatedGroup returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessageLite other) {

@@ -1878,6 +1964,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(RepeatedGroup other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.RepeatedGroup.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasA) {

               A = other.A;

             }

@@ -1889,6 +1976,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             uint tag;

             string field_name;

             while (input.ReadTag(out tag, out field_name)) {

@@ -1931,11 +2019,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetA(value); }

           }

           public Builder SetA(int value) {

+            PrepareBuilder();

             result.hasA = true;

             result.a_ = value;

             return this;

           }

           public Builder ClearA() {

+            PrepareBuilder();

             result.hasA = false;

             result.a_ = 0;

             return this;

@@ -3542,7 +3632,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestAllTypesLite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -3552,21 +3642,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestAllTypesLite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestAllTypesLite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestAllTypesLite result = new TestAllTypesLite();

+      bool builderIsReadOnly;

+      TestAllTypesLite result;

+      

+      private TestAllTypesLite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestAllTypesLite original = result;

+          result = new TestAllTypesLite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestAllTypesLite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestAllTypesLite();

+        result = DefaultInstance ?? new TestAllTypesLite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override TestAllTypesLite DefaultInstanceForType {

@@ -3574,8 +3691,8 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestAllTypesLite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.repeatedInt32_.MakeReadOnly();

         result.repeatedInt64_.MakeReadOnly();

@@ -3601,9 +3718,8 @@ namespace Google.ProtocolBuffers.TestProtos {
         result.repeatedImportEnum_.MakeReadOnly();

         result.repeatedStringPiece_.MakeReadOnly();

         result.repeatedCord_.MakeReadOnly();

-        TestAllTypesLite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -3617,6 +3733,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestAllTypesLite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasOptionalInt32) {

           OptionalInt32 = other.OptionalInt32;

         }

@@ -3829,6 +3946,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -4202,11 +4320,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalInt32(value); }

       }

       public Builder SetOptionalInt32(int value) {

+        PrepareBuilder();

         result.hasOptionalInt32 = true;

         result.optionalInt32_ = value;

         return this;

       }

       public Builder ClearOptionalInt32() {

+        PrepareBuilder();

         result.hasOptionalInt32 = false;

         result.optionalInt32_ = 0;

         return this;

@@ -4220,11 +4340,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalInt64(value); }

       }

       public Builder SetOptionalInt64(long value) {

+        PrepareBuilder();

         result.hasOptionalInt64 = true;

         result.optionalInt64_ = value;

         return this;

       }

       public Builder ClearOptionalInt64() {

+        PrepareBuilder();

         result.hasOptionalInt64 = false;

         result.optionalInt64_ = 0L;

         return this;

@@ -4240,11 +4362,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetOptionalUint32(uint value) {

+        PrepareBuilder();

         result.hasOptionalUint32 = true;

         result.optionalUint32_ = value;

         return this;

       }

       public Builder ClearOptionalUint32() {

+        PrepareBuilder();

         result.hasOptionalUint32 = false;

         result.optionalUint32_ = 0;

         return this;

@@ -4260,11 +4384,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetOptionalUint64(ulong value) {

+        PrepareBuilder();

         result.hasOptionalUint64 = true;

         result.optionalUint64_ = value;

         return this;

       }

       public Builder ClearOptionalUint64() {

+        PrepareBuilder();

         result.hasOptionalUint64 = false;

         result.optionalUint64_ = 0UL;

         return this;

@@ -4278,11 +4404,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalSint32(value); }

       }

       public Builder SetOptionalSint32(int value) {

+        PrepareBuilder();

         result.hasOptionalSint32 = true;

         result.optionalSint32_ = value;

         return this;

       }

       public Builder ClearOptionalSint32() {

+        PrepareBuilder();

         result.hasOptionalSint32 = false;

         result.optionalSint32_ = 0;

         return this;

@@ -4296,11 +4424,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalSint64(value); }

       }

       public Builder SetOptionalSint64(long value) {

+        PrepareBuilder();

         result.hasOptionalSint64 = true;

         result.optionalSint64_ = value;

         return this;

       }

       public Builder ClearOptionalSint64() {

+        PrepareBuilder();

         result.hasOptionalSint64 = false;

         result.optionalSint64_ = 0;

         return this;

@@ -4316,11 +4446,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetOptionalFixed32(uint value) {

+        PrepareBuilder();

         result.hasOptionalFixed32 = true;

         result.optionalFixed32_ = value;

         return this;

       }

       public Builder ClearOptionalFixed32() {

+        PrepareBuilder();

         result.hasOptionalFixed32 = false;

         result.optionalFixed32_ = 0;

         return this;

@@ -4336,11 +4468,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetOptionalFixed64(ulong value) {

+        PrepareBuilder();

         result.hasOptionalFixed64 = true;

         result.optionalFixed64_ = value;

         return this;

       }

       public Builder ClearOptionalFixed64() {

+        PrepareBuilder();

         result.hasOptionalFixed64 = false;

         result.optionalFixed64_ = 0;

         return this;

@@ -4354,11 +4488,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalSfixed32(value); }

       }

       public Builder SetOptionalSfixed32(int value) {

+        PrepareBuilder();

         result.hasOptionalSfixed32 = true;

         result.optionalSfixed32_ = value;

         return this;

       }

       public Builder ClearOptionalSfixed32() {

+        PrepareBuilder();

         result.hasOptionalSfixed32 = false;

         result.optionalSfixed32_ = 0;

         return this;

@@ -4372,11 +4508,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalSfixed64(value); }

       }

       public Builder SetOptionalSfixed64(long value) {

+        PrepareBuilder();

         result.hasOptionalSfixed64 = true;

         result.optionalSfixed64_ = value;

         return this;

       }

       public Builder ClearOptionalSfixed64() {

+        PrepareBuilder();

         result.hasOptionalSfixed64 = false;

         result.optionalSfixed64_ = 0;

         return this;

@@ -4390,11 +4528,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalFloat(value); }

       }

       public Builder SetOptionalFloat(float value) {

+        PrepareBuilder();

         result.hasOptionalFloat = true;

         result.optionalFloat_ = value;

         return this;

       }

       public Builder ClearOptionalFloat() {

+        PrepareBuilder();

         result.hasOptionalFloat = false;

         result.optionalFloat_ = 0F;

         return this;

@@ -4408,11 +4548,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalDouble(value); }

       }

       public Builder SetOptionalDouble(double value) {

+        PrepareBuilder();

         result.hasOptionalDouble = true;

         result.optionalDouble_ = value;

         return this;

       }

       public Builder ClearOptionalDouble() {

+        PrepareBuilder();

         result.hasOptionalDouble = false;

         result.optionalDouble_ = 0D;

         return this;

@@ -4426,11 +4568,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalBool(value); }

       }

       public Builder SetOptionalBool(bool value) {

+        PrepareBuilder();

         result.hasOptionalBool = true;

         result.optionalBool_ = value;

         return this;

       }

       public Builder ClearOptionalBool() {

+        PrepareBuilder();

         result.hasOptionalBool = false;

         result.optionalBool_ = false;

         return this;

@@ -4445,11 +4589,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalString(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalString = true;

         result.optionalString_ = value;

         return this;

       }

       public Builder ClearOptionalString() {

+        PrepareBuilder();

         result.hasOptionalString = false;

         result.optionalString_ = "";

         return this;

@@ -4464,11 +4610,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalBytes(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalBytes = true;

         result.optionalBytes_ = value;

         return this;

       }

       public Builder ClearOptionalBytes() {

+        PrepareBuilder();

         result.hasOptionalBytes = false;

         result.optionalBytes_ = pb::ByteString.Empty;

         return this;

@@ -4483,18 +4631,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalGroup(global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.OptionalGroup value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalGroup = true;

         result.optionalGroup_ = value;

         return this;

       }

       public Builder SetOptionalGroup(global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.OptionalGroup.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalGroup = true;

         result.optionalGroup_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalGroup(global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.OptionalGroup value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalGroup &&

             result.optionalGroup_ != global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.OptionalGroup.DefaultInstance) {

             result.optionalGroup_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.OptionalGroup.CreateBuilder(result.optionalGroup_).MergeFrom(value).BuildPartial();

@@ -4505,6 +4656,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalGroup() {

+        PrepareBuilder();

         result.hasOptionalGroup = false;

         result.optionalGroup_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.OptionalGroup.DefaultInstance;

         return this;

@@ -4519,18 +4671,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalNestedMessage = true;

         result.optionalNestedMessage_ = value;

         return this;

       }

       public Builder SetOptionalNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalNestedMessage = true;

         result.optionalNestedMessage_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalNestedMessage &&

             result.optionalNestedMessage_ != global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage.DefaultInstance) {

             result.optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage.CreateBuilder(result.optionalNestedMessage_).MergeFrom(value).BuildPartial();

@@ -4541,6 +4696,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalNestedMessage() {

+        PrepareBuilder();

         result.hasOptionalNestedMessage = false;

         result.optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage.DefaultInstance;

         return this;

@@ -4555,18 +4711,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalForeignMessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalForeignMessage = true;

         result.optionalForeignMessage_ = value;

         return this;

       }

       public Builder SetOptionalForeignMessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalForeignMessage = true;

         result.optionalForeignMessage_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalForeignMessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalForeignMessage &&

             result.optionalForeignMessage_ != global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite.DefaultInstance) {

             result.optionalForeignMessage_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite.CreateBuilder(result.optionalForeignMessage_).MergeFrom(value).BuildPartial();

@@ -4577,6 +4736,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalForeignMessage() {

+        PrepareBuilder();

         result.hasOptionalForeignMessage = false;

         result.optionalForeignMessage_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite.DefaultInstance;

         return this;

@@ -4591,18 +4751,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalImportMessage(global::Google.ProtocolBuffers.TestProtos.ImportMessageLite value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalImportMessage = true;

         result.optionalImportMessage_ = value;

         return this;

       }

       public Builder SetOptionalImportMessage(global::Google.ProtocolBuffers.TestProtos.ImportMessageLite.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalImportMessage = true;

         result.optionalImportMessage_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalImportMessage(global::Google.ProtocolBuffers.TestProtos.ImportMessageLite value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalImportMessage &&

             result.optionalImportMessage_ != global::Google.ProtocolBuffers.TestProtos.ImportMessageLite.DefaultInstance) {

             result.optionalImportMessage_ = global::Google.ProtocolBuffers.TestProtos.ImportMessageLite.CreateBuilder(result.optionalImportMessage_).MergeFrom(value).BuildPartial();

@@ -4613,6 +4776,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalImportMessage() {

+        PrepareBuilder();

         result.hasOptionalImportMessage = false;

         result.optionalImportMessage_ = global::Google.ProtocolBuffers.TestProtos.ImportMessageLite.DefaultInstance;

         return this;

@@ -4626,11 +4790,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalNestedEnum(value); }

       }

       public Builder SetOptionalNestedEnum(global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedEnum value) {

+        PrepareBuilder();

         result.hasOptionalNestedEnum = true;

         result.optionalNestedEnum_ = value;

         return this;

       }

       public Builder ClearOptionalNestedEnum() {

+        PrepareBuilder();

         result.hasOptionalNestedEnum = false;

         result.optionalNestedEnum_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedEnum.FOO;

         return this;

@@ -4644,11 +4810,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalForeignEnum(value); }

       }

       public Builder SetOptionalForeignEnum(global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite value) {

+        PrepareBuilder();

         result.hasOptionalForeignEnum = true;

         result.optionalForeignEnum_ = value;

         return this;

       }

       public Builder ClearOptionalForeignEnum() {

+        PrepareBuilder();

         result.hasOptionalForeignEnum = false;

         result.optionalForeignEnum_ = global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite.FOREIGN_LITE_FOO;

         return this;

@@ -4662,11 +4830,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalImportEnum(value); }

       }

       public Builder SetOptionalImportEnum(global::Google.ProtocolBuffers.TestProtos.ImportEnumLite value) {

+        PrepareBuilder();

         result.hasOptionalImportEnum = true;

         result.optionalImportEnum_ = value;

         return this;

       }

       public Builder ClearOptionalImportEnum() {

+        PrepareBuilder();

         result.hasOptionalImportEnum = false;

         result.optionalImportEnum_ = global::Google.ProtocolBuffers.TestProtos.ImportEnumLite.IMPORT_LITE_FOO;

         return this;

@@ -4681,11 +4851,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalStringPiece(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalStringPiece = true;

         result.optionalStringPiece_ = value;

         return this;

       }

       public Builder ClearOptionalStringPiece() {

+        PrepareBuilder();

         result.hasOptionalStringPiece = false;

         result.optionalStringPiece_ = "";

         return this;

@@ -4700,18 +4872,20 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalCord(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalCord = true;

         result.optionalCord_ = value;

         return this;

       }

       public Builder ClearOptionalCord() {

+        PrepareBuilder();

         result.hasOptionalCord = false;

         result.optionalCord_ = "";

         return this;

       }

       

       public pbc::IPopsicleList<int> RepeatedInt32List {

-        get { return result.repeatedInt32_; }

+        get { return PrepareBuilder().repeatedInt32_; }

       }

       public int RepeatedInt32Count {

         get { return result.RepeatedInt32Count; }

@@ -4720,24 +4894,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedInt32(index);

       }

       public Builder SetRepeatedInt32(int index, int value) {

+        PrepareBuilder();

         result.repeatedInt32_[index] = value;

         return this;

       }

       public Builder AddRepeatedInt32(int value) {

+        PrepareBuilder();

         result.repeatedInt32_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedInt32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedInt32_);

         return this;

       }

       public Builder ClearRepeatedInt32() {

+        PrepareBuilder();

         result.repeatedInt32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> RepeatedInt64List {

-        get { return result.repeatedInt64_; }

+        get { return PrepareBuilder().repeatedInt64_; }

       }

       public int RepeatedInt64Count {

         get { return result.RepeatedInt64Count; }

@@ -4746,25 +4924,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedInt64(index);

       }

       public Builder SetRepeatedInt64(int index, long value) {

+        PrepareBuilder();

         result.repeatedInt64_[index] = value;

         return this;

       }

       public Builder AddRepeatedInt64(long value) {

+        PrepareBuilder();

         result.repeatedInt64_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedInt64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedInt64_);

         return this;

       }

       public Builder ClearRepeatedInt64() {

+        PrepareBuilder();

         result.repeatedInt64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> RepeatedUint32List {

-        get { return result.repeatedUint32_; }

+        get { return PrepareBuilder().repeatedUint32_; }

       }

       public int RepeatedUint32Count {

         get { return result.RepeatedUint32Count; }

@@ -4775,27 +4957,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedUint32(int index, uint value) {

+        PrepareBuilder();

         result.repeatedUint32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedUint32(uint value) {

+        PrepareBuilder();

         result.repeatedUint32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedUint32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedUint32_);

         return this;

       }

       public Builder ClearRepeatedUint32() {

+        PrepareBuilder();

         result.repeatedUint32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> RepeatedUint64List {

-        get { return result.repeatedUint64_; }

+        get { return PrepareBuilder().repeatedUint64_; }

       }

       public int RepeatedUint64Count {

         get { return result.RepeatedUint64Count; }

@@ -4806,26 +4992,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedUint64(int index, ulong value) {

+        PrepareBuilder();

         result.repeatedUint64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedUint64(ulong value) {

+        PrepareBuilder();

         result.repeatedUint64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedUint64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedUint64_);

         return this;

       }

       public Builder ClearRepeatedUint64() {

+        PrepareBuilder();

         result.repeatedUint64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> RepeatedSint32List {

-        get { return result.repeatedSint32_; }

+        get { return PrepareBuilder().repeatedSint32_; }

       }

       public int RepeatedSint32Count {

         get { return result.RepeatedSint32Count; }

@@ -4834,24 +5024,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedSint32(index);

       }

       public Builder SetRepeatedSint32(int index, int value) {

+        PrepareBuilder();

         result.repeatedSint32_[index] = value;

         return this;

       }

       public Builder AddRepeatedSint32(int value) {

+        PrepareBuilder();

         result.repeatedSint32_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedSint32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedSint32_);

         return this;

       }

       public Builder ClearRepeatedSint32() {

+        PrepareBuilder();

         result.repeatedSint32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> RepeatedSint64List {

-        get { return result.repeatedSint64_; }

+        get { return PrepareBuilder().repeatedSint64_; }

       }

       public int RepeatedSint64Count {

         get { return result.RepeatedSint64Count; }

@@ -4860,25 +5054,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedSint64(index);

       }

       public Builder SetRepeatedSint64(int index, long value) {

+        PrepareBuilder();

         result.repeatedSint64_[index] = value;

         return this;

       }

       public Builder AddRepeatedSint64(long value) {

+        PrepareBuilder();

         result.repeatedSint64_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedSint64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedSint64_);

         return this;

       }

       public Builder ClearRepeatedSint64() {

+        PrepareBuilder();

         result.repeatedSint64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> RepeatedFixed32List {

-        get { return result.repeatedFixed32_; }

+        get { return PrepareBuilder().repeatedFixed32_; }

       }

       public int RepeatedFixed32Count {

         get { return result.RepeatedFixed32Count; }

@@ -4889,27 +5087,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedFixed32(int index, uint value) {

+        PrepareBuilder();

         result.repeatedFixed32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedFixed32(uint value) {

+        PrepareBuilder();

         result.repeatedFixed32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedFixed32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedFixed32_);

         return this;

       }

       public Builder ClearRepeatedFixed32() {

+        PrepareBuilder();

         result.repeatedFixed32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> RepeatedFixed64List {

-        get { return result.repeatedFixed64_; }

+        get { return PrepareBuilder().repeatedFixed64_; }

       }

       public int RepeatedFixed64Count {

         get { return result.RepeatedFixed64Count; }

@@ -4920,26 +5122,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedFixed64(int index, ulong value) {

+        PrepareBuilder();

         result.repeatedFixed64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedFixed64(ulong value) {

+        PrepareBuilder();

         result.repeatedFixed64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedFixed64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedFixed64_);

         return this;

       }

       public Builder ClearRepeatedFixed64() {

+        PrepareBuilder();

         result.repeatedFixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> RepeatedSfixed32List {

-        get { return result.repeatedSfixed32_; }

+        get { return PrepareBuilder().repeatedSfixed32_; }

       }

       public int RepeatedSfixed32Count {

         get { return result.RepeatedSfixed32Count; }

@@ -4948,24 +5154,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedSfixed32(index);

       }

       public Builder SetRepeatedSfixed32(int index, int value) {

+        PrepareBuilder();

         result.repeatedSfixed32_[index] = value;

         return this;

       }

       public Builder AddRepeatedSfixed32(int value) {

+        PrepareBuilder();

         result.repeatedSfixed32_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedSfixed32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedSfixed32_);

         return this;

       }

       public Builder ClearRepeatedSfixed32() {

+        PrepareBuilder();

         result.repeatedSfixed32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> RepeatedSfixed64List {

-        get { return result.repeatedSfixed64_; }

+        get { return PrepareBuilder().repeatedSfixed64_; }

       }

       public int RepeatedSfixed64Count {

         get { return result.RepeatedSfixed64Count; }

@@ -4974,24 +5184,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedSfixed64(index);

       }

       public Builder SetRepeatedSfixed64(int index, long value) {

+        PrepareBuilder();

         result.repeatedSfixed64_[index] = value;

         return this;

       }

       public Builder AddRepeatedSfixed64(long value) {

+        PrepareBuilder();

         result.repeatedSfixed64_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedSfixed64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedSfixed64_);

         return this;

       }

       public Builder ClearRepeatedSfixed64() {

+        PrepareBuilder();

         result.repeatedSfixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<float> RepeatedFloatList {

-        get { return result.repeatedFloat_; }

+        get { return PrepareBuilder().repeatedFloat_; }

       }

       public int RepeatedFloatCount {

         get { return result.RepeatedFloatCount; }

@@ -5000,24 +5214,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedFloat(index);

       }

       public Builder SetRepeatedFloat(int index, float value) {

+        PrepareBuilder();

         result.repeatedFloat_[index] = value;

         return this;

       }

       public Builder AddRepeatedFloat(float value) {

+        PrepareBuilder();

         result.repeatedFloat_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedFloat(scg::IEnumerable<float> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedFloat_);

         return this;

       }

       public Builder ClearRepeatedFloat() {

+        PrepareBuilder();

         result.repeatedFloat_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<double> RepeatedDoubleList {

-        get { return result.repeatedDouble_; }

+        get { return PrepareBuilder().repeatedDouble_; }

       }

       public int RepeatedDoubleCount {

         get { return result.RepeatedDoubleCount; }

@@ -5026,24 +5244,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedDouble(index);

       }

       public Builder SetRepeatedDouble(int index, double value) {

+        PrepareBuilder();

         result.repeatedDouble_[index] = value;

         return this;

       }

       public Builder AddRepeatedDouble(double value) {

+        PrepareBuilder();

         result.repeatedDouble_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedDouble(scg::IEnumerable<double> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedDouble_);

         return this;

       }

       public Builder ClearRepeatedDouble() {

+        PrepareBuilder();

         result.repeatedDouble_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<bool> RepeatedBoolList {

-        get { return result.repeatedBool_; }

+        get { return PrepareBuilder().repeatedBool_; }

       }

       public int RepeatedBoolCount {

         get { return result.RepeatedBoolCount; }

@@ -5052,24 +5274,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedBool(index);

       }

       public Builder SetRepeatedBool(int index, bool value) {

+        PrepareBuilder();

         result.repeatedBool_[index] = value;

         return this;

       }

       public Builder AddRepeatedBool(bool value) {

+        PrepareBuilder();

         result.repeatedBool_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedBool(scg::IEnumerable<bool> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedBool_);

         return this;

       }

       public Builder ClearRepeatedBool() {

+        PrepareBuilder();

         result.repeatedBool_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedStringList {

-        get { return result.repeatedString_; }

+        get { return PrepareBuilder().repeatedString_; }

       }

       public int RepeatedStringCount {

         get { return result.RepeatedStringCount; }

@@ -5079,25 +5305,29 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedString(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedString_[index] = value;

         return this;

       }

       public Builder AddRepeatedString(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedString_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedString(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedString_);

         return this;

       }

       public Builder ClearRepeatedString() {

+        PrepareBuilder();

         result.repeatedString_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<pb::ByteString> RepeatedBytesList {

-        get { return result.repeatedBytes_; }

+        get { return PrepareBuilder().repeatedBytes_; }

       }

       public int RepeatedBytesCount {

         get { return result.RepeatedBytesCount; }

@@ -5107,25 +5337,29 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedBytes(int index, pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedBytes_[index] = value;

         return this;

       }

       public Builder AddRepeatedBytes(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedBytes_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedBytes(scg::IEnumerable<pb::ByteString> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedBytes_);

         return this;

       }

       public Builder ClearRepeatedBytes() {

+        PrepareBuilder();

         result.repeatedBytes_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.RepeatedGroup> RepeatedGroupList {

-        get { return result.repeatedGroup_; }

+        get { return PrepareBuilder().repeatedGroup_; }

       }

       public int RepeatedGroupCount {

         get { return result.RepeatedGroupCount; }

@@ -5135,35 +5369,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedGroup(int index, global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.RepeatedGroup value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedGroup_[index] = value;

         return this;

       }

       public Builder SetRepeatedGroup(int index, global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.RepeatedGroup.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedGroup_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedGroup(global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.RepeatedGroup value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedGroup_.Add(value);

         return this;

       }

       public Builder AddRepeatedGroup(global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.RepeatedGroup.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedGroup_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedGroup(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.RepeatedGroup> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedGroup_);

         return this;

       }

       public Builder ClearRepeatedGroup() {

+        PrepareBuilder();

         result.repeatedGroup_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage> RepeatedNestedMessageList {

-        get { return result.repeatedNestedMessage_; }

+        get { return PrepareBuilder().repeatedNestedMessage_; }

       }

       public int RepeatedNestedMessageCount {

         get { return result.RepeatedNestedMessageCount; }

@@ -5173,35 +5413,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedNestedMessage(int index, global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedNestedMessage_[index] = value;

         return this;

       }

       public Builder SetRepeatedNestedMessage(int index, global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedNestedMessage_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedNestedMessage_.Add(value);

         return this;

       }

       public Builder AddRepeatedNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedNestedMessage_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedNestedMessage(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedNestedMessage_);

         return this;

       }

       public Builder ClearRepeatedNestedMessage() {

+        PrepareBuilder();

         result.repeatedNestedMessage_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite> RepeatedForeignMessageList {

-        get { return result.repeatedForeignMessage_; }

+        get { return PrepareBuilder().repeatedForeignMessage_; }

       }

       public int RepeatedForeignMessageCount {

         get { return result.RepeatedForeignMessageCount; }

@@ -5211,35 +5457,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedForeignMessage(int index, global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedForeignMessage_[index] = value;

         return this;

       }

       public Builder SetRepeatedForeignMessage(int index, global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedForeignMessage_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedForeignMessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedForeignMessage_.Add(value);

         return this;

       }

       public Builder AddRepeatedForeignMessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedForeignMessage_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedForeignMessage(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedForeignMessage_);

         return this;

       }

       public Builder ClearRepeatedForeignMessage() {

+        PrepareBuilder();

         result.repeatedForeignMessage_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ImportMessageLite> RepeatedImportMessageList {

-        get { return result.repeatedImportMessage_; }

+        get { return PrepareBuilder().repeatedImportMessage_; }

       }

       public int RepeatedImportMessageCount {

         get { return result.RepeatedImportMessageCount; }

@@ -5249,35 +5501,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedImportMessage(int index, global::Google.ProtocolBuffers.TestProtos.ImportMessageLite value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedImportMessage_[index] = value;

         return this;

       }

       public Builder SetRepeatedImportMessage(int index, global::Google.ProtocolBuffers.TestProtos.ImportMessageLite.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedImportMessage_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedImportMessage(global::Google.ProtocolBuffers.TestProtos.ImportMessageLite value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedImportMessage_.Add(value);

         return this;

       }

       public Builder AddRepeatedImportMessage(global::Google.ProtocolBuffers.TestProtos.ImportMessageLite.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedImportMessage_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedImportMessage(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ImportMessageLite> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedImportMessage_);

         return this;

       }

       public Builder ClearRepeatedImportMessage() {

+        PrepareBuilder();

         result.repeatedImportMessage_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedEnum> RepeatedNestedEnumList {

-        get { return result.repeatedNestedEnum_; }

+        get { return PrepareBuilder().repeatedNestedEnum_; }

       }

       public int RepeatedNestedEnumCount {

         get { return result.RepeatedNestedEnumCount; }

@@ -5286,24 +5544,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedNestedEnum(index);

       }

       public Builder SetRepeatedNestedEnum(int index, global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedEnum value) {

+        PrepareBuilder();

         result.repeatedNestedEnum_[index] = value;

         return this;

       }

       public Builder AddRepeatedNestedEnum(global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedEnum value) {

+        PrepareBuilder();

         result.repeatedNestedEnum_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedNestedEnum(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedEnum> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedNestedEnum_);

         return this;

       }

       public Builder ClearRepeatedNestedEnum() {

+        PrepareBuilder();

         result.repeatedNestedEnum_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite> RepeatedForeignEnumList {

-        get { return result.repeatedForeignEnum_; }

+        get { return PrepareBuilder().repeatedForeignEnum_; }

       }

       public int RepeatedForeignEnumCount {

         get { return result.RepeatedForeignEnumCount; }

@@ -5312,24 +5574,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedForeignEnum(index);

       }

       public Builder SetRepeatedForeignEnum(int index, global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite value) {

+        PrepareBuilder();

         result.repeatedForeignEnum_[index] = value;

         return this;

       }

       public Builder AddRepeatedForeignEnum(global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite value) {

+        PrepareBuilder();

         result.repeatedForeignEnum_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedForeignEnum(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedForeignEnum_);

         return this;

       }

       public Builder ClearRepeatedForeignEnum() {

+        PrepareBuilder();

         result.repeatedForeignEnum_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ImportEnumLite> RepeatedImportEnumList {

-        get { return result.repeatedImportEnum_; }

+        get { return PrepareBuilder().repeatedImportEnum_; }

       }

       public int RepeatedImportEnumCount {

         get { return result.RepeatedImportEnumCount; }

@@ -5338,24 +5604,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedImportEnum(index);

       }

       public Builder SetRepeatedImportEnum(int index, global::Google.ProtocolBuffers.TestProtos.ImportEnumLite value) {

+        PrepareBuilder();

         result.repeatedImportEnum_[index] = value;

         return this;

       }

       public Builder AddRepeatedImportEnum(global::Google.ProtocolBuffers.TestProtos.ImportEnumLite value) {

+        PrepareBuilder();

         result.repeatedImportEnum_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedImportEnum(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ImportEnumLite> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedImportEnum_);

         return this;

       }

       public Builder ClearRepeatedImportEnum() {

+        PrepareBuilder();

         result.repeatedImportEnum_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedStringPieceList {

-        get { return result.repeatedStringPiece_; }

+        get { return PrepareBuilder().repeatedStringPiece_; }

       }

       public int RepeatedStringPieceCount {

         get { return result.RepeatedStringPieceCount; }

@@ -5365,25 +5635,29 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedStringPiece(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedStringPiece_[index] = value;

         return this;

       }

       public Builder AddRepeatedStringPiece(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedStringPiece_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedStringPiece(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedStringPiece_);

         return this;

       }

       public Builder ClearRepeatedStringPiece() {

+        PrepareBuilder();

         result.repeatedStringPiece_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedCordList {

-        get { return result.repeatedCord_; }

+        get { return PrepareBuilder().repeatedCord_; }

       }

       public int RepeatedCordCount {

         get { return result.RepeatedCordCount; }

@@ -5393,19 +5667,23 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedCord(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedCord_[index] = value;

         return this;

       }

       public Builder AddRepeatedCord(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedCord_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedCord(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedCord_);

         return this;

       }

       public Builder ClearRepeatedCord() {

+        PrepareBuilder();

         result.repeatedCord_.Clear();

         return this;

       }

@@ -5418,11 +5696,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultInt32(value); }

       }

       public Builder SetDefaultInt32(int value) {

+        PrepareBuilder();

         result.hasDefaultInt32 = true;

         result.defaultInt32_ = value;

         return this;

       }

       public Builder ClearDefaultInt32() {

+        PrepareBuilder();

         result.hasDefaultInt32 = false;

         result.defaultInt32_ = 41;

         return this;

@@ -5436,11 +5716,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultInt64(value); }

       }

       public Builder SetDefaultInt64(long value) {

+        PrepareBuilder();

         result.hasDefaultInt64 = true;

         result.defaultInt64_ = value;

         return this;

       }

       public Builder ClearDefaultInt64() {

+        PrepareBuilder();

         result.hasDefaultInt64 = false;

         result.defaultInt64_ = 42L;

         return this;

@@ -5456,11 +5738,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetDefaultUint32(uint value) {

+        PrepareBuilder();

         result.hasDefaultUint32 = true;

         result.defaultUint32_ = value;

         return this;

       }

       public Builder ClearDefaultUint32() {

+        PrepareBuilder();

         result.hasDefaultUint32 = false;

         result.defaultUint32_ = 43;

         return this;

@@ -5476,11 +5760,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetDefaultUint64(ulong value) {

+        PrepareBuilder();

         result.hasDefaultUint64 = true;

         result.defaultUint64_ = value;

         return this;

       }

       public Builder ClearDefaultUint64() {

+        PrepareBuilder();

         result.hasDefaultUint64 = false;

         result.defaultUint64_ = 44UL;

         return this;

@@ -5494,11 +5780,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultSint32(value); }

       }

       public Builder SetDefaultSint32(int value) {

+        PrepareBuilder();

         result.hasDefaultSint32 = true;

         result.defaultSint32_ = value;

         return this;

       }

       public Builder ClearDefaultSint32() {

+        PrepareBuilder();

         result.hasDefaultSint32 = false;

         result.defaultSint32_ = -45;

         return this;

@@ -5512,11 +5800,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultSint64(value); }

       }

       public Builder SetDefaultSint64(long value) {

+        PrepareBuilder();

         result.hasDefaultSint64 = true;

         result.defaultSint64_ = value;

         return this;

       }

       public Builder ClearDefaultSint64() {

+        PrepareBuilder();

         result.hasDefaultSint64 = false;

         result.defaultSint64_ = 46;

         return this;

@@ -5532,11 +5822,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetDefaultFixed32(uint value) {

+        PrepareBuilder();

         result.hasDefaultFixed32 = true;

         result.defaultFixed32_ = value;

         return this;

       }

       public Builder ClearDefaultFixed32() {

+        PrepareBuilder();

         result.hasDefaultFixed32 = false;

         result.defaultFixed32_ = 47;

         return this;

@@ -5552,11 +5844,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetDefaultFixed64(ulong value) {

+        PrepareBuilder();

         result.hasDefaultFixed64 = true;

         result.defaultFixed64_ = value;

         return this;

       }

       public Builder ClearDefaultFixed64() {

+        PrepareBuilder();

         result.hasDefaultFixed64 = false;

         result.defaultFixed64_ = 48;

         return this;

@@ -5570,11 +5864,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultSfixed32(value); }

       }

       public Builder SetDefaultSfixed32(int value) {

+        PrepareBuilder();

         result.hasDefaultSfixed32 = true;

         result.defaultSfixed32_ = value;

         return this;

       }

       public Builder ClearDefaultSfixed32() {

+        PrepareBuilder();

         result.hasDefaultSfixed32 = false;

         result.defaultSfixed32_ = 49;

         return this;

@@ -5588,11 +5884,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultSfixed64(value); }

       }

       public Builder SetDefaultSfixed64(long value) {

+        PrepareBuilder();

         result.hasDefaultSfixed64 = true;

         result.defaultSfixed64_ = value;

         return this;

       }

       public Builder ClearDefaultSfixed64() {

+        PrepareBuilder();

         result.hasDefaultSfixed64 = false;

         result.defaultSfixed64_ = -50;

         return this;

@@ -5606,11 +5904,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultFloat(value); }

       }

       public Builder SetDefaultFloat(float value) {

+        PrepareBuilder();

         result.hasDefaultFloat = true;

         result.defaultFloat_ = value;

         return this;

       }

       public Builder ClearDefaultFloat() {

+        PrepareBuilder();

         result.hasDefaultFloat = false;

         result.defaultFloat_ = 51.5F;

         return this;

@@ -5624,11 +5924,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultDouble(value); }

       }

       public Builder SetDefaultDouble(double value) {

+        PrepareBuilder();

         result.hasDefaultDouble = true;

         result.defaultDouble_ = value;

         return this;

       }

       public Builder ClearDefaultDouble() {

+        PrepareBuilder();

         result.hasDefaultDouble = false;

         result.defaultDouble_ = 52000D;

         return this;

@@ -5642,11 +5944,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultBool(value); }

       }

       public Builder SetDefaultBool(bool value) {

+        PrepareBuilder();

         result.hasDefaultBool = true;

         result.defaultBool_ = value;

         return this;

       }

       public Builder ClearDefaultBool() {

+        PrepareBuilder();

         result.hasDefaultBool = false;

         result.defaultBool_ = true;

         return this;

@@ -5661,11 +5965,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetDefaultString(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasDefaultString = true;

         result.defaultString_ = value;

         return this;

       }

       public Builder ClearDefaultString() {

+        PrepareBuilder();

         result.hasDefaultString = false;

         result.defaultString_ = "hello";

         return this;

@@ -5680,11 +5986,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetDefaultBytes(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasDefaultBytes = true;

         result.defaultBytes_ = value;

         return this;

       }

       public Builder ClearDefaultBytes() {

+        PrepareBuilder();

         result.hasDefaultBytes = false;

         result.defaultBytes_ = pb::ByteString.FromBase64("d29ybGQ=");

         return this;

@@ -5698,11 +6006,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultNestedEnum(value); }

       }

       public Builder SetDefaultNestedEnum(global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedEnum value) {

+        PrepareBuilder();

         result.hasDefaultNestedEnum = true;

         result.defaultNestedEnum_ = value;

         return this;

       }

       public Builder ClearDefaultNestedEnum() {

+        PrepareBuilder();

         result.hasDefaultNestedEnum = false;

         result.defaultNestedEnum_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedEnum.BAR;

         return this;

@@ -5716,11 +6026,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultForeignEnum(value); }

       }

       public Builder SetDefaultForeignEnum(global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite value) {

+        PrepareBuilder();

         result.hasDefaultForeignEnum = true;

         result.defaultForeignEnum_ = value;

         return this;

       }

       public Builder ClearDefaultForeignEnum() {

+        PrepareBuilder();

         result.hasDefaultForeignEnum = false;

         result.defaultForeignEnum_ = global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite.FOREIGN_LITE_BAR;

         return this;

@@ -5734,11 +6046,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultImportEnum(value); }

       }

       public Builder SetDefaultImportEnum(global::Google.ProtocolBuffers.TestProtos.ImportEnumLite value) {

+        PrepareBuilder();

         result.hasDefaultImportEnum = true;

         result.defaultImportEnum_ = value;

         return this;

       }

       public Builder ClearDefaultImportEnum() {

+        PrepareBuilder();

         result.hasDefaultImportEnum = false;

         result.defaultImportEnum_ = global::Google.ProtocolBuffers.TestProtos.ImportEnumLite.IMPORT_LITE_BAR;

         return this;

@@ -5753,11 +6067,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetDefaultStringPiece(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasDefaultStringPiece = true;

         result.defaultStringPiece_ = value;

         return this;

       }

       public Builder ClearDefaultStringPiece() {

+        PrepareBuilder();

         result.hasDefaultStringPiece = false;

         result.defaultStringPiece_ = "abc";

         return this;

@@ -5772,11 +6088,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetDefaultCord(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasDefaultCord = true;

         result.defaultCord_ = value;

         return this;

       }

       public Builder ClearDefaultCord() {

+        PrepareBuilder();

         result.hasDefaultCord = false;

         result.defaultCord_ = "123";

         return this;

@@ -5898,7 +6216,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(ForeignMessageLite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -5908,21 +6226,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new ForeignMessageLite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(ForeignMessageLite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      ForeignMessageLite result;

+      

+      private ForeignMessageLite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          ForeignMessageLite original = result;

+          result = new ForeignMessageLite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      ForeignMessageLite result = new ForeignMessageLite();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override ForeignMessageLite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new ForeignMessageLite();

+        result = DefaultInstance ?? new ForeignMessageLite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override ForeignMessageLite DefaultInstanceForType {

@@ -5930,12 +6275,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ForeignMessageLite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        ForeignMessageLite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -5949,6 +6293,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(ForeignMessageLite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasC) {

           C = other.C;

         }

@@ -5960,6 +6305,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -6002,11 +6348,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetC(value); }

       }

       public Builder SetC(int value) {

+        PrepareBuilder();

         result.hasC = true;

         result.c_ = value;

         return this;

       }

       public Builder ClearC() {

+        PrepareBuilder();

         result.hasC = false;

         result.c_ = 0;

         return this;

@@ -6566,7 +6914,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestPackedTypesLite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -6576,21 +6924,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestPackedTypesLite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestPackedTypesLite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestPackedTypesLite result;

       

-      TestPackedTypesLite result = new TestPackedTypesLite();

+      private TestPackedTypesLite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestPackedTypesLite original = result;

+          result = new TestPackedTypesLite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestPackedTypesLite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestPackedTypesLite();

+        result = DefaultInstance ?? new TestPackedTypesLite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override TestPackedTypesLite DefaultInstanceForType {

@@ -6598,8 +6973,8 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestPackedTypesLite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.packedInt32_.MakeReadOnly();

         result.packedInt64_.MakeReadOnly();

@@ -6615,9 +6990,8 @@ namespace Google.ProtocolBuffers.TestProtos {
         result.packedDouble_.MakeReadOnly();

         result.packedBool_.MakeReadOnly();

         result.packedEnum_.MakeReadOnly();

-        TestPackedTypesLite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -6631,6 +7005,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestPackedTypesLite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestPackedTypesLite.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.packedInt32_.Count != 0) {

           base.AddRange(other.packedInt32_, result.packedInt32_);

         }

@@ -6681,6 +7056,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -6783,7 +7159,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       

       public pbc::IPopsicleList<int> PackedInt32List {

-        get { return result.packedInt32_; }

+        get { return PrepareBuilder().packedInt32_; }

       }

       public int PackedInt32Count {

         get { return result.PackedInt32Count; }

@@ -6792,24 +7168,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedInt32(index);

       }

       public Builder SetPackedInt32(int index, int value) {

+        PrepareBuilder();

         result.packedInt32_[index] = value;

         return this;

       }

       public Builder AddPackedInt32(int value) {

+        PrepareBuilder();

         result.packedInt32_.Add(value);

         return this;

       }

       public Builder AddRangePackedInt32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedInt32_);

         return this;

       }

       public Builder ClearPackedInt32() {

+        PrepareBuilder();

         result.packedInt32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> PackedInt64List {

-        get { return result.packedInt64_; }

+        get { return PrepareBuilder().packedInt64_; }

       }

       public int PackedInt64Count {

         get { return result.PackedInt64Count; }

@@ -6818,25 +7198,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedInt64(index);

       }

       public Builder SetPackedInt64(int index, long value) {

+        PrepareBuilder();

         result.packedInt64_[index] = value;

         return this;

       }

       public Builder AddPackedInt64(long value) {

+        PrepareBuilder();

         result.packedInt64_.Add(value);

         return this;

       }

       public Builder AddRangePackedInt64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedInt64_);

         return this;

       }

       public Builder ClearPackedInt64() {

+        PrepareBuilder();

         result.packedInt64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> PackedUint32List {

-        get { return result.packedUint32_; }

+        get { return PrepareBuilder().packedUint32_; }

       }

       public int PackedUint32Count {

         get { return result.PackedUint32Count; }

@@ -6847,27 +7231,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetPackedUint32(int index, uint value) {

+        PrepareBuilder();

         result.packedUint32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddPackedUint32(uint value) {

+        PrepareBuilder();

         result.packedUint32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangePackedUint32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedUint32_);

         return this;

       }

       public Builder ClearPackedUint32() {

+        PrepareBuilder();

         result.packedUint32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> PackedUint64List {

-        get { return result.packedUint64_; }

+        get { return PrepareBuilder().packedUint64_; }

       }

       public int PackedUint64Count {

         get { return result.PackedUint64Count; }

@@ -6878,26 +7266,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetPackedUint64(int index, ulong value) {

+        PrepareBuilder();

         result.packedUint64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddPackedUint64(ulong value) {

+        PrepareBuilder();

         result.packedUint64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangePackedUint64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedUint64_);

         return this;

       }

       public Builder ClearPackedUint64() {

+        PrepareBuilder();

         result.packedUint64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> PackedSint32List {

-        get { return result.packedSint32_; }

+        get { return PrepareBuilder().packedSint32_; }

       }

       public int PackedSint32Count {

         get { return result.PackedSint32Count; }

@@ -6906,24 +7298,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedSint32(index);

       }

       public Builder SetPackedSint32(int index, int value) {

+        PrepareBuilder();

         result.packedSint32_[index] = value;

         return this;

       }

       public Builder AddPackedSint32(int value) {

+        PrepareBuilder();

         result.packedSint32_.Add(value);

         return this;

       }

       public Builder AddRangePackedSint32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedSint32_);

         return this;

       }

       public Builder ClearPackedSint32() {

+        PrepareBuilder();

         result.packedSint32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> PackedSint64List {

-        get { return result.packedSint64_; }

+        get { return PrepareBuilder().packedSint64_; }

       }

       public int PackedSint64Count {

         get { return result.PackedSint64Count; }

@@ -6932,25 +7328,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedSint64(index);

       }

       public Builder SetPackedSint64(int index, long value) {

+        PrepareBuilder();

         result.packedSint64_[index] = value;

         return this;

       }

       public Builder AddPackedSint64(long value) {

+        PrepareBuilder();

         result.packedSint64_.Add(value);

         return this;

       }

       public Builder AddRangePackedSint64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedSint64_);

         return this;

       }

       public Builder ClearPackedSint64() {

+        PrepareBuilder();

         result.packedSint64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> PackedFixed32List {

-        get { return result.packedFixed32_; }

+        get { return PrepareBuilder().packedFixed32_; }

       }

       public int PackedFixed32Count {

         get { return result.PackedFixed32Count; }

@@ -6961,27 +7361,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetPackedFixed32(int index, uint value) {

+        PrepareBuilder();

         result.packedFixed32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddPackedFixed32(uint value) {

+        PrepareBuilder();

         result.packedFixed32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangePackedFixed32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedFixed32_);

         return this;

       }

       public Builder ClearPackedFixed32() {

+        PrepareBuilder();

         result.packedFixed32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> PackedFixed64List {

-        get { return result.packedFixed64_; }

+        get { return PrepareBuilder().packedFixed64_; }

       }

       public int PackedFixed64Count {

         get { return result.PackedFixed64Count; }

@@ -6992,26 +7396,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetPackedFixed64(int index, ulong value) {

+        PrepareBuilder();

         result.packedFixed64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddPackedFixed64(ulong value) {

+        PrepareBuilder();

         result.packedFixed64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangePackedFixed64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedFixed64_);

         return this;

       }

       public Builder ClearPackedFixed64() {

+        PrepareBuilder();

         result.packedFixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> PackedSfixed32List {

-        get { return result.packedSfixed32_; }

+        get { return PrepareBuilder().packedSfixed32_; }

       }

       public int PackedSfixed32Count {

         get { return result.PackedSfixed32Count; }

@@ -7020,24 +7428,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedSfixed32(index);

       }

       public Builder SetPackedSfixed32(int index, int value) {

+        PrepareBuilder();

         result.packedSfixed32_[index] = value;

         return this;

       }

       public Builder AddPackedSfixed32(int value) {

+        PrepareBuilder();

         result.packedSfixed32_.Add(value);

         return this;

       }

       public Builder AddRangePackedSfixed32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedSfixed32_);

         return this;

       }

       public Builder ClearPackedSfixed32() {

+        PrepareBuilder();

         result.packedSfixed32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> PackedSfixed64List {

-        get { return result.packedSfixed64_; }

+        get { return PrepareBuilder().packedSfixed64_; }

       }

       public int PackedSfixed64Count {

         get { return result.PackedSfixed64Count; }

@@ -7046,24 +7458,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedSfixed64(index);

       }

       public Builder SetPackedSfixed64(int index, long value) {

+        PrepareBuilder();

         result.packedSfixed64_[index] = value;

         return this;

       }

       public Builder AddPackedSfixed64(long value) {

+        PrepareBuilder();

         result.packedSfixed64_.Add(value);

         return this;

       }

       public Builder AddRangePackedSfixed64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedSfixed64_);

         return this;

       }

       public Builder ClearPackedSfixed64() {

+        PrepareBuilder();

         result.packedSfixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<float> PackedFloatList {

-        get { return result.packedFloat_; }

+        get { return PrepareBuilder().packedFloat_; }

       }

       public int PackedFloatCount {

         get { return result.PackedFloatCount; }

@@ -7072,24 +7488,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedFloat(index);

       }

       public Builder SetPackedFloat(int index, float value) {

+        PrepareBuilder();

         result.packedFloat_[index] = value;

         return this;

       }

       public Builder AddPackedFloat(float value) {

+        PrepareBuilder();

         result.packedFloat_.Add(value);

         return this;

       }

       public Builder AddRangePackedFloat(scg::IEnumerable<float> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedFloat_);

         return this;

       }

       public Builder ClearPackedFloat() {

+        PrepareBuilder();

         result.packedFloat_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<double> PackedDoubleList {

-        get { return result.packedDouble_; }

+        get { return PrepareBuilder().packedDouble_; }

       }

       public int PackedDoubleCount {

         get { return result.PackedDoubleCount; }

@@ -7098,24 +7518,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedDouble(index);

       }

       public Builder SetPackedDouble(int index, double value) {

+        PrepareBuilder();

         result.packedDouble_[index] = value;

         return this;

       }

       public Builder AddPackedDouble(double value) {

+        PrepareBuilder();

         result.packedDouble_.Add(value);

         return this;

       }

       public Builder AddRangePackedDouble(scg::IEnumerable<double> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedDouble_);

         return this;

       }

       public Builder ClearPackedDouble() {

+        PrepareBuilder();

         result.packedDouble_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<bool> PackedBoolList {

-        get { return result.packedBool_; }

+        get { return PrepareBuilder().packedBool_; }

       }

       public int PackedBoolCount {

         get { return result.PackedBoolCount; }

@@ -7124,24 +7548,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedBool(index);

       }

       public Builder SetPackedBool(int index, bool value) {

+        PrepareBuilder();

         result.packedBool_[index] = value;

         return this;

       }

       public Builder AddPackedBool(bool value) {

+        PrepareBuilder();

         result.packedBool_.Add(value);

         return this;

       }

       public Builder AddRangePackedBool(scg::IEnumerable<bool> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedBool_);

         return this;

       }

       public Builder ClearPackedBool() {

+        PrepareBuilder();

         result.packedBool_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite> PackedEnumList {

-        get { return result.packedEnum_; }

+        get { return PrepareBuilder().packedEnum_; }

       }

       public int PackedEnumCount {

         get { return result.PackedEnumCount; }

@@ -7150,18 +7578,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedEnum(index);

       }

       public Builder SetPackedEnum(int index, global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite value) {

+        PrepareBuilder();

         result.packedEnum_[index] = value;

         return this;

       }

       public Builder AddPackedEnum(global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite value) {

+        PrepareBuilder();

         result.packedEnum_.Add(value);

         return this;

       }

       public Builder AddRangePackedEnum(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedEnum_);

         return this;

       }

       public Builder ClearPackedEnum() {

+        PrepareBuilder();

         result.packedEnum_.Clear();

         return this;

       }

@@ -7665,7 +8097,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestUnpackedTypesLite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -7675,21 +8107,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestUnpackedTypesLite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestUnpackedTypesLite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestUnpackedTypesLite result = new TestUnpackedTypesLite();

+      bool builderIsReadOnly;

+      TestUnpackedTypesLite result;

+      

+      private TestUnpackedTypesLite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestUnpackedTypesLite original = result;

+          result = new TestUnpackedTypesLite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestUnpackedTypesLite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestUnpackedTypesLite();

+        result = DefaultInstance ?? new TestUnpackedTypesLite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override TestUnpackedTypesLite DefaultInstanceForType {

@@ -7697,8 +8156,8 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestUnpackedTypesLite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.unpackedInt32_.MakeReadOnly();

         result.unpackedInt64_.MakeReadOnly();

@@ -7714,9 +8173,8 @@ namespace Google.ProtocolBuffers.TestProtos {
         result.unpackedDouble_.MakeReadOnly();

         result.unpackedBool_.MakeReadOnly();

         result.unpackedEnum_.MakeReadOnly();

-        TestUnpackedTypesLite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -7730,6 +8188,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestUnpackedTypesLite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestUnpackedTypesLite.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.unpackedInt32_.Count != 0) {

           base.AddRange(other.unpackedInt32_, result.unpackedInt32_);

         }

@@ -7780,6 +8239,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -7882,7 +8342,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       

       public pbc::IPopsicleList<int> UnpackedInt32List {

-        get { return result.unpackedInt32_; }

+        get { return PrepareBuilder().unpackedInt32_; }

       }

       public int UnpackedInt32Count {

         get { return result.UnpackedInt32Count; }

@@ -7891,24 +8351,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedInt32(index);

       }

       public Builder SetUnpackedInt32(int index, int value) {

+        PrepareBuilder();

         result.unpackedInt32_[index] = value;

         return this;

       }

       public Builder AddUnpackedInt32(int value) {

+        PrepareBuilder();

         result.unpackedInt32_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedInt32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedInt32_);

         return this;

       }

       public Builder ClearUnpackedInt32() {

+        PrepareBuilder();

         result.unpackedInt32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> UnpackedInt64List {

-        get { return result.unpackedInt64_; }

+        get { return PrepareBuilder().unpackedInt64_; }

       }

       public int UnpackedInt64Count {

         get { return result.UnpackedInt64Count; }

@@ -7917,25 +8381,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedInt64(index);

       }

       public Builder SetUnpackedInt64(int index, long value) {

+        PrepareBuilder();

         result.unpackedInt64_[index] = value;

         return this;

       }

       public Builder AddUnpackedInt64(long value) {

+        PrepareBuilder();

         result.unpackedInt64_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedInt64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedInt64_);

         return this;

       }

       public Builder ClearUnpackedInt64() {

+        PrepareBuilder();

         result.unpackedInt64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> UnpackedUint32List {

-        get { return result.unpackedUint32_; }

+        get { return PrepareBuilder().unpackedUint32_; }

       }

       public int UnpackedUint32Count {

         get { return result.UnpackedUint32Count; }

@@ -7946,27 +8414,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetUnpackedUint32(int index, uint value) {

+        PrepareBuilder();

         result.unpackedUint32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddUnpackedUint32(uint value) {

+        PrepareBuilder();

         result.unpackedUint32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeUnpackedUint32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedUint32_);

         return this;

       }

       public Builder ClearUnpackedUint32() {

+        PrepareBuilder();

         result.unpackedUint32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> UnpackedUint64List {

-        get { return result.unpackedUint64_; }

+        get { return PrepareBuilder().unpackedUint64_; }

       }

       public int UnpackedUint64Count {

         get { return result.UnpackedUint64Count; }

@@ -7977,26 +8449,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetUnpackedUint64(int index, ulong value) {

+        PrepareBuilder();

         result.unpackedUint64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddUnpackedUint64(ulong value) {

+        PrepareBuilder();

         result.unpackedUint64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeUnpackedUint64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedUint64_);

         return this;

       }

       public Builder ClearUnpackedUint64() {

+        PrepareBuilder();

         result.unpackedUint64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> UnpackedSint32List {

-        get { return result.unpackedSint32_; }

+        get { return PrepareBuilder().unpackedSint32_; }

       }

       public int UnpackedSint32Count {

         get { return result.UnpackedSint32Count; }

@@ -8005,24 +8481,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedSint32(index);

       }

       public Builder SetUnpackedSint32(int index, int value) {

+        PrepareBuilder();

         result.unpackedSint32_[index] = value;

         return this;

       }

       public Builder AddUnpackedSint32(int value) {

+        PrepareBuilder();

         result.unpackedSint32_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedSint32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedSint32_);

         return this;

       }

       public Builder ClearUnpackedSint32() {

+        PrepareBuilder();

         result.unpackedSint32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> UnpackedSint64List {

-        get { return result.unpackedSint64_; }

+        get { return PrepareBuilder().unpackedSint64_; }

       }

       public int UnpackedSint64Count {

         get { return result.UnpackedSint64Count; }

@@ -8031,25 +8511,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedSint64(index);

       }

       public Builder SetUnpackedSint64(int index, long value) {

+        PrepareBuilder();

         result.unpackedSint64_[index] = value;

         return this;

       }

       public Builder AddUnpackedSint64(long value) {

+        PrepareBuilder();

         result.unpackedSint64_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedSint64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedSint64_);

         return this;

       }

       public Builder ClearUnpackedSint64() {

+        PrepareBuilder();

         result.unpackedSint64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> UnpackedFixed32List {

-        get { return result.unpackedFixed32_; }

+        get { return PrepareBuilder().unpackedFixed32_; }

       }

       public int UnpackedFixed32Count {

         get { return result.UnpackedFixed32Count; }

@@ -8060,27 +8544,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetUnpackedFixed32(int index, uint value) {

+        PrepareBuilder();

         result.unpackedFixed32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddUnpackedFixed32(uint value) {

+        PrepareBuilder();

         result.unpackedFixed32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeUnpackedFixed32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedFixed32_);

         return this;

       }

       public Builder ClearUnpackedFixed32() {

+        PrepareBuilder();

         result.unpackedFixed32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> UnpackedFixed64List {

-        get { return result.unpackedFixed64_; }

+        get { return PrepareBuilder().unpackedFixed64_; }

       }

       public int UnpackedFixed64Count {

         get { return result.UnpackedFixed64Count; }

@@ -8091,26 +8579,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetUnpackedFixed64(int index, ulong value) {

+        PrepareBuilder();

         result.unpackedFixed64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddUnpackedFixed64(ulong value) {

+        PrepareBuilder();

         result.unpackedFixed64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeUnpackedFixed64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedFixed64_);

         return this;

       }

       public Builder ClearUnpackedFixed64() {

+        PrepareBuilder();

         result.unpackedFixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> UnpackedSfixed32List {

-        get { return result.unpackedSfixed32_; }

+        get { return PrepareBuilder().unpackedSfixed32_; }

       }

       public int UnpackedSfixed32Count {

         get { return result.UnpackedSfixed32Count; }

@@ -8119,24 +8611,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedSfixed32(index);

       }

       public Builder SetUnpackedSfixed32(int index, int value) {

+        PrepareBuilder();

         result.unpackedSfixed32_[index] = value;

         return this;

       }

       public Builder AddUnpackedSfixed32(int value) {

+        PrepareBuilder();

         result.unpackedSfixed32_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedSfixed32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedSfixed32_);

         return this;

       }

       public Builder ClearUnpackedSfixed32() {

+        PrepareBuilder();

         result.unpackedSfixed32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> UnpackedSfixed64List {

-        get { return result.unpackedSfixed64_; }

+        get { return PrepareBuilder().unpackedSfixed64_; }

       }

       public int UnpackedSfixed64Count {

         get { return result.UnpackedSfixed64Count; }

@@ -8145,24 +8641,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedSfixed64(index);

       }

       public Builder SetUnpackedSfixed64(int index, long value) {

+        PrepareBuilder();

         result.unpackedSfixed64_[index] = value;

         return this;

       }

       public Builder AddUnpackedSfixed64(long value) {

+        PrepareBuilder();

         result.unpackedSfixed64_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedSfixed64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedSfixed64_);

         return this;

       }

       public Builder ClearUnpackedSfixed64() {

+        PrepareBuilder();

         result.unpackedSfixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<float> UnpackedFloatList {

-        get { return result.unpackedFloat_; }

+        get { return PrepareBuilder().unpackedFloat_; }

       }

       public int UnpackedFloatCount {

         get { return result.UnpackedFloatCount; }

@@ -8171,24 +8671,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedFloat(index);

       }

       public Builder SetUnpackedFloat(int index, float value) {

+        PrepareBuilder();

         result.unpackedFloat_[index] = value;

         return this;

       }

       public Builder AddUnpackedFloat(float value) {

+        PrepareBuilder();

         result.unpackedFloat_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedFloat(scg::IEnumerable<float> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedFloat_);

         return this;

       }

       public Builder ClearUnpackedFloat() {

+        PrepareBuilder();

         result.unpackedFloat_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<double> UnpackedDoubleList {

-        get { return result.unpackedDouble_; }

+        get { return PrepareBuilder().unpackedDouble_; }

       }

       public int UnpackedDoubleCount {

         get { return result.UnpackedDoubleCount; }

@@ -8197,24 +8701,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedDouble(index);

       }

       public Builder SetUnpackedDouble(int index, double value) {

+        PrepareBuilder();

         result.unpackedDouble_[index] = value;

         return this;

       }

       public Builder AddUnpackedDouble(double value) {

+        PrepareBuilder();

         result.unpackedDouble_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedDouble(scg::IEnumerable<double> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedDouble_);

         return this;

       }

       public Builder ClearUnpackedDouble() {

+        PrepareBuilder();

         result.unpackedDouble_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<bool> UnpackedBoolList {

-        get { return result.unpackedBool_; }

+        get { return PrepareBuilder().unpackedBool_; }

       }

       public int UnpackedBoolCount {

         get { return result.UnpackedBoolCount; }

@@ -8223,24 +8731,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedBool(index);

       }

       public Builder SetUnpackedBool(int index, bool value) {

+        PrepareBuilder();

         result.unpackedBool_[index] = value;

         return this;

       }

       public Builder AddUnpackedBool(bool value) {

+        PrepareBuilder();

         result.unpackedBool_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedBool(scg::IEnumerable<bool> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedBool_);

         return this;

       }

       public Builder ClearUnpackedBool() {

+        PrepareBuilder();

         result.unpackedBool_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite> UnpackedEnumList {

-        get { return result.unpackedEnum_; }

+        get { return PrepareBuilder().unpackedEnum_; }

       }

       public int UnpackedEnumCount {

         get { return result.UnpackedEnumCount; }

@@ -8249,18 +8761,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedEnum(index);

       }

       public Builder SetUnpackedEnum(int index, global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite value) {

+        PrepareBuilder();

         result.unpackedEnum_[index] = value;

         return this;

       }

       public Builder AddUnpackedEnum(global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite value) {

+        PrepareBuilder();

         result.unpackedEnum_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedEnum(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignEnumLite> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedEnum_);

         return this;

       }

       public Builder ClearUnpackedEnum() {

+        PrepareBuilder();

         result.unpackedEnum_.Clear();

         return this;

       }

@@ -8369,7 +8885,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestAllExtensionsLite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -8379,21 +8895,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestAllExtensionsLite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestAllExtensionsLite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestAllExtensionsLite result = new TestAllExtensionsLite();

+      bool builderIsReadOnly;

+      TestAllExtensionsLite result;

+      

+      private TestAllExtensionsLite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestAllExtensionsLite original = result;

+          result = new TestAllExtensionsLite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestAllExtensionsLite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestAllExtensionsLite();

+        result = DefaultInstance ?? new TestAllExtensionsLite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override TestAllExtensionsLite DefaultInstanceForType {

@@ -8401,12 +8944,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestAllExtensionsLite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestAllExtensionsLite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -8420,6 +8962,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestAllExtensionsLite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestAllExtensionsLite.DefaultInstance) return this;

+        PrepareBuilder();

           this.MergeExtensionFields(other);

         return this;

       }

@@ -8429,6 +8972,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -8575,7 +9119,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(OptionalGroup_extension_lite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -8585,21 +9129,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new OptionalGroup_extension_lite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(OptionalGroup_extension_lite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      OptionalGroup_extension_lite result;

+      

+      private OptionalGroup_extension_lite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          OptionalGroup_extension_lite original = result;

+          result = new OptionalGroup_extension_lite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      OptionalGroup_extension_lite result = new OptionalGroup_extension_lite();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override OptionalGroup_extension_lite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new OptionalGroup_extension_lite();

+        result = DefaultInstance ?? new OptionalGroup_extension_lite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override OptionalGroup_extension_lite DefaultInstanceForType {

@@ -8607,12 +9178,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OptionalGroup_extension_lite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        OptionalGroup_extension_lite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -8626,6 +9196,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(OptionalGroup_extension_lite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.OptionalGroup_extension_lite.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           A = other.A;

         }

@@ -8637,6 +9208,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -8679,11 +9251,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetA(value); }

       }

       public Builder SetA(int value) {

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = 0;

         return this;

@@ -8805,7 +9379,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(RepeatedGroup_extension_lite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -8815,21 +9389,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new RepeatedGroup_extension_lite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(RepeatedGroup_extension_lite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      RepeatedGroup_extension_lite result;

+      

+      private RepeatedGroup_extension_lite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          RepeatedGroup_extension_lite original = result;

+          result = new RepeatedGroup_extension_lite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      RepeatedGroup_extension_lite result = new RepeatedGroup_extension_lite();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override RepeatedGroup_extension_lite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new RepeatedGroup_extension_lite();

+        result = DefaultInstance ?? new RepeatedGroup_extension_lite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override RepeatedGroup_extension_lite DefaultInstanceForType {

@@ -8837,12 +9438,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override RepeatedGroup_extension_lite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        RepeatedGroup_extension_lite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -8856,6 +9456,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(RepeatedGroup_extension_lite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.RepeatedGroup_extension_lite.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           A = other.A;

         }

@@ -8867,6 +9468,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -8909,11 +9511,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetA(value); }

       }

       public Builder SetA(int value) {

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = 0;

         return this;

@@ -9023,7 +9627,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestPackedExtensionsLite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -9033,21 +9637,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestPackedExtensionsLite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestPackedExtensionsLite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestPackedExtensionsLite result = new TestPackedExtensionsLite();

+      bool builderIsReadOnly;

+      TestPackedExtensionsLite result;

+      

+      private TestPackedExtensionsLite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestPackedExtensionsLite original = result;

+          result = new TestPackedExtensionsLite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestPackedExtensionsLite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestPackedExtensionsLite();

+        result = DefaultInstance ?? new TestPackedExtensionsLite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override TestPackedExtensionsLite DefaultInstanceForType {

@@ -9055,12 +9686,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestPackedExtensionsLite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestPackedExtensionsLite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -9074,6 +9704,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestPackedExtensionsLite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestPackedExtensionsLite.DefaultInstance) return this;

+        PrepareBuilder();

           this.MergeExtensionFields(other);

         return this;

       }

@@ -9083,6 +9714,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -9217,7 +9849,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestUnpackedExtensionsLite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -9227,21 +9859,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestUnpackedExtensionsLite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestUnpackedExtensionsLite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestUnpackedExtensionsLite result;

       

-      TestUnpackedExtensionsLite result = new TestUnpackedExtensionsLite();

+      private TestUnpackedExtensionsLite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestUnpackedExtensionsLite original = result;

+          result = new TestUnpackedExtensionsLite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestUnpackedExtensionsLite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestUnpackedExtensionsLite();

+        result = DefaultInstance ?? new TestUnpackedExtensionsLite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override TestUnpackedExtensionsLite DefaultInstanceForType {

@@ -9249,12 +9908,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestUnpackedExtensionsLite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestUnpackedExtensionsLite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -9268,6 +9926,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestUnpackedExtensionsLite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestUnpackedExtensionsLite.DefaultInstance) return this;

+        PrepareBuilder();

           this.MergeExtensionFields(other);

         return this;

       }

@@ -9277,6 +9936,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -9406,7 +10066,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestNestedExtensionLite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -9416,21 +10076,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestNestedExtensionLite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestNestedExtensionLite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestNestedExtensionLite result;

+      

+      private TestNestedExtensionLite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestNestedExtensionLite original = result;

+          result = new TestNestedExtensionLite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestNestedExtensionLite result = new TestNestedExtensionLite();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestNestedExtensionLite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestNestedExtensionLite();

+        result = DefaultInstance ?? new TestNestedExtensionLite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override TestNestedExtensionLite DefaultInstanceForType {

@@ -9438,12 +10125,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestNestedExtensionLite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestNestedExtensionLite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -9457,6 +10143,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestNestedExtensionLite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestNestedExtensionLite.DefaultInstance) return this;

+        PrepareBuilder();

         return this;

       }

       

@@ -9465,6 +10152,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -9611,7 +10299,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestDeprecatedLite prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -9621,21 +10309,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestDeprecatedLite();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestDeprecatedLite cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestDeprecatedLite result = new TestDeprecatedLite();

+      bool builderIsReadOnly;

+      TestDeprecatedLite result;

+      

+      private TestDeprecatedLite PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestDeprecatedLite original = result;

+          result = new TestDeprecatedLite();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestDeprecatedLite MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestDeprecatedLite();

+        result = DefaultInstance ?? new TestDeprecatedLite();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override TestDeprecatedLite DefaultInstanceForType {

@@ -9643,12 +10358,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestDeprecatedLite BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestDeprecatedLite returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessageLite other) {

@@ -9662,6 +10376,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestDeprecatedLite other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestDeprecatedLite.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasDeprecatedField) {

           DeprecatedField = other.DeprecatedField;

         }

@@ -9673,6 +10388,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         uint tag;

         string field_name;

         while (input.ReadTag(out tag, out field_name)) {

@@ -9715,11 +10431,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDeprecatedField(value); }

       }

       public Builder SetDeprecatedField(int value) {

+        PrepareBuilder();

         result.hasDeprecatedField = true;

         result.deprecatedField_ = value;

         return this;

       }

       public Builder ClearDeprecatedField() {

+        PrepareBuilder();

         result.hasDeprecatedField = false;

         result.deprecatedField_ = 0;

         return this;

diff --git a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestProtoFile.cs b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestProtoFile.cs
index d8824d49..aeea673d 100644
--- a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestProtoFile.cs
+++ b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestProtoFile.cs
@@ -1260,7 +1260,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(NestedMessage prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1270,21 +1270,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new NestedMessage();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(NestedMessage cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          NestedMessage result;

+          

+          private NestedMessage PrepareBuilder() {

+            if (builderIsReadOnly) {

+              NestedMessage original = result;

+              result = new NestedMessage();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

           

-          NestedMessage result = new NestedMessage();

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override NestedMessage MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new NestedMessage();

+            result = DefaultInstance ?? new NestedMessage();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -1296,12 +1323,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override NestedMessage BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            NestedMessage returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -1315,6 +1341,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(NestedMessage other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasBb) {

               Bb = other.Bb;

             }

@@ -1327,6 +1354,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -1382,11 +1410,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetBb(value); }

           }

           public Builder SetBb(int value) {

+            PrepareBuilder();

             result.hasBb = true;

             result.bb_ = value;

             return this;

           }

           public Builder ClearBb() {

+            PrepareBuilder();

             result.hasBb = false;

             result.bb_ = 0;

             return this;

@@ -1499,7 +1529,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(OptionalGroup prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1509,21 +1539,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new OptionalGroup();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(OptionalGroup cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          OptionalGroup result;

+          

+          private OptionalGroup PrepareBuilder() {

+            if (builderIsReadOnly) {

+              OptionalGroup original = result;

+              result = new OptionalGroup();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

           

-          OptionalGroup result = new OptionalGroup();

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override OptionalGroup MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new OptionalGroup();

+            result = DefaultInstance ?? new OptionalGroup();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -1535,12 +1592,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override OptionalGroup BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            OptionalGroup returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -1554,6 +1610,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(OptionalGroup other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasA) {

               A = other.A;

             }

@@ -1566,6 +1623,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -1621,11 +1679,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetA(value); }

           }

           public Builder SetA(int value) {

+            PrepareBuilder();

             result.hasA = true;

             result.a_ = value;

             return this;

           }

           public Builder ClearA() {

+            PrepareBuilder();

             result.hasA = false;

             result.a_ = 0;

             return this;

@@ -1738,7 +1798,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(RepeatedGroup prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -1748,21 +1808,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new RepeatedGroup();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(RepeatedGroup cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          RepeatedGroup result;

           

-          RepeatedGroup result = new RepeatedGroup();

+          private RepeatedGroup PrepareBuilder() {

+            if (builderIsReadOnly) {

+              RepeatedGroup original = result;

+              result = new RepeatedGroup();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override RepeatedGroup MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new RepeatedGroup();

+            result = DefaultInstance ?? new RepeatedGroup();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -1774,12 +1861,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override RepeatedGroup BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            RepeatedGroup returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -1793,6 +1879,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(RepeatedGroup other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.RepeatedGroup.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasA) {

               A = other.A;

             }

@@ -1805,6 +1892,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -1860,11 +1948,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetA(value); }

           }

           public Builder SetA(int value) {

+            PrepareBuilder();

             result.hasA = true;

             result.a_ = value;

             return this;

           }

           public Builder ClearA() {

+            PrepareBuilder();

             result.hasA = false;

             result.a_ = 0;

             return this;

@@ -3181,7 +3271,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestAllTypes prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -3191,21 +3281,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestAllTypes();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestAllTypes cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestAllTypes result;

+      

+      private TestAllTypes PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestAllTypes original = result;

+          result = new TestAllTypes();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestAllTypes result = new TestAllTypes();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestAllTypes MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestAllTypes();

+        result = DefaultInstance ?? new TestAllTypes();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -3217,8 +3334,8 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestAllTypes BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.repeatedInt32_.MakeReadOnly();

         result.repeatedInt64_.MakeReadOnly();

@@ -3244,9 +3361,8 @@ namespace Google.ProtocolBuffers.TestProtos {
         result.repeatedImportEnum_.MakeReadOnly();

         result.repeatedStringPiece_.MakeReadOnly();

         result.repeatedCord_.MakeReadOnly();

-        TestAllTypes returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -3260,6 +3376,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestAllTypes other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestAllTypes.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasOptionalInt32) {

           OptionalInt32 = other.OptionalInt32;

         }

@@ -3473,6 +3590,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -3907,11 +4025,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalInt32(value); }

       }

       public Builder SetOptionalInt32(int value) {

+        PrepareBuilder();

         result.hasOptionalInt32 = true;

         result.optionalInt32_ = value;

         return this;

       }

       public Builder ClearOptionalInt32() {

+        PrepareBuilder();

         result.hasOptionalInt32 = false;

         result.optionalInt32_ = 0;

         return this;

@@ -3925,11 +4045,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalInt64(value); }

       }

       public Builder SetOptionalInt64(long value) {

+        PrepareBuilder();

         result.hasOptionalInt64 = true;

         result.optionalInt64_ = value;

         return this;

       }

       public Builder ClearOptionalInt64() {

+        PrepareBuilder();

         result.hasOptionalInt64 = false;

         result.optionalInt64_ = 0L;

         return this;

@@ -3945,11 +4067,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetOptionalUint32(uint value) {

+        PrepareBuilder();

         result.hasOptionalUint32 = true;

         result.optionalUint32_ = value;

         return this;

       }

       public Builder ClearOptionalUint32() {

+        PrepareBuilder();

         result.hasOptionalUint32 = false;

         result.optionalUint32_ = 0;

         return this;

@@ -3965,11 +4089,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetOptionalUint64(ulong value) {

+        PrepareBuilder();

         result.hasOptionalUint64 = true;

         result.optionalUint64_ = value;

         return this;

       }

       public Builder ClearOptionalUint64() {

+        PrepareBuilder();

         result.hasOptionalUint64 = false;

         result.optionalUint64_ = 0UL;

         return this;

@@ -3983,11 +4109,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalSint32(value); }

       }

       public Builder SetOptionalSint32(int value) {

+        PrepareBuilder();

         result.hasOptionalSint32 = true;

         result.optionalSint32_ = value;

         return this;

       }

       public Builder ClearOptionalSint32() {

+        PrepareBuilder();

         result.hasOptionalSint32 = false;

         result.optionalSint32_ = 0;

         return this;

@@ -4001,11 +4129,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalSint64(value); }

       }

       public Builder SetOptionalSint64(long value) {

+        PrepareBuilder();

         result.hasOptionalSint64 = true;

         result.optionalSint64_ = value;

         return this;

       }

       public Builder ClearOptionalSint64() {

+        PrepareBuilder();

         result.hasOptionalSint64 = false;

         result.optionalSint64_ = 0;

         return this;

@@ -4021,11 +4151,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetOptionalFixed32(uint value) {

+        PrepareBuilder();

         result.hasOptionalFixed32 = true;

         result.optionalFixed32_ = value;

         return this;

       }

       public Builder ClearOptionalFixed32() {

+        PrepareBuilder();

         result.hasOptionalFixed32 = false;

         result.optionalFixed32_ = 0;

         return this;

@@ -4041,11 +4173,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetOptionalFixed64(ulong value) {

+        PrepareBuilder();

         result.hasOptionalFixed64 = true;

         result.optionalFixed64_ = value;

         return this;

       }

       public Builder ClearOptionalFixed64() {

+        PrepareBuilder();

         result.hasOptionalFixed64 = false;

         result.optionalFixed64_ = 0;

         return this;

@@ -4059,11 +4193,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalSfixed32(value); }

       }

       public Builder SetOptionalSfixed32(int value) {

+        PrepareBuilder();

         result.hasOptionalSfixed32 = true;

         result.optionalSfixed32_ = value;

         return this;

       }

       public Builder ClearOptionalSfixed32() {

+        PrepareBuilder();

         result.hasOptionalSfixed32 = false;

         result.optionalSfixed32_ = 0;

         return this;

@@ -4077,11 +4213,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalSfixed64(value); }

       }

       public Builder SetOptionalSfixed64(long value) {

+        PrepareBuilder();

         result.hasOptionalSfixed64 = true;

         result.optionalSfixed64_ = value;

         return this;

       }

       public Builder ClearOptionalSfixed64() {

+        PrepareBuilder();

         result.hasOptionalSfixed64 = false;

         result.optionalSfixed64_ = 0;

         return this;

@@ -4095,11 +4233,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalFloat(value); }

       }

       public Builder SetOptionalFloat(float value) {

+        PrepareBuilder();

         result.hasOptionalFloat = true;

         result.optionalFloat_ = value;

         return this;

       }

       public Builder ClearOptionalFloat() {

+        PrepareBuilder();

         result.hasOptionalFloat = false;

         result.optionalFloat_ = 0F;

         return this;

@@ -4113,11 +4253,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalDouble(value); }

       }

       public Builder SetOptionalDouble(double value) {

+        PrepareBuilder();

         result.hasOptionalDouble = true;

         result.optionalDouble_ = value;

         return this;

       }

       public Builder ClearOptionalDouble() {

+        PrepareBuilder();

         result.hasOptionalDouble = false;

         result.optionalDouble_ = 0D;

         return this;

@@ -4131,11 +4273,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalBool(value); }

       }

       public Builder SetOptionalBool(bool value) {

+        PrepareBuilder();

         result.hasOptionalBool = true;

         result.optionalBool_ = value;

         return this;

       }

       public Builder ClearOptionalBool() {

+        PrepareBuilder();

         result.hasOptionalBool = false;

         result.optionalBool_ = false;

         return this;

@@ -4150,11 +4294,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalString(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalString = true;

         result.optionalString_ = value;

         return this;

       }

       public Builder ClearOptionalString() {

+        PrepareBuilder();

         result.hasOptionalString = false;

         result.optionalString_ = "";

         return this;

@@ -4169,11 +4315,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalBytes(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalBytes = true;

         result.optionalBytes_ = value;

         return this;

       }

       public Builder ClearOptionalBytes() {

+        PrepareBuilder();

         result.hasOptionalBytes = false;

         result.optionalBytes_ = pb::ByteString.Empty;

         return this;

@@ -4188,18 +4336,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalGroup(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalGroup = true;

         result.optionalGroup_ = value;

         return this;

       }

       public Builder SetOptionalGroup(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalGroup = true;

         result.optionalGroup_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalGroup(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalGroup &&

             result.optionalGroup_ != global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.DefaultInstance) {

             result.optionalGroup_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.CreateBuilder(result.optionalGroup_).MergeFrom(value).BuildPartial();

@@ -4210,6 +4361,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalGroup() {

+        PrepareBuilder();

         result.hasOptionalGroup = false;

         result.optionalGroup_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.DefaultInstance;

         return this;

@@ -4224,18 +4376,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalNestedMessage = true;

         result.optionalNestedMessage_ = value;

         return this;

       }

       public Builder SetOptionalNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalNestedMessage = true;

         result.optionalNestedMessage_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalNestedMessage &&

             result.optionalNestedMessage_ != global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance) {

             result.optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.CreateBuilder(result.optionalNestedMessage_).MergeFrom(value).BuildPartial();

@@ -4246,6 +4401,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalNestedMessage() {

+        PrepareBuilder();

         result.hasOptionalNestedMessage = false;

         result.optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance;

         return this;

@@ -4260,18 +4416,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalForeignMessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalForeignMessage = true;

         result.optionalForeignMessage_ = value;

         return this;

       }

       public Builder SetOptionalForeignMessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalForeignMessage = true;

         result.optionalForeignMessage_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalForeignMessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalForeignMessage &&

             result.optionalForeignMessage_ != global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance) {

             result.optionalForeignMessage_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.CreateBuilder(result.optionalForeignMessage_).MergeFrom(value).BuildPartial();

@@ -4282,6 +4441,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalForeignMessage() {

+        PrepareBuilder();

         result.hasOptionalForeignMessage = false;

         result.optionalForeignMessage_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

         return this;

@@ -4296,18 +4456,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalImportMessage(global::Google.ProtocolBuffers.TestProtos.ImportMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalImportMessage = true;

         result.optionalImportMessage_ = value;

         return this;

       }

       public Builder SetOptionalImportMessage(global::Google.ProtocolBuffers.TestProtos.ImportMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalImportMessage = true;

         result.optionalImportMessage_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalImportMessage(global::Google.ProtocolBuffers.TestProtos.ImportMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalImportMessage &&

             result.optionalImportMessage_ != global::Google.ProtocolBuffers.TestProtos.ImportMessage.DefaultInstance) {

             result.optionalImportMessage_ = global::Google.ProtocolBuffers.TestProtos.ImportMessage.CreateBuilder(result.optionalImportMessage_).MergeFrom(value).BuildPartial();

@@ -4318,6 +4481,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalImportMessage() {

+        PrepareBuilder();

         result.hasOptionalImportMessage = false;

         result.optionalImportMessage_ = global::Google.ProtocolBuffers.TestProtos.ImportMessage.DefaultInstance;

         return this;

@@ -4331,11 +4495,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalNestedEnum(value); }

       }

       public Builder SetOptionalNestedEnum(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum value) {

+        PrepareBuilder();

         result.hasOptionalNestedEnum = true;

         result.optionalNestedEnum_ = value;

         return this;

       }

       public Builder ClearOptionalNestedEnum() {

+        PrepareBuilder();

         result.hasOptionalNestedEnum = false;

         result.optionalNestedEnum_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum.FOO;

         return this;

@@ -4349,11 +4515,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalForeignEnum(value); }

       }

       public Builder SetOptionalForeignEnum(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.hasOptionalForeignEnum = true;

         result.optionalForeignEnum_ = value;

         return this;

       }

       public Builder ClearOptionalForeignEnum() {

+        PrepareBuilder();

         result.hasOptionalForeignEnum = false;

         result.optionalForeignEnum_ = global::Google.ProtocolBuffers.TestProtos.ForeignEnum.FOREIGN_FOO;

         return this;

@@ -4367,11 +4535,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalImportEnum(value); }

       }

       public Builder SetOptionalImportEnum(global::Google.ProtocolBuffers.TestProtos.ImportEnum value) {

+        PrepareBuilder();

         result.hasOptionalImportEnum = true;

         result.optionalImportEnum_ = value;

         return this;

       }

       public Builder ClearOptionalImportEnum() {

+        PrepareBuilder();

         result.hasOptionalImportEnum = false;

         result.optionalImportEnum_ = global::Google.ProtocolBuffers.TestProtos.ImportEnum.IMPORT_FOO;

         return this;

@@ -4386,11 +4556,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalStringPiece(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalStringPiece = true;

         result.optionalStringPiece_ = value;

         return this;

       }

       public Builder ClearOptionalStringPiece() {

+        PrepareBuilder();

         result.hasOptionalStringPiece = false;

         result.optionalStringPiece_ = "";

         return this;

@@ -4405,18 +4577,20 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalCord(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalCord = true;

         result.optionalCord_ = value;

         return this;

       }

       public Builder ClearOptionalCord() {

+        PrepareBuilder();

         result.hasOptionalCord = false;

         result.optionalCord_ = "";

         return this;

       }

       

       public pbc::IPopsicleList<int> RepeatedInt32List {

-        get { return result.repeatedInt32_; }

+        get { return PrepareBuilder().repeatedInt32_; }

       }

       public int RepeatedInt32Count {

         get { return result.RepeatedInt32Count; }

@@ -4425,24 +4599,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedInt32(index);

       }

       public Builder SetRepeatedInt32(int index, int value) {

+        PrepareBuilder();

         result.repeatedInt32_[index] = value;

         return this;

       }

       public Builder AddRepeatedInt32(int value) {

+        PrepareBuilder();

         result.repeatedInt32_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedInt32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedInt32_);

         return this;

       }

       public Builder ClearRepeatedInt32() {

+        PrepareBuilder();

         result.repeatedInt32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> RepeatedInt64List {

-        get { return result.repeatedInt64_; }

+        get { return PrepareBuilder().repeatedInt64_; }

       }

       public int RepeatedInt64Count {

         get { return result.RepeatedInt64Count; }

@@ -4451,25 +4629,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedInt64(index);

       }

       public Builder SetRepeatedInt64(int index, long value) {

+        PrepareBuilder();

         result.repeatedInt64_[index] = value;

         return this;

       }

       public Builder AddRepeatedInt64(long value) {

+        PrepareBuilder();

         result.repeatedInt64_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedInt64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedInt64_);

         return this;

       }

       public Builder ClearRepeatedInt64() {

+        PrepareBuilder();

         result.repeatedInt64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> RepeatedUint32List {

-        get { return result.repeatedUint32_; }

+        get { return PrepareBuilder().repeatedUint32_; }

       }

       public int RepeatedUint32Count {

         get { return result.RepeatedUint32Count; }

@@ -4480,27 +4662,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedUint32(int index, uint value) {

+        PrepareBuilder();

         result.repeatedUint32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedUint32(uint value) {

+        PrepareBuilder();

         result.repeatedUint32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedUint32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedUint32_);

         return this;

       }

       public Builder ClearRepeatedUint32() {

+        PrepareBuilder();

         result.repeatedUint32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> RepeatedUint64List {

-        get { return result.repeatedUint64_; }

+        get { return PrepareBuilder().repeatedUint64_; }

       }

       public int RepeatedUint64Count {

         get { return result.RepeatedUint64Count; }

@@ -4511,26 +4697,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedUint64(int index, ulong value) {

+        PrepareBuilder();

         result.repeatedUint64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedUint64(ulong value) {

+        PrepareBuilder();

         result.repeatedUint64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedUint64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedUint64_);

         return this;

       }

       public Builder ClearRepeatedUint64() {

+        PrepareBuilder();

         result.repeatedUint64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> RepeatedSint32List {

-        get { return result.repeatedSint32_; }

+        get { return PrepareBuilder().repeatedSint32_; }

       }

       public int RepeatedSint32Count {

         get { return result.RepeatedSint32Count; }

@@ -4539,24 +4729,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedSint32(index);

       }

       public Builder SetRepeatedSint32(int index, int value) {

+        PrepareBuilder();

         result.repeatedSint32_[index] = value;

         return this;

       }

       public Builder AddRepeatedSint32(int value) {

+        PrepareBuilder();

         result.repeatedSint32_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedSint32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedSint32_);

         return this;

       }

       public Builder ClearRepeatedSint32() {

+        PrepareBuilder();

         result.repeatedSint32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> RepeatedSint64List {

-        get { return result.repeatedSint64_; }

+        get { return PrepareBuilder().repeatedSint64_; }

       }

       public int RepeatedSint64Count {

         get { return result.RepeatedSint64Count; }

@@ -4565,25 +4759,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedSint64(index);

       }

       public Builder SetRepeatedSint64(int index, long value) {

+        PrepareBuilder();

         result.repeatedSint64_[index] = value;

         return this;

       }

       public Builder AddRepeatedSint64(long value) {

+        PrepareBuilder();

         result.repeatedSint64_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedSint64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedSint64_);

         return this;

       }

       public Builder ClearRepeatedSint64() {

+        PrepareBuilder();

         result.repeatedSint64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> RepeatedFixed32List {

-        get { return result.repeatedFixed32_; }

+        get { return PrepareBuilder().repeatedFixed32_; }

       }

       public int RepeatedFixed32Count {

         get { return result.RepeatedFixed32Count; }

@@ -4594,27 +4792,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedFixed32(int index, uint value) {

+        PrepareBuilder();

         result.repeatedFixed32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedFixed32(uint value) {

+        PrepareBuilder();

         result.repeatedFixed32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedFixed32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedFixed32_);

         return this;

       }

       public Builder ClearRepeatedFixed32() {

+        PrepareBuilder();

         result.repeatedFixed32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> RepeatedFixed64List {

-        get { return result.repeatedFixed64_; }

+        get { return PrepareBuilder().repeatedFixed64_; }

       }

       public int RepeatedFixed64Count {

         get { return result.RepeatedFixed64Count; }

@@ -4625,26 +4827,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedFixed64(int index, ulong value) {

+        PrepareBuilder();

         result.repeatedFixed64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedFixed64(ulong value) {

+        PrepareBuilder();

         result.repeatedFixed64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedFixed64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedFixed64_);

         return this;

       }

       public Builder ClearRepeatedFixed64() {

+        PrepareBuilder();

         result.repeatedFixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> RepeatedSfixed32List {

-        get { return result.repeatedSfixed32_; }

+        get { return PrepareBuilder().repeatedSfixed32_; }

       }

       public int RepeatedSfixed32Count {

         get { return result.RepeatedSfixed32Count; }

@@ -4653,24 +4859,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedSfixed32(index);

       }

       public Builder SetRepeatedSfixed32(int index, int value) {

+        PrepareBuilder();

         result.repeatedSfixed32_[index] = value;

         return this;

       }

       public Builder AddRepeatedSfixed32(int value) {

+        PrepareBuilder();

         result.repeatedSfixed32_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedSfixed32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedSfixed32_);

         return this;

       }

       public Builder ClearRepeatedSfixed32() {

+        PrepareBuilder();

         result.repeatedSfixed32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> RepeatedSfixed64List {

-        get { return result.repeatedSfixed64_; }

+        get { return PrepareBuilder().repeatedSfixed64_; }

       }

       public int RepeatedSfixed64Count {

         get { return result.RepeatedSfixed64Count; }

@@ -4679,24 +4889,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedSfixed64(index);

       }

       public Builder SetRepeatedSfixed64(int index, long value) {

+        PrepareBuilder();

         result.repeatedSfixed64_[index] = value;

         return this;

       }

       public Builder AddRepeatedSfixed64(long value) {

+        PrepareBuilder();

         result.repeatedSfixed64_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedSfixed64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedSfixed64_);

         return this;

       }

       public Builder ClearRepeatedSfixed64() {

+        PrepareBuilder();

         result.repeatedSfixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<float> RepeatedFloatList {

-        get { return result.repeatedFloat_; }

+        get { return PrepareBuilder().repeatedFloat_; }

       }

       public int RepeatedFloatCount {

         get { return result.RepeatedFloatCount; }

@@ -4705,24 +4919,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedFloat(index);

       }

       public Builder SetRepeatedFloat(int index, float value) {

+        PrepareBuilder();

         result.repeatedFloat_[index] = value;

         return this;

       }

       public Builder AddRepeatedFloat(float value) {

+        PrepareBuilder();

         result.repeatedFloat_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedFloat(scg::IEnumerable<float> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedFloat_);

         return this;

       }

       public Builder ClearRepeatedFloat() {

+        PrepareBuilder();

         result.repeatedFloat_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<double> RepeatedDoubleList {

-        get { return result.repeatedDouble_; }

+        get { return PrepareBuilder().repeatedDouble_; }

       }

       public int RepeatedDoubleCount {

         get { return result.RepeatedDoubleCount; }

@@ -4731,24 +4949,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedDouble(index);

       }

       public Builder SetRepeatedDouble(int index, double value) {

+        PrepareBuilder();

         result.repeatedDouble_[index] = value;

         return this;

       }

       public Builder AddRepeatedDouble(double value) {

+        PrepareBuilder();

         result.repeatedDouble_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedDouble(scg::IEnumerable<double> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedDouble_);

         return this;

       }

       public Builder ClearRepeatedDouble() {

+        PrepareBuilder();

         result.repeatedDouble_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<bool> RepeatedBoolList {

-        get { return result.repeatedBool_; }

+        get { return PrepareBuilder().repeatedBool_; }

       }

       public int RepeatedBoolCount {

         get { return result.RepeatedBoolCount; }

@@ -4757,24 +4979,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedBool(index);

       }

       public Builder SetRepeatedBool(int index, bool value) {

+        PrepareBuilder();

         result.repeatedBool_[index] = value;

         return this;

       }

       public Builder AddRepeatedBool(bool value) {

+        PrepareBuilder();

         result.repeatedBool_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedBool(scg::IEnumerable<bool> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedBool_);

         return this;

       }

       public Builder ClearRepeatedBool() {

+        PrepareBuilder();

         result.repeatedBool_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedStringList {

-        get { return result.repeatedString_; }

+        get { return PrepareBuilder().repeatedString_; }

       }

       public int RepeatedStringCount {

         get { return result.RepeatedStringCount; }

@@ -4784,25 +5010,29 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedString(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedString_[index] = value;

         return this;

       }

       public Builder AddRepeatedString(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedString_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedString(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedString_);

         return this;

       }

       public Builder ClearRepeatedString() {

+        PrepareBuilder();

         result.repeatedString_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<pb::ByteString> RepeatedBytesList {

-        get { return result.repeatedBytes_; }

+        get { return PrepareBuilder().repeatedBytes_; }

       }

       public int RepeatedBytesCount {

         get { return result.RepeatedBytesCount; }

@@ -4812,25 +5042,29 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedBytes(int index, pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedBytes_[index] = value;

         return this;

       }

       public Builder AddRepeatedBytes(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedBytes_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedBytes(scg::IEnumerable<pb::ByteString> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedBytes_);

         return this;

       }

       public Builder ClearRepeatedBytes() {

+        PrepareBuilder();

         result.repeatedBytes_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.RepeatedGroup> RepeatedGroupList {

-        get { return result.repeatedGroup_; }

+        get { return PrepareBuilder().repeatedGroup_; }

       }

       public int RepeatedGroupCount {

         get { return result.RepeatedGroupCount; }

@@ -4840,35 +5074,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedGroup(int index, global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.RepeatedGroup value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedGroup_[index] = value;

         return this;

       }

       public Builder SetRepeatedGroup(int index, global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.RepeatedGroup.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedGroup_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedGroup(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.RepeatedGroup value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedGroup_.Add(value);

         return this;

       }

       public Builder AddRepeatedGroup(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.RepeatedGroup.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedGroup_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedGroup(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.RepeatedGroup> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedGroup_);

         return this;

       }

       public Builder ClearRepeatedGroup() {

+        PrepareBuilder();

         result.repeatedGroup_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage> RepeatedNestedMessageList {

-        get { return result.repeatedNestedMessage_; }

+        get { return PrepareBuilder().repeatedNestedMessage_; }

       }

       public int RepeatedNestedMessageCount {

         get { return result.RepeatedNestedMessageCount; }

@@ -4878,35 +5118,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedNestedMessage(int index, global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedNestedMessage_[index] = value;

         return this;

       }

       public Builder SetRepeatedNestedMessage(int index, global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedNestedMessage_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedNestedMessage_.Add(value);

         return this;

       }

       public Builder AddRepeatedNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedNestedMessage_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedNestedMessage(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedNestedMessage_);

         return this;

       }

       public Builder ClearRepeatedNestedMessage() {

+        PrepareBuilder();

         result.repeatedNestedMessage_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignMessage> RepeatedForeignMessageList {

-        get { return result.repeatedForeignMessage_; }

+        get { return PrepareBuilder().repeatedForeignMessage_; }

       }

       public int RepeatedForeignMessageCount {

         get { return result.RepeatedForeignMessageCount; }

@@ -4916,35 +5162,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedForeignMessage(int index, global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedForeignMessage_[index] = value;

         return this;

       }

       public Builder SetRepeatedForeignMessage(int index, global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedForeignMessage_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedForeignMessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedForeignMessage_.Add(value);

         return this;

       }

       public Builder AddRepeatedForeignMessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedForeignMessage_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedForeignMessage(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignMessage> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedForeignMessage_);

         return this;

       }

       public Builder ClearRepeatedForeignMessage() {

+        PrepareBuilder();

         result.repeatedForeignMessage_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ImportMessage> RepeatedImportMessageList {

-        get { return result.repeatedImportMessage_; }

+        get { return PrepareBuilder().repeatedImportMessage_; }

       }

       public int RepeatedImportMessageCount {

         get { return result.RepeatedImportMessageCount; }

@@ -4954,35 +5206,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedImportMessage(int index, global::Google.ProtocolBuffers.TestProtos.ImportMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedImportMessage_[index] = value;

         return this;

       }

       public Builder SetRepeatedImportMessage(int index, global::Google.ProtocolBuffers.TestProtos.ImportMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedImportMessage_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedImportMessage(global::Google.ProtocolBuffers.TestProtos.ImportMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedImportMessage_.Add(value);

         return this;

       }

       public Builder AddRepeatedImportMessage(global::Google.ProtocolBuffers.TestProtos.ImportMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedImportMessage_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedImportMessage(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ImportMessage> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedImportMessage_);

         return this;

       }

       public Builder ClearRepeatedImportMessage() {

+        PrepareBuilder();

         result.repeatedImportMessage_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum> RepeatedNestedEnumList {

-        get { return result.repeatedNestedEnum_; }

+        get { return PrepareBuilder().repeatedNestedEnum_; }

       }

       public int RepeatedNestedEnumCount {

         get { return result.RepeatedNestedEnumCount; }

@@ -4991,24 +5249,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedNestedEnum(index);

       }

       public Builder SetRepeatedNestedEnum(int index, global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum value) {

+        PrepareBuilder();

         result.repeatedNestedEnum_[index] = value;

         return this;

       }

       public Builder AddRepeatedNestedEnum(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum value) {

+        PrepareBuilder();

         result.repeatedNestedEnum_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedNestedEnum(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedNestedEnum_);

         return this;

       }

       public Builder ClearRepeatedNestedEnum() {

+        PrepareBuilder();

         result.repeatedNestedEnum_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> RepeatedForeignEnumList {

-        get { return result.repeatedForeignEnum_; }

+        get { return PrepareBuilder().repeatedForeignEnum_; }

       }

       public int RepeatedForeignEnumCount {

         get { return result.RepeatedForeignEnumCount; }

@@ -5017,24 +5279,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedForeignEnum(index);

       }

       public Builder SetRepeatedForeignEnum(int index, global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.repeatedForeignEnum_[index] = value;

         return this;

       }

       public Builder AddRepeatedForeignEnum(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.repeatedForeignEnum_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedForeignEnum(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedForeignEnum_);

         return this;

       }

       public Builder ClearRepeatedForeignEnum() {

+        PrepareBuilder();

         result.repeatedForeignEnum_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ImportEnum> RepeatedImportEnumList {

-        get { return result.repeatedImportEnum_; }

+        get { return PrepareBuilder().repeatedImportEnum_; }

       }

       public int RepeatedImportEnumCount {

         get { return result.RepeatedImportEnumCount; }

@@ -5043,24 +5309,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedImportEnum(index);

       }

       public Builder SetRepeatedImportEnum(int index, global::Google.ProtocolBuffers.TestProtos.ImportEnum value) {

+        PrepareBuilder();

         result.repeatedImportEnum_[index] = value;

         return this;

       }

       public Builder AddRepeatedImportEnum(global::Google.ProtocolBuffers.TestProtos.ImportEnum value) {

+        PrepareBuilder();

         result.repeatedImportEnum_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedImportEnum(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ImportEnum> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedImportEnum_);

         return this;

       }

       public Builder ClearRepeatedImportEnum() {

+        PrepareBuilder();

         result.repeatedImportEnum_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedStringPieceList {

-        get { return result.repeatedStringPiece_; }

+        get { return PrepareBuilder().repeatedStringPiece_; }

       }

       public int RepeatedStringPieceCount {

         get { return result.RepeatedStringPieceCount; }

@@ -5070,25 +5340,29 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedStringPiece(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedStringPiece_[index] = value;

         return this;

       }

       public Builder AddRepeatedStringPiece(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedStringPiece_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedStringPiece(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedStringPiece_);

         return this;

       }

       public Builder ClearRepeatedStringPiece() {

+        PrepareBuilder();

         result.repeatedStringPiece_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedCordList {

-        get { return result.repeatedCord_; }

+        get { return PrepareBuilder().repeatedCord_; }

       }

       public int RepeatedCordCount {

         get { return result.RepeatedCordCount; }

@@ -5098,19 +5372,23 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedCord(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedCord_[index] = value;

         return this;

       }

       public Builder AddRepeatedCord(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedCord_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedCord(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedCord_);

         return this;

       }

       public Builder ClearRepeatedCord() {

+        PrepareBuilder();

         result.repeatedCord_.Clear();

         return this;

       }

@@ -5123,11 +5401,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultInt32(value); }

       }

       public Builder SetDefaultInt32(int value) {

+        PrepareBuilder();

         result.hasDefaultInt32 = true;

         result.defaultInt32_ = value;

         return this;

       }

       public Builder ClearDefaultInt32() {

+        PrepareBuilder();

         result.hasDefaultInt32 = false;

         result.defaultInt32_ = 41;

         return this;

@@ -5141,11 +5421,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultInt64(value); }

       }

       public Builder SetDefaultInt64(long value) {

+        PrepareBuilder();

         result.hasDefaultInt64 = true;

         result.defaultInt64_ = value;

         return this;

       }

       public Builder ClearDefaultInt64() {

+        PrepareBuilder();

         result.hasDefaultInt64 = false;

         result.defaultInt64_ = 42L;

         return this;

@@ -5161,11 +5443,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetDefaultUint32(uint value) {

+        PrepareBuilder();

         result.hasDefaultUint32 = true;

         result.defaultUint32_ = value;

         return this;

       }

       public Builder ClearDefaultUint32() {

+        PrepareBuilder();

         result.hasDefaultUint32 = false;

         result.defaultUint32_ = 43;

         return this;

@@ -5181,11 +5465,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetDefaultUint64(ulong value) {

+        PrepareBuilder();

         result.hasDefaultUint64 = true;

         result.defaultUint64_ = value;

         return this;

       }

       public Builder ClearDefaultUint64() {

+        PrepareBuilder();

         result.hasDefaultUint64 = false;

         result.defaultUint64_ = 44UL;

         return this;

@@ -5199,11 +5485,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultSint32(value); }

       }

       public Builder SetDefaultSint32(int value) {

+        PrepareBuilder();

         result.hasDefaultSint32 = true;

         result.defaultSint32_ = value;

         return this;

       }

       public Builder ClearDefaultSint32() {

+        PrepareBuilder();

         result.hasDefaultSint32 = false;

         result.defaultSint32_ = -45;

         return this;

@@ -5217,11 +5505,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultSint64(value); }

       }

       public Builder SetDefaultSint64(long value) {

+        PrepareBuilder();

         result.hasDefaultSint64 = true;

         result.defaultSint64_ = value;

         return this;

       }

       public Builder ClearDefaultSint64() {

+        PrepareBuilder();

         result.hasDefaultSint64 = false;

         result.defaultSint64_ = 46;

         return this;

@@ -5237,11 +5527,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetDefaultFixed32(uint value) {

+        PrepareBuilder();

         result.hasDefaultFixed32 = true;

         result.defaultFixed32_ = value;

         return this;

       }

       public Builder ClearDefaultFixed32() {

+        PrepareBuilder();

         result.hasDefaultFixed32 = false;

         result.defaultFixed32_ = 47;

         return this;

@@ -5257,11 +5549,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetDefaultFixed64(ulong value) {

+        PrepareBuilder();

         result.hasDefaultFixed64 = true;

         result.defaultFixed64_ = value;

         return this;

       }

       public Builder ClearDefaultFixed64() {

+        PrepareBuilder();

         result.hasDefaultFixed64 = false;

         result.defaultFixed64_ = 48;

         return this;

@@ -5275,11 +5569,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultSfixed32(value); }

       }

       public Builder SetDefaultSfixed32(int value) {

+        PrepareBuilder();

         result.hasDefaultSfixed32 = true;

         result.defaultSfixed32_ = value;

         return this;

       }

       public Builder ClearDefaultSfixed32() {

+        PrepareBuilder();

         result.hasDefaultSfixed32 = false;

         result.defaultSfixed32_ = 49;

         return this;

@@ -5293,11 +5589,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultSfixed64(value); }

       }

       public Builder SetDefaultSfixed64(long value) {

+        PrepareBuilder();

         result.hasDefaultSfixed64 = true;

         result.defaultSfixed64_ = value;

         return this;

       }

       public Builder ClearDefaultSfixed64() {

+        PrepareBuilder();

         result.hasDefaultSfixed64 = false;

         result.defaultSfixed64_ = -50;

         return this;

@@ -5311,11 +5609,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultFloat(value); }

       }

       public Builder SetDefaultFloat(float value) {

+        PrepareBuilder();

         result.hasDefaultFloat = true;

         result.defaultFloat_ = value;

         return this;

       }

       public Builder ClearDefaultFloat() {

+        PrepareBuilder();

         result.hasDefaultFloat = false;

         result.defaultFloat_ = 51.5F;

         return this;

@@ -5329,11 +5629,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultDouble(value); }

       }

       public Builder SetDefaultDouble(double value) {

+        PrepareBuilder();

         result.hasDefaultDouble = true;

         result.defaultDouble_ = value;

         return this;

       }

       public Builder ClearDefaultDouble() {

+        PrepareBuilder();

         result.hasDefaultDouble = false;

         result.defaultDouble_ = 52000D;

         return this;

@@ -5347,11 +5649,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultBool(value); }

       }

       public Builder SetDefaultBool(bool value) {

+        PrepareBuilder();

         result.hasDefaultBool = true;

         result.defaultBool_ = value;

         return this;

       }

       public Builder ClearDefaultBool() {

+        PrepareBuilder();

         result.hasDefaultBool = false;

         result.defaultBool_ = true;

         return this;

@@ -5366,11 +5670,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetDefaultString(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasDefaultString = true;

         result.defaultString_ = value;

         return this;

       }

       public Builder ClearDefaultString() {

+        PrepareBuilder();

         result.hasDefaultString = false;

         result.defaultString_ = "hello";

         return this;

@@ -5385,11 +5691,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetDefaultBytes(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasDefaultBytes = true;

         result.defaultBytes_ = value;

         return this;

       }

       public Builder ClearDefaultBytes() {

+        PrepareBuilder();

         result.hasDefaultBytes = false;

         result.defaultBytes_ = (pb::ByteString) global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Descriptor.Fields[62].DefaultValue;

         return this;

@@ -5403,11 +5711,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultNestedEnum(value); }

       }

       public Builder SetDefaultNestedEnum(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum value) {

+        PrepareBuilder();

         result.hasDefaultNestedEnum = true;

         result.defaultNestedEnum_ = value;

         return this;

       }

       public Builder ClearDefaultNestedEnum() {

+        PrepareBuilder();

         result.hasDefaultNestedEnum = false;

         result.defaultNestedEnum_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedEnum.BAR;

         return this;

@@ -5421,11 +5731,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultForeignEnum(value); }

       }

       public Builder SetDefaultForeignEnum(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.hasDefaultForeignEnum = true;

         result.defaultForeignEnum_ = value;

         return this;

       }

       public Builder ClearDefaultForeignEnum() {

+        PrepareBuilder();

         result.hasDefaultForeignEnum = false;

         result.defaultForeignEnum_ = global::Google.ProtocolBuffers.TestProtos.ForeignEnum.FOREIGN_BAR;

         return this;

@@ -5439,11 +5751,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDefaultImportEnum(value); }

       }

       public Builder SetDefaultImportEnum(global::Google.ProtocolBuffers.TestProtos.ImportEnum value) {

+        PrepareBuilder();

         result.hasDefaultImportEnum = true;

         result.defaultImportEnum_ = value;

         return this;

       }

       public Builder ClearDefaultImportEnum() {

+        PrepareBuilder();

         result.hasDefaultImportEnum = false;

         result.defaultImportEnum_ = global::Google.ProtocolBuffers.TestProtos.ImportEnum.IMPORT_BAR;

         return this;

@@ -5458,11 +5772,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetDefaultStringPiece(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasDefaultStringPiece = true;

         result.defaultStringPiece_ = value;

         return this;

       }

       public Builder ClearDefaultStringPiece() {

+        PrepareBuilder();

         result.hasDefaultStringPiece = false;

         result.defaultStringPiece_ = "abc";

         return this;

@@ -5477,11 +5793,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetDefaultCord(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasDefaultCord = true;

         result.defaultCord_ = value;

         return this;

       }

       public Builder ClearDefaultCord() {

+        PrepareBuilder();

         result.hasDefaultCord = false;

         result.defaultCord_ = "123";

         return this;

@@ -5594,7 +5912,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestDeprecatedFields prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -5604,21 +5922,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestDeprecatedFields();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestDeprecatedFields cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestDeprecatedFields result;

       

-      TestDeprecatedFields result = new TestDeprecatedFields();

+      private TestDeprecatedFields PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestDeprecatedFields original = result;

+          result = new TestDeprecatedFields();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestDeprecatedFields MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestDeprecatedFields();

+        result = DefaultInstance ?? new TestDeprecatedFields();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -5630,12 +5975,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestDeprecatedFields BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestDeprecatedFields returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -5649,6 +5993,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestDeprecatedFields other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestDeprecatedFields.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasDeprecatedInt32) {

           DeprecatedInt32 = other.DeprecatedInt32;

         }

@@ -5661,6 +6006,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -5716,11 +6062,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDeprecatedInt32(value); }

       }

       public Builder SetDeprecatedInt32(int value) {

+        PrepareBuilder();

         result.hasDeprecatedInt32 = true;

         result.deprecatedInt32_ = value;

         return this;

       }

       public Builder ClearDeprecatedInt32() {

+        PrepareBuilder();

         result.hasDeprecatedInt32 = false;

         result.deprecatedInt32_ = 0;

         return this;

@@ -5833,7 +6181,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(ForeignMessage prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -5843,21 +6191,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new ForeignMessage();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(ForeignMessage cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      ForeignMessage result = new ForeignMessage();

+      bool builderIsReadOnly;

+      ForeignMessage result;

+      

+      private ForeignMessage PrepareBuilder() {

+        if (builderIsReadOnly) {

+          ForeignMessage original = result;

+          result = new ForeignMessage();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override ForeignMessage MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new ForeignMessage();

+        result = DefaultInstance ?? new ForeignMessage();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -5869,12 +6244,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ForeignMessage BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        ForeignMessage returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -5888,6 +6262,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(ForeignMessage other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasC) {

           C = other.C;

         }

@@ -5900,6 +6275,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -5955,11 +6331,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetC(value); }

       }

       public Builder SetC(int value) {

+        PrepareBuilder();

         result.hasC = true;

         result.c_ = value;

         return this;

       }

       public Builder ClearC() {

+        PrepareBuilder();

         result.hasC = false;

         result.c_ = 0;

         return this;

@@ -6060,7 +6438,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestAllExtensions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -6070,21 +6448,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestAllExtensions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestAllExtensions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestAllExtensions result = new TestAllExtensions();

+      bool builderIsReadOnly;

+      TestAllExtensions result;

+      

+      private TestAllExtensions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestAllExtensions original = result;

+          result = new TestAllExtensions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestAllExtensions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestAllExtensions();

+        result = DefaultInstance ?? new TestAllExtensions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -6096,12 +6501,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestAllExtensions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestAllExtensions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -6115,6 +6519,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestAllExtensions other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestAllExtensions.DefaultInstance) return this;

+        PrepareBuilder();

           this.MergeExtensionFields(other);

         this.MergeUnknownFields(other.UnknownFields);

         return this;

@@ -6125,6 +6530,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -6275,7 +6681,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(OptionalGroup_extension prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -6285,21 +6691,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new OptionalGroup_extension();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(OptionalGroup_extension cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      OptionalGroup_extension result = new OptionalGroup_extension();

+      bool builderIsReadOnly;

+      OptionalGroup_extension result;

+      

+      private OptionalGroup_extension PrepareBuilder() {

+        if (builderIsReadOnly) {

+          OptionalGroup_extension original = result;

+          result = new OptionalGroup_extension();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override OptionalGroup_extension MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new OptionalGroup_extension();

+        result = DefaultInstance ?? new OptionalGroup_extension();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -6311,12 +6744,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OptionalGroup_extension BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        OptionalGroup_extension returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -6330,6 +6762,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(OptionalGroup_extension other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.OptionalGroup_extension.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           A = other.A;

         }

@@ -6342,6 +6775,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -6397,11 +6831,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetA(value); }

       }

       public Builder SetA(int value) {

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = 0;

         return this;

@@ -6514,7 +6950,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(RepeatedGroup_extension prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -6524,21 +6960,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new RepeatedGroup_extension();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(RepeatedGroup_extension cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      RepeatedGroup_extension result;

+      

+      private RepeatedGroup_extension PrepareBuilder() {

+        if (builderIsReadOnly) {

+          RepeatedGroup_extension original = result;

+          result = new RepeatedGroup_extension();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      RepeatedGroup_extension result = new RepeatedGroup_extension();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override RepeatedGroup_extension MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new RepeatedGroup_extension();

+        result = DefaultInstance ?? new RepeatedGroup_extension();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -6550,12 +7013,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override RepeatedGroup_extension BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        RepeatedGroup_extension returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -6569,6 +7031,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(RepeatedGroup_extension other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.RepeatedGroup_extension.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           A = other.A;

         }

@@ -6581,6 +7044,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -6636,11 +7100,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetA(value); }

       }

       public Builder SetA(int value) {

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = 0;

         return this;

@@ -6739,7 +7205,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestNestedExtension prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -6749,21 +7215,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestNestedExtension();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestNestedExtension cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestNestedExtension result;

+      

+      private TestNestedExtension PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestNestedExtension original = result;

+          result = new TestNestedExtension();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestNestedExtension result = new TestNestedExtension();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestNestedExtension MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestNestedExtension();

+        result = DefaultInstance ?? new TestNestedExtension();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -6775,12 +7268,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestNestedExtension BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestNestedExtension returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -6794,6 +7286,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestNestedExtension other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestNestedExtension.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -6803,6 +7296,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -7472,7 +7966,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestRequired prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -7482,21 +7976,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestRequired();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestRequired cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestRequired result = new TestRequired();

+      bool builderIsReadOnly;

+      TestRequired result;

+      

+      private TestRequired PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestRequired original = result;

+          result = new TestRequired();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestRequired MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestRequired();

+        result = DefaultInstance ?? new TestRequired();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -7508,12 +8029,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRequired BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestRequired returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -7527,6 +8047,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestRequired other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestRequired.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           A = other.A;

         }

@@ -7635,6 +8156,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -7818,11 +8340,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetA(value); }

       }

       public Builder SetA(int value) {

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = 0;

         return this;

@@ -7836,11 +8360,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy2(value); }

       }

       public Builder SetDummy2(int value) {

+        PrepareBuilder();

         result.hasDummy2 = true;

         result.dummy2_ = value;

         return this;

       }

       public Builder ClearDummy2() {

+        PrepareBuilder();

         result.hasDummy2 = false;

         result.dummy2_ = 0;

         return this;

@@ -7854,11 +8380,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetB(value); }

       }

       public Builder SetB(int value) {

+        PrepareBuilder();

         result.hasB = true;

         result.b_ = value;

         return this;

       }

       public Builder ClearB() {

+        PrepareBuilder();

         result.hasB = false;

         result.b_ = 0;

         return this;

@@ -7872,11 +8400,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy4(value); }

       }

       public Builder SetDummy4(int value) {

+        PrepareBuilder();

         result.hasDummy4 = true;

         result.dummy4_ = value;

         return this;

       }

       public Builder ClearDummy4() {

+        PrepareBuilder();

         result.hasDummy4 = false;

         result.dummy4_ = 0;

         return this;

@@ -7890,11 +8420,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy5(value); }

       }

       public Builder SetDummy5(int value) {

+        PrepareBuilder();

         result.hasDummy5 = true;

         result.dummy5_ = value;

         return this;

       }

       public Builder ClearDummy5() {

+        PrepareBuilder();

         result.hasDummy5 = false;

         result.dummy5_ = 0;

         return this;

@@ -7908,11 +8440,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy6(value); }

       }

       public Builder SetDummy6(int value) {

+        PrepareBuilder();

         result.hasDummy6 = true;

         result.dummy6_ = value;

         return this;

       }

       public Builder ClearDummy6() {

+        PrepareBuilder();

         result.hasDummy6 = false;

         result.dummy6_ = 0;

         return this;

@@ -7926,11 +8460,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy7(value); }

       }

       public Builder SetDummy7(int value) {

+        PrepareBuilder();

         result.hasDummy7 = true;

         result.dummy7_ = value;

         return this;

       }

       public Builder ClearDummy7() {

+        PrepareBuilder();

         result.hasDummy7 = false;

         result.dummy7_ = 0;

         return this;

@@ -7944,11 +8480,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy8(value); }

       }

       public Builder SetDummy8(int value) {

+        PrepareBuilder();

         result.hasDummy8 = true;

         result.dummy8_ = value;

         return this;

       }

       public Builder ClearDummy8() {

+        PrepareBuilder();

         result.hasDummy8 = false;

         result.dummy8_ = 0;

         return this;

@@ -7962,11 +8500,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy9(value); }

       }

       public Builder SetDummy9(int value) {

+        PrepareBuilder();

         result.hasDummy9 = true;

         result.dummy9_ = value;

         return this;

       }

       public Builder ClearDummy9() {

+        PrepareBuilder();

         result.hasDummy9 = false;

         result.dummy9_ = 0;

         return this;

@@ -7980,11 +8520,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy10(value); }

       }

       public Builder SetDummy10(int value) {

+        PrepareBuilder();

         result.hasDummy10 = true;

         result.dummy10_ = value;

         return this;

       }

       public Builder ClearDummy10() {

+        PrepareBuilder();

         result.hasDummy10 = false;

         result.dummy10_ = 0;

         return this;

@@ -7998,11 +8540,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy11(value); }

       }

       public Builder SetDummy11(int value) {

+        PrepareBuilder();

         result.hasDummy11 = true;

         result.dummy11_ = value;

         return this;

       }

       public Builder ClearDummy11() {

+        PrepareBuilder();

         result.hasDummy11 = false;

         result.dummy11_ = 0;

         return this;

@@ -8016,11 +8560,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy12(value); }

       }

       public Builder SetDummy12(int value) {

+        PrepareBuilder();

         result.hasDummy12 = true;

         result.dummy12_ = value;

         return this;

       }

       public Builder ClearDummy12() {

+        PrepareBuilder();

         result.hasDummy12 = false;

         result.dummy12_ = 0;

         return this;

@@ -8034,11 +8580,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy13(value); }

       }

       public Builder SetDummy13(int value) {

+        PrepareBuilder();

         result.hasDummy13 = true;

         result.dummy13_ = value;

         return this;

       }

       public Builder ClearDummy13() {

+        PrepareBuilder();

         result.hasDummy13 = false;

         result.dummy13_ = 0;

         return this;

@@ -8052,11 +8600,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy14(value); }

       }

       public Builder SetDummy14(int value) {

+        PrepareBuilder();

         result.hasDummy14 = true;

         result.dummy14_ = value;

         return this;

       }

       public Builder ClearDummy14() {

+        PrepareBuilder();

         result.hasDummy14 = false;

         result.dummy14_ = 0;

         return this;

@@ -8070,11 +8620,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy15(value); }

       }

       public Builder SetDummy15(int value) {

+        PrepareBuilder();

         result.hasDummy15 = true;

         result.dummy15_ = value;

         return this;

       }

       public Builder ClearDummy15() {

+        PrepareBuilder();

         result.hasDummy15 = false;

         result.dummy15_ = 0;

         return this;

@@ -8088,11 +8640,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy16(value); }

       }

       public Builder SetDummy16(int value) {

+        PrepareBuilder();

         result.hasDummy16 = true;

         result.dummy16_ = value;

         return this;

       }

       public Builder ClearDummy16() {

+        PrepareBuilder();

         result.hasDummy16 = false;

         result.dummy16_ = 0;

         return this;

@@ -8106,11 +8660,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy17(value); }

       }

       public Builder SetDummy17(int value) {

+        PrepareBuilder();

         result.hasDummy17 = true;

         result.dummy17_ = value;

         return this;

       }

       public Builder ClearDummy17() {

+        PrepareBuilder();

         result.hasDummy17 = false;

         result.dummy17_ = 0;

         return this;

@@ -8124,11 +8680,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy18(value); }

       }

       public Builder SetDummy18(int value) {

+        PrepareBuilder();

         result.hasDummy18 = true;

         result.dummy18_ = value;

         return this;

       }

       public Builder ClearDummy18() {

+        PrepareBuilder();

         result.hasDummy18 = false;

         result.dummy18_ = 0;

         return this;

@@ -8142,11 +8700,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy19(value); }

       }

       public Builder SetDummy19(int value) {

+        PrepareBuilder();

         result.hasDummy19 = true;

         result.dummy19_ = value;

         return this;

       }

       public Builder ClearDummy19() {

+        PrepareBuilder();

         result.hasDummy19 = false;

         result.dummy19_ = 0;

         return this;

@@ -8160,11 +8720,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy20(value); }

       }

       public Builder SetDummy20(int value) {

+        PrepareBuilder();

         result.hasDummy20 = true;

         result.dummy20_ = value;

         return this;

       }

       public Builder ClearDummy20() {

+        PrepareBuilder();

         result.hasDummy20 = false;

         result.dummy20_ = 0;

         return this;

@@ -8178,11 +8740,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy21(value); }

       }

       public Builder SetDummy21(int value) {

+        PrepareBuilder();

         result.hasDummy21 = true;

         result.dummy21_ = value;

         return this;

       }

       public Builder ClearDummy21() {

+        PrepareBuilder();

         result.hasDummy21 = false;

         result.dummy21_ = 0;

         return this;

@@ -8196,11 +8760,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy22(value); }

       }

       public Builder SetDummy22(int value) {

+        PrepareBuilder();

         result.hasDummy22 = true;

         result.dummy22_ = value;

         return this;

       }

       public Builder ClearDummy22() {

+        PrepareBuilder();

         result.hasDummy22 = false;

         result.dummy22_ = 0;

         return this;

@@ -8214,11 +8780,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy23(value); }

       }

       public Builder SetDummy23(int value) {

+        PrepareBuilder();

         result.hasDummy23 = true;

         result.dummy23_ = value;

         return this;

       }

       public Builder ClearDummy23() {

+        PrepareBuilder();

         result.hasDummy23 = false;

         result.dummy23_ = 0;

         return this;

@@ -8232,11 +8800,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy24(value); }

       }

       public Builder SetDummy24(int value) {

+        PrepareBuilder();

         result.hasDummy24 = true;

         result.dummy24_ = value;

         return this;

       }

       public Builder ClearDummy24() {

+        PrepareBuilder();

         result.hasDummy24 = false;

         result.dummy24_ = 0;

         return this;

@@ -8250,11 +8820,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy25(value); }

       }

       public Builder SetDummy25(int value) {

+        PrepareBuilder();

         result.hasDummy25 = true;

         result.dummy25_ = value;

         return this;

       }

       public Builder ClearDummy25() {

+        PrepareBuilder();

         result.hasDummy25 = false;

         result.dummy25_ = 0;

         return this;

@@ -8268,11 +8840,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy26(value); }

       }

       public Builder SetDummy26(int value) {

+        PrepareBuilder();

         result.hasDummy26 = true;

         result.dummy26_ = value;

         return this;

       }

       public Builder ClearDummy26() {

+        PrepareBuilder();

         result.hasDummy26 = false;

         result.dummy26_ = 0;

         return this;

@@ -8286,11 +8860,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy27(value); }

       }

       public Builder SetDummy27(int value) {

+        PrepareBuilder();

         result.hasDummy27 = true;

         result.dummy27_ = value;

         return this;

       }

       public Builder ClearDummy27() {

+        PrepareBuilder();

         result.hasDummy27 = false;

         result.dummy27_ = 0;

         return this;

@@ -8304,11 +8880,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy28(value); }

       }

       public Builder SetDummy28(int value) {

+        PrepareBuilder();

         result.hasDummy28 = true;

         result.dummy28_ = value;

         return this;

       }

       public Builder ClearDummy28() {

+        PrepareBuilder();

         result.hasDummy28 = false;

         result.dummy28_ = 0;

         return this;

@@ -8322,11 +8900,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy29(value); }

       }

       public Builder SetDummy29(int value) {

+        PrepareBuilder();

         result.hasDummy29 = true;

         result.dummy29_ = value;

         return this;

       }

       public Builder ClearDummy29() {

+        PrepareBuilder();

         result.hasDummy29 = false;

         result.dummy29_ = 0;

         return this;

@@ -8340,11 +8920,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy30(value); }

       }

       public Builder SetDummy30(int value) {

+        PrepareBuilder();

         result.hasDummy30 = true;

         result.dummy30_ = value;

         return this;

       }

       public Builder ClearDummy30() {

+        PrepareBuilder();

         result.hasDummy30 = false;

         result.dummy30_ = 0;

         return this;

@@ -8358,11 +8940,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy31(value); }

       }

       public Builder SetDummy31(int value) {

+        PrepareBuilder();

         result.hasDummy31 = true;

         result.dummy31_ = value;

         return this;

       }

       public Builder ClearDummy31() {

+        PrepareBuilder();

         result.hasDummy31 = false;

         result.dummy31_ = 0;

         return this;

@@ -8376,11 +8960,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy32(value); }

       }

       public Builder SetDummy32(int value) {

+        PrepareBuilder();

         result.hasDummy32 = true;

         result.dummy32_ = value;

         return this;

       }

       public Builder ClearDummy32() {

+        PrepareBuilder();

         result.hasDummy32 = false;

         result.dummy32_ = 0;

         return this;

@@ -8394,11 +8980,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetC(value); }

       }

       public Builder SetC(int value) {

+        PrepareBuilder();

         result.hasC = true;

         result.c_ = value;

         return this;

       }

       public Builder ClearC() {

+        PrepareBuilder();

         result.hasC = false;

         result.c_ = 0;

         return this;

@@ -8551,7 +9139,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestRequiredForeign prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -8561,21 +9149,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestRequiredForeign();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestRequiredForeign cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestRequiredForeign result;

       

-      TestRequiredForeign result = new TestRequiredForeign();

+      private TestRequiredForeign PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestRequiredForeign original = result;

+          result = new TestRequiredForeign();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestRequiredForeign MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestRequiredForeign();

+        result = DefaultInstance ?? new TestRequiredForeign();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -8587,13 +9202,12 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRequiredForeign BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.repeatedMessage_.MakeReadOnly();

-        TestRequiredForeign returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -8607,6 +9221,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestRequiredForeign other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestRequiredForeign.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasOptionalMessage) {

           MergeOptionalMessage(other.OptionalMessage);

         }

@@ -8625,6 +9240,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -8694,18 +9310,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalMessage(global::Google.ProtocolBuffers.TestProtos.TestRequired value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalMessage = true;

         result.optionalMessage_ = value;

         return this;

       }

       public Builder SetOptionalMessage(global::Google.ProtocolBuffers.TestProtos.TestRequired.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalMessage = true;

         result.optionalMessage_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalMessage(global::Google.ProtocolBuffers.TestProtos.TestRequired value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalMessage &&

             result.optionalMessage_ != global::Google.ProtocolBuffers.TestProtos.TestRequired.DefaultInstance) {

             result.optionalMessage_ = global::Google.ProtocolBuffers.TestProtos.TestRequired.CreateBuilder(result.optionalMessage_).MergeFrom(value).BuildPartial();

@@ -8716,13 +9335,14 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalMessage() {

+        PrepareBuilder();

         result.hasOptionalMessage = false;

         result.optionalMessage_ = global::Google.ProtocolBuffers.TestProtos.TestRequired.DefaultInstance;

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.TestRequired> RepeatedMessageList {

-        get { return result.repeatedMessage_; }

+        get { return PrepareBuilder().repeatedMessage_; }

       }

       public int RepeatedMessageCount {

         get { return result.RepeatedMessageCount; }

@@ -8732,29 +9352,35 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedMessage(int index, global::Google.ProtocolBuffers.TestProtos.TestRequired value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedMessage_[index] = value;

         return this;

       }

       public Builder SetRepeatedMessage(int index, global::Google.ProtocolBuffers.TestProtos.TestRequired.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedMessage_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedMessage(global::Google.ProtocolBuffers.TestProtos.TestRequired value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedMessage_.Add(value);

         return this;

       }

       public Builder AddRepeatedMessage(global::Google.ProtocolBuffers.TestProtos.TestRequired.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedMessage_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedMessage(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.TestRequired> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedMessage_);

         return this;

       }

       public Builder ClearRepeatedMessage() {

+        PrepareBuilder();

         result.repeatedMessage_.Clear();

         return this;

       }

@@ -8767,11 +9393,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDummy(value); }

       }

       public Builder SetDummy(int value) {

+        PrepareBuilder();

         result.hasDummy = true;

         result.dummy_ = value;

         return this;

       }

       public Builder ClearDummy() {

+        PrepareBuilder();

         result.hasDummy = false;

         result.dummy_ = 0;

         return this;

@@ -8884,7 +9512,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestForeignNested prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -8894,21 +9522,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestForeignNested();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestForeignNested cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestForeignNested result = new TestForeignNested();

+      bool builderIsReadOnly;

+      TestForeignNested result;

+      

+      private TestForeignNested PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestForeignNested original = result;

+          result = new TestForeignNested();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestForeignNested MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestForeignNested();

+        result = DefaultInstance ?? new TestForeignNested();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -8920,12 +9575,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestForeignNested BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestForeignNested returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -8939,6 +9593,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestForeignNested other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestForeignNested.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasForeignNested) {

           MergeForeignNested(other.ForeignNested);

         }

@@ -8951,6 +9606,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -9012,18 +9668,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetForeignNested(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasForeignNested = true;

         result.foreignNested_ = value;

         return this;

       }

       public Builder SetForeignNested(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasForeignNested = true;

         result.foreignNested_ = builderForValue.Build();

         return this;

       }

       public Builder MergeForeignNested(global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasForeignNested &&

             result.foreignNested_ != global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance) {

             result.foreignNested_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.CreateBuilder(result.foreignNested_).MergeFrom(value).BuildPartial();

@@ -9034,6 +9693,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearForeignNested() {

+        PrepareBuilder();

         result.hasForeignNested = false;

         result.foreignNested_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance;

         return this;

@@ -9130,7 +9790,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestEmptyMessage prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -9140,21 +9800,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestEmptyMessage();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestEmptyMessage cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestEmptyMessage result;

+      

+      private TestEmptyMessage PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestEmptyMessage original = result;

+          result = new TestEmptyMessage();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestEmptyMessage result = new TestEmptyMessage();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestEmptyMessage MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestEmptyMessage();

+        result = DefaultInstance ?? new TestEmptyMessage();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -9166,12 +9853,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestEmptyMessage BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestEmptyMessage returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -9185,6 +9871,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestEmptyMessage other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestEmptyMessage.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -9194,6 +9881,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -9332,7 +10020,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestEmptyMessageWithExtensions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -9342,21 +10030,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestEmptyMessageWithExtensions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestEmptyMessageWithExtensions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestEmptyMessageWithExtensions result = new TestEmptyMessageWithExtensions();

+      bool builderIsReadOnly;

+      TestEmptyMessageWithExtensions result;

+      

+      private TestEmptyMessageWithExtensions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestEmptyMessageWithExtensions original = result;

+          result = new TestEmptyMessageWithExtensions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestEmptyMessageWithExtensions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestEmptyMessageWithExtensions();

+        result = DefaultInstance ?? new TestEmptyMessageWithExtensions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -9368,12 +10083,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestEmptyMessageWithExtensions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestEmptyMessageWithExtensions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -9387,6 +10101,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestEmptyMessageWithExtensions other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestEmptyMessageWithExtensions.DefaultInstance) return this;

+        PrepareBuilder();

           this.MergeExtensionFields(other);

         this.MergeUnknownFields(other.UnknownFields);

         return this;

@@ -9397,6 +10112,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -9537,7 +10253,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestMultipleExtensionRanges prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -9547,21 +10263,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestMultipleExtensionRanges();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestMultipleExtensionRanges cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestMultipleExtensionRanges result;

+      

+      private TestMultipleExtensionRanges PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestMultipleExtensionRanges original = result;

+          result = new TestMultipleExtensionRanges();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestMultipleExtensionRanges result = new TestMultipleExtensionRanges();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestMultipleExtensionRanges MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestMultipleExtensionRanges();

+        result = DefaultInstance ?? new TestMultipleExtensionRanges();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -9573,12 +10316,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMultipleExtensionRanges BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestMultipleExtensionRanges returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -9592,6 +10334,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestMultipleExtensionRanges other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestMultipleExtensionRanges.DefaultInstance) return this;

+        PrepareBuilder();

           this.MergeExtensionFields(other);

         this.MergeUnknownFields(other.UnknownFields);

         return this;

@@ -9602,6 +10345,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -9768,7 +10512,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestReallyLargeTagNumber prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -9778,21 +10522,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestReallyLargeTagNumber();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestReallyLargeTagNumber cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestReallyLargeTagNumber result;

+      

+      private TestReallyLargeTagNumber PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestReallyLargeTagNumber original = result;

+          result = new TestReallyLargeTagNumber();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestReallyLargeTagNumber result = new TestReallyLargeTagNumber();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestReallyLargeTagNumber MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestReallyLargeTagNumber();

+        result = DefaultInstance ?? new TestReallyLargeTagNumber();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -9804,12 +10575,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestReallyLargeTagNumber BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestReallyLargeTagNumber returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -9823,6 +10593,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestReallyLargeTagNumber other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestReallyLargeTagNumber.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           A = other.A;

         }

@@ -9838,6 +10609,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -9897,11 +10669,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetA(value); }

       }

       public Builder SetA(int value) {

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = 0;

         return this;

@@ -9915,11 +10689,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetBb(value); }

       }

       public Builder SetBb(int value) {

+        PrepareBuilder();

         result.hasBb = true;

         result.bb_ = value;

         return this;

       }

       public Builder ClearBb() {

+        PrepareBuilder();

         result.hasBb = false;

         result.bb_ = 0;

         return this;

@@ -10048,7 +10824,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestRecursiveMessage prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -10058,21 +10834,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestRecursiveMessage();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestRecursiveMessage cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestRecursiveMessage result = new TestRecursiveMessage();

+      bool builderIsReadOnly;

+      TestRecursiveMessage result;

+      

+      private TestRecursiveMessage PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestRecursiveMessage original = result;

+          result = new TestRecursiveMessage();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestRecursiveMessage MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestRecursiveMessage();

+        result = DefaultInstance ?? new TestRecursiveMessage();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -10084,12 +10887,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRecursiveMessage BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestRecursiveMessage returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -10103,6 +10905,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestRecursiveMessage other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           MergeA(other.A);

         }

@@ -10118,6 +10921,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -10183,18 +10987,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetA(global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder SetA(global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = builderForValue.Build();

         return this;

       }

       public Builder MergeA(global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasA &&

             result.a_ != global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.DefaultInstance) {

             result.a_ = global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.CreateBuilder(result.a_).MergeFrom(value).BuildPartial();

@@ -10205,6 +11012,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.DefaultInstance;

         return this;

@@ -10218,11 +11026,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetI(value); }

       }

       public Builder SetI(int value) {

+        PrepareBuilder();

         result.hasI = true;

         result.i_ = value;

         return this;

       }

       public Builder ClearI() {

+        PrepareBuilder();

         result.hasI = false;

         result.i_ = 0;

         return this;

@@ -10335,7 +11145,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestMutualRecursionA prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -10345,21 +11155,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestMutualRecursionA();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestMutualRecursionA cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestMutualRecursionA result;

+      

+      private TestMutualRecursionA PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestMutualRecursionA original = result;

+          result = new TestMutualRecursionA();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestMutualRecursionA result = new TestMutualRecursionA();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestMutualRecursionA MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestMutualRecursionA();

+        result = DefaultInstance ?? new TestMutualRecursionA();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -10371,12 +11208,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMutualRecursionA BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestMutualRecursionA returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -10390,6 +11226,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestMutualRecursionA other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasBb) {

           MergeBb(other.Bb);

         }

@@ -10402,6 +11239,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -10463,18 +11301,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetBb(global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasBb = true;

         result.bb_ = value;

         return this;

       }

       public Builder SetBb(global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasBb = true;

         result.bb_ = builderForValue.Build();

         return this;

       }

       public Builder MergeBb(global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasBb &&

             result.bb_ != global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.DefaultInstance) {

             result.bb_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.CreateBuilder(result.bb_).MergeFrom(value).BuildPartial();

@@ -10485,6 +11326,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearBb() {

+        PrepareBuilder();

         result.hasBb = false;

         result.bb_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.DefaultInstance;

         return this;

@@ -10613,7 +11455,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestMutualRecursionB prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -10623,21 +11465,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestMutualRecursionB();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestMutualRecursionB cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestMutualRecursionB result = new TestMutualRecursionB();

+      bool builderIsReadOnly;

+      TestMutualRecursionB result;

+      

+      private TestMutualRecursionB PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestMutualRecursionB original = result;

+          result = new TestMutualRecursionB();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestMutualRecursionB MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestMutualRecursionB();

+        result = DefaultInstance ?? new TestMutualRecursionB();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -10649,12 +11518,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMutualRecursionB BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestMutualRecursionB returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -10668,6 +11536,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestMutualRecursionB other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           MergeA(other.A);

         }

@@ -10683,6 +11552,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -10748,18 +11618,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetA(global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder SetA(global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = builderForValue.Build();

         return this;

       }

       public Builder MergeA(global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasA &&

             result.a_ != global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.DefaultInstance) {

             result.a_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.CreateBuilder(result.a_).MergeFrom(value).BuildPartial();

@@ -10770,6 +11643,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.DefaultInstance;

         return this;

@@ -10783,11 +11657,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOptionalInt32(value); }

       }

       public Builder SetOptionalInt32(int value) {

+        PrepareBuilder();

         result.hasOptionalInt32 = true;

         result.optionalInt32_ = value;

         return this;

       }

       public Builder ClearOptionalInt32() {

+        PrepareBuilder();

         result.hasOptionalInt32 = false;

         result.optionalInt32_ = 0;

         return this;

@@ -10932,7 +11808,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(Foo prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -10942,21 +11818,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new Foo();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(Foo cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          Foo result;

           

-          Foo result = new Foo();

+          private Foo PrepareBuilder() {

+            if (builderIsReadOnly) {

+              Foo original = result;

+              result = new Foo();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override Foo MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new Foo();

+            result = DefaultInstance ?? new Foo();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -10968,12 +11871,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Foo BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            Foo returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -10987,6 +11889,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(Foo other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasA) {

               A = other.A;

             }

@@ -10999,6 +11902,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -11054,11 +11958,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetA(value); }

           }

           public Builder SetA(int value) {

+            PrepareBuilder();

             result.hasA = true;

             result.a_ = value;

             return this;

           }

           public Builder ClearA() {

+            PrepareBuilder();

             result.hasA = false;

             result.a_ = 0;

             return this;

@@ -11171,7 +12077,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(Bar prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -11181,21 +12087,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new Bar();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(Bar cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

           

-          Bar result = new Bar();

+          bool builderIsReadOnly;

+          Bar result;

+          

+          private Bar PrepareBuilder() {

+            if (builderIsReadOnly) {

+              Bar original = result;

+              result = new Bar();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override Bar MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new Bar();

+            result = DefaultInstance ?? new Bar();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -11207,12 +12140,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Bar BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            Bar returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -11226,6 +12158,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(Bar other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasA) {

               A = other.A;

             }

@@ -11238,6 +12171,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -11293,11 +12227,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetA(value); }

           }

           public Builder SetA(int value) {

+            PrepareBuilder();

             result.hasA = true;

             result.a_ = value;

             return this;

           }

           public Builder ClearA() {

+            PrepareBuilder();

             result.hasA = false;

             result.a_ = 0;

             return this;

@@ -11418,7 +12354,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestDupFieldNumber prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -11428,21 +12364,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestDupFieldNumber();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestDupFieldNumber cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestDupFieldNumber result = new TestDupFieldNumber();

+      bool builderIsReadOnly;

+      TestDupFieldNumber result;

+      

+      private TestDupFieldNumber PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestDupFieldNumber original = result;

+          result = new TestDupFieldNumber();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestDupFieldNumber MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestDupFieldNumber();

+        result = DefaultInstance ?? new TestDupFieldNumber();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -11454,12 +12417,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestDupFieldNumber BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestDupFieldNumber returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -11473,6 +12435,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestDupFieldNumber other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasA) {

           A = other.A;

         }

@@ -11491,6 +12454,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -11564,11 +12528,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetA(value); }

       }

       public Builder SetA(int value) {

+        PrepareBuilder();

         result.hasA = true;

         result.a_ = value;

         return this;

       }

       public Builder ClearA() {

+        PrepareBuilder();

         result.hasA = false;

         result.a_ = 0;

         return this;

@@ -11583,18 +12549,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetFoo(global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasFoo = true;

         result.foo_ = value;

         return this;

       }

       public Builder SetFoo(global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasFoo = true;

         result.foo_ = builderForValue.Build();

         return this;

       }

       public Builder MergeFoo(global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasFoo &&

             result.foo_ != global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.DefaultInstance) {

             result.foo_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.CreateBuilder(result.foo_).MergeFrom(value).BuildPartial();

@@ -11605,6 +12574,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearFoo() {

+        PrepareBuilder();

         result.hasFoo = false;

         result.foo_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.DefaultInstance;

         return this;

@@ -11619,18 +12589,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetBar(global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasBar = true;

         result.bar_ = value;

         return this;

       }

       public Builder SetBar(global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasBar = true;

         result.bar_ = builderForValue.Build();

         return this;

       }

       public Builder MergeBar(global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasBar &&

             result.bar_ != global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.DefaultInstance) {

             result.bar_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.CreateBuilder(result.bar_).MergeFrom(value).BuildPartial();

@@ -11641,6 +12614,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearBar() {

+        PrepareBuilder();

         result.hasBar = false;

         result.bar_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.DefaultInstance;

         return this;

@@ -11810,7 +12784,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(NestedMessage prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -11820,21 +12794,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new NestedMessage();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(NestedMessage cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          NestedMessage result;

+          

+          private NestedMessage PrepareBuilder() {

+            if (builderIsReadOnly) {

+              NestedMessage original = result;

+              result = new NestedMessage();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

           

-          NestedMessage result = new NestedMessage();

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override NestedMessage MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new NestedMessage();

+            result = DefaultInstance ?? new NestedMessage();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -11846,14 +12847,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override NestedMessage BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

             result.nestedmessageRepeatedInt32_.MakeReadOnly();

             result.nestedmessageRepeatedForeignmessage_.MakeReadOnly();

-            NestedMessage returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -11867,6 +12867,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(NestedMessage other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.nestedmessageRepeatedInt32_.Count != 0) {

               base.AddRange(other.nestedmessageRepeatedInt32_, result.nestedmessageRepeatedInt32_);

             }

@@ -11882,6 +12883,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -11935,7 +12937,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           

           public pbc::IPopsicleList<int> NestedmessageRepeatedInt32List {

-            get { return result.nestedmessageRepeatedInt32_; }

+            get { return PrepareBuilder().nestedmessageRepeatedInt32_; }

           }

           public int NestedmessageRepeatedInt32Count {

             get { return result.NestedmessageRepeatedInt32Count; }

@@ -11944,24 +12946,28 @@ namespace Google.ProtocolBuffers.TestProtos {
             return result.GetNestedmessageRepeatedInt32(index);

           }

           public Builder SetNestedmessageRepeatedInt32(int index, int value) {

+            PrepareBuilder();

             result.nestedmessageRepeatedInt32_[index] = value;

             return this;

           }

           public Builder AddNestedmessageRepeatedInt32(int value) {

+            PrepareBuilder();

             result.nestedmessageRepeatedInt32_.Add(value);

             return this;

           }

           public Builder AddRangeNestedmessageRepeatedInt32(scg::IEnumerable<int> values) {

+            PrepareBuilder();

             base.AddRange(values, result.nestedmessageRepeatedInt32_);

             return this;

           }

           public Builder ClearNestedmessageRepeatedInt32() {

+            PrepareBuilder();

             result.nestedmessageRepeatedInt32_.Clear();

             return this;

           }

           

           public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignMessage> NestedmessageRepeatedForeignmessageList {

-            get { return result.nestedmessageRepeatedForeignmessage_; }

+            get { return PrepareBuilder().nestedmessageRepeatedForeignmessage_; }

           }

           public int NestedmessageRepeatedForeignmessageCount {

             get { return result.NestedmessageRepeatedForeignmessageCount; }

@@ -11971,29 +12977,35 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           public Builder SetNestedmessageRepeatedForeignmessage(int index, global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.nestedmessageRepeatedForeignmessage_[index] = value;

             return this;

           }

           public Builder SetNestedmessageRepeatedForeignmessage(int index, global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

             pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+            PrepareBuilder();

             result.nestedmessageRepeatedForeignmessage_[index] = builderForValue.Build();

             return this;

           }

           public Builder AddNestedmessageRepeatedForeignmessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

             pb::ThrowHelper.ThrowIfNull(value, "value");

+            PrepareBuilder();

             result.nestedmessageRepeatedForeignmessage_.Add(value);

             return this;

           }

           public Builder AddNestedmessageRepeatedForeignmessage(global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

             pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+            PrepareBuilder();

             result.nestedmessageRepeatedForeignmessage_.Add(builderForValue.Build());

             return this;

           }

           public Builder AddRangeNestedmessageRepeatedForeignmessage(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignMessage> values) {

+            PrepareBuilder();

             base.AddRange(values, result.nestedmessageRepeatedForeignmessage_);

             return this;

           }

           public Builder ClearNestedmessageRepeatedForeignmessage() {

+            PrepareBuilder();

             result.nestedmessageRepeatedForeignmessage_.Clear();

             return this;

           }

@@ -12081,7 +13093,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestNestedMessageHasBits prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -12091,21 +13103,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestNestedMessageHasBits();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestNestedMessageHasBits cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestNestedMessageHasBits result = new TestNestedMessageHasBits();

+      bool builderIsReadOnly;

+      TestNestedMessageHasBits result;

+      

+      private TestNestedMessageHasBits PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestNestedMessageHasBits original = result;

+          result = new TestNestedMessageHasBits();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestNestedMessageHasBits MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestNestedMessageHasBits();

+        result = DefaultInstance ?? new TestNestedMessageHasBits();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -12117,12 +13156,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestNestedMessageHasBits BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestNestedMessageHasBits returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -12136,6 +13174,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestNestedMessageHasBits other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasOptionalNestedMessage) {

           MergeOptionalNestedMessage(other.OptionalNestedMessage);

         }

@@ -12148,6 +13187,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -12209,18 +13249,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetOptionalNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasOptionalNestedMessage = true;

         result.optionalNestedMessage_ = value;

         return this;

       }

       public Builder SetOptionalNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasOptionalNestedMessage = true;

         result.optionalNestedMessage_ = builderForValue.Build();

         return this;

       }

       public Builder MergeOptionalNestedMessage(global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasOptionalNestedMessage &&

             result.optionalNestedMessage_ != global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.DefaultInstance) {

             result.optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.CreateBuilder(result.optionalNestedMessage_).MergeFrom(value).BuildPartial();

@@ -12231,6 +13274,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearOptionalNestedMessage() {

+        PrepareBuilder();

         result.hasOptionalNestedMessage = false;

         result.optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.DefaultInstance;

         return this;

@@ -12558,7 +13602,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestCamelCaseFieldNames prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -12568,21 +13612,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestCamelCaseFieldNames();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestCamelCaseFieldNames cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestCamelCaseFieldNames result;

+      

+      private TestCamelCaseFieldNames PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestCamelCaseFieldNames original = result;

+          result = new TestCamelCaseFieldNames();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestCamelCaseFieldNames result = new TestCamelCaseFieldNames();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestCamelCaseFieldNames MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestCamelCaseFieldNames();

+        result = DefaultInstance ?? new TestCamelCaseFieldNames();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -12594,8 +13665,8 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestCamelCaseFieldNames BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.repeatedPrimitiveField_.MakeReadOnly();

         result.repeatedStringField_.MakeReadOnly();

@@ -12603,9 +13674,8 @@ namespace Google.ProtocolBuffers.TestProtos {
         result.repeatedMessageField_.MakeReadOnly();

         result.repeatedStringPieceField_.MakeReadOnly();

         result.repeatedCordField_.MakeReadOnly();

-        TestCamelCaseFieldNames returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -12619,6 +13689,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestCamelCaseFieldNames other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestCamelCaseFieldNames.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasPrimitiveField) {

           PrimitiveField = other.PrimitiveField;

         }

@@ -12664,6 +13735,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -12787,11 +13859,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetPrimitiveField(value); }

       }

       public Builder SetPrimitiveField(int value) {

+        PrepareBuilder();

         result.hasPrimitiveField = true;

         result.primitiveField_ = value;

         return this;

       }

       public Builder ClearPrimitiveField() {

+        PrepareBuilder();

         result.hasPrimitiveField = false;

         result.primitiveField_ = 0;

         return this;

@@ -12806,11 +13880,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetStringField(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasStringField = true;

         result.stringField_ = value;

         return this;

       }

       public Builder ClearStringField() {

+        PrepareBuilder();

         result.hasStringField = false;

         result.stringField_ = "";

         return this;

@@ -12824,11 +13900,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetEnumField(value); }

       }

       public Builder SetEnumField(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.hasEnumField = true;

         result.enumField_ = value;

         return this;

       }

       public Builder ClearEnumField() {

+        PrepareBuilder();

         result.hasEnumField = false;

         result.enumField_ = global::Google.ProtocolBuffers.TestProtos.ForeignEnum.FOREIGN_FOO;

         return this;

@@ -12843,18 +13921,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetMessageField(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasMessageField = true;

         result.messageField_ = value;

         return this;

       }

       public Builder SetMessageField(global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasMessageField = true;

         result.messageField_ = builderForValue.Build();

         return this;

       }

       public Builder MergeMessageField(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasMessageField &&

             result.messageField_ != global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance) {

             result.messageField_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.CreateBuilder(result.messageField_).MergeFrom(value).BuildPartial();

@@ -12865,6 +13946,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearMessageField() {

+        PrepareBuilder();

         result.hasMessageField = false;

         result.messageField_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

         return this;

@@ -12879,11 +13961,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetStringPieceField(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasStringPieceField = true;

         result.stringPieceField_ = value;

         return this;

       }

       public Builder ClearStringPieceField() {

+        PrepareBuilder();

         result.hasStringPieceField = false;

         result.stringPieceField_ = "";

         return this;

@@ -12898,18 +13982,20 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetCordField(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasCordField = true;

         result.cordField_ = value;

         return this;

       }

       public Builder ClearCordField() {

+        PrepareBuilder();

         result.hasCordField = false;

         result.cordField_ = "";

         return this;

       }

       

       public pbc::IPopsicleList<int> RepeatedPrimitiveFieldList {

-        get { return result.repeatedPrimitiveField_; }

+        get { return PrepareBuilder().repeatedPrimitiveField_; }

       }

       public int RepeatedPrimitiveFieldCount {

         get { return result.RepeatedPrimitiveFieldCount; }

@@ -12918,24 +14004,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedPrimitiveField(index);

       }

       public Builder SetRepeatedPrimitiveField(int index, int value) {

+        PrepareBuilder();

         result.repeatedPrimitiveField_[index] = value;

         return this;

       }

       public Builder AddRepeatedPrimitiveField(int value) {

+        PrepareBuilder();

         result.repeatedPrimitiveField_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedPrimitiveField(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedPrimitiveField_);

         return this;

       }

       public Builder ClearRepeatedPrimitiveField() {

+        PrepareBuilder();

         result.repeatedPrimitiveField_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedStringFieldList {

-        get { return result.repeatedStringField_; }

+        get { return PrepareBuilder().repeatedStringField_; }

       }

       public int RepeatedStringFieldCount {

         get { return result.RepeatedStringFieldCount; }

@@ -12945,25 +14035,29 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedStringField(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedStringField_[index] = value;

         return this;

       }

       public Builder AddRepeatedStringField(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedStringField_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedStringField(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedStringField_);

         return this;

       }

       public Builder ClearRepeatedStringField() {

+        PrepareBuilder();

         result.repeatedStringField_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> RepeatedEnumFieldList {

-        get { return result.repeatedEnumField_; }

+        get { return PrepareBuilder().repeatedEnumField_; }

       }

       public int RepeatedEnumFieldCount {

         get { return result.RepeatedEnumFieldCount; }

@@ -12972,24 +14066,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedEnumField(index);

       }

       public Builder SetRepeatedEnumField(int index, global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.repeatedEnumField_[index] = value;

         return this;

       }

       public Builder AddRepeatedEnumField(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.repeatedEnumField_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedEnumField(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedEnumField_);

         return this;

       }

       public Builder ClearRepeatedEnumField() {

+        PrepareBuilder();

         result.repeatedEnumField_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignMessage> RepeatedMessageFieldList {

-        get { return result.repeatedMessageField_; }

+        get { return PrepareBuilder().repeatedMessageField_; }

       }

       public int RepeatedMessageFieldCount {

         get { return result.RepeatedMessageFieldCount; }

@@ -12999,35 +14097,41 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedMessageField(int index, global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedMessageField_[index] = value;

         return this;

       }

       public Builder SetRepeatedMessageField(int index, global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedMessageField_[index] = builderForValue.Build();

         return this;

       }

       public Builder AddRepeatedMessageField(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedMessageField_.Add(value);

         return this;

       }

       public Builder AddRepeatedMessageField(global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.repeatedMessageField_.Add(builderForValue.Build());

         return this;

       }

       public Builder AddRangeRepeatedMessageField(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignMessage> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedMessageField_);

         return this;

       }

       public Builder ClearRepeatedMessageField() {

+        PrepareBuilder();

         result.repeatedMessageField_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedStringPieceFieldList {

-        get { return result.repeatedStringPieceField_; }

+        get { return PrepareBuilder().repeatedStringPieceField_; }

       }

       public int RepeatedStringPieceFieldCount {

         get { return result.RepeatedStringPieceFieldCount; }

@@ -13037,25 +14141,29 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedStringPieceField(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedStringPieceField_[index] = value;

         return this;

       }

       public Builder AddRepeatedStringPieceField(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedStringPieceField_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedStringPieceField(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedStringPieceField_);

         return this;

       }

       public Builder ClearRepeatedStringPieceField() {

+        PrepareBuilder();

         result.repeatedStringPieceField_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedCordFieldList {

-        get { return result.repeatedCordField_; }

+        get { return PrepareBuilder().repeatedCordField_; }

       }

       public int RepeatedCordFieldCount {

         get { return result.RepeatedCordFieldCount; }

@@ -13065,19 +14173,23 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedCordField(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedCordField_[index] = value;

         return this;

       }

       public Builder AddRepeatedCordField(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedCordField_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedCordField(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedCordField_);

         return this;

       }

       public Builder ClearRepeatedCordField() {

+        PrepareBuilder();

         result.repeatedCordField_.Clear();

         return this;

       }

@@ -13226,7 +14338,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestFieldOrderings prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -13236,21 +14348,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestFieldOrderings();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestFieldOrderings cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestFieldOrderings result;

+      

+      private TestFieldOrderings PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestFieldOrderings original = result;

+          result = new TestFieldOrderings();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestFieldOrderings result = new TestFieldOrderings();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestFieldOrderings MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestFieldOrderings();

+        result = DefaultInstance ?? new TestFieldOrderings();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -13262,12 +14401,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestFieldOrderings BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestFieldOrderings returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -13281,6 +14419,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestFieldOrderings other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestFieldOrderings.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasMyString) {

           MyString = other.MyString;

         }

@@ -13300,6 +14439,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -13364,11 +14504,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetMyString(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasMyString = true;

         result.myString_ = value;

         return this;

       }

       public Builder ClearMyString() {

+        PrepareBuilder();

         result.hasMyString = false;

         result.myString_ = "";

         return this;

@@ -13382,11 +14524,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetMyInt(value); }

       }

       public Builder SetMyInt(long value) {

+        PrepareBuilder();

         result.hasMyInt = true;

         result.myInt_ = value;

         return this;

       }

       public Builder ClearMyInt() {

+        PrepareBuilder();

         result.hasMyInt = false;

         result.myInt_ = 0L;

         return this;

@@ -13400,11 +14544,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetMyFloat(value); }

       }

       public Builder SetMyFloat(float value) {

+        PrepareBuilder();

         result.hasMyFloat = true;

         result.myFloat_ = value;

         return this;

       }

       public Builder ClearMyFloat() {

+        PrepareBuilder();

         result.hasMyFloat = false;

         result.myFloat_ = 0F;

         return this;

@@ -13807,7 +14953,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestExtremeDefaultValues prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -13817,21 +14963,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestExtremeDefaultValues();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestExtremeDefaultValues cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestExtremeDefaultValues result;

       

-      TestExtremeDefaultValues result = new TestExtremeDefaultValues();

+      private TestExtremeDefaultValues PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestExtremeDefaultValues original = result;

+          result = new TestExtremeDefaultValues();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestExtremeDefaultValues MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestExtremeDefaultValues();

+        result = DefaultInstance ?? new TestExtremeDefaultValues();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -13843,12 +15016,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestExtremeDefaultValues BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestExtremeDefaultValues returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -13862,6 +15034,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestExtremeDefaultValues other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestExtremeDefaultValues.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasEscapedBytes) {

           EscapedBytes = other.EscapedBytes;

         }

@@ -13928,6 +15101,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -14056,11 +15230,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetEscapedBytes(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasEscapedBytes = true;

         result.escapedBytes_ = value;

         return this;

       }

       public Builder ClearEscapedBytes() {

+        PrepareBuilder();

         result.hasEscapedBytes = false;

         result.escapedBytes_ = (pb::ByteString) global::Google.ProtocolBuffers.TestProtos.TestExtremeDefaultValues.Descriptor.Fields[0].DefaultValue;

         return this;

@@ -14076,11 +15252,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetLargeUint32(uint value) {

+        PrepareBuilder();

         result.hasLargeUint32 = true;

         result.largeUint32_ = value;

         return this;

       }

       public Builder ClearLargeUint32() {

+        PrepareBuilder();

         result.hasLargeUint32 = false;

         result.largeUint32_ = 4294967295;

         return this;

@@ -14096,11 +15274,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetLargeUint64(ulong value) {

+        PrepareBuilder();

         result.hasLargeUint64 = true;

         result.largeUint64_ = value;

         return this;

       }

       public Builder ClearLargeUint64() {

+        PrepareBuilder();

         result.hasLargeUint64 = false;

         result.largeUint64_ = 18446744073709551615UL;

         return this;

@@ -14114,11 +15294,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetSmallInt32(value); }

       }

       public Builder SetSmallInt32(int value) {

+        PrepareBuilder();

         result.hasSmallInt32 = true;

         result.smallInt32_ = value;

         return this;

       }

       public Builder ClearSmallInt32() {

+        PrepareBuilder();

         result.hasSmallInt32 = false;

         result.smallInt32_ = -2147483647;

         return this;

@@ -14132,11 +15314,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetSmallInt64(value); }

       }

       public Builder SetSmallInt64(long value) {

+        PrepareBuilder();

         result.hasSmallInt64 = true;

         result.smallInt64_ = value;

         return this;

       }

       public Builder ClearSmallInt64() {

+        PrepareBuilder();

         result.hasSmallInt64 = false;

         result.smallInt64_ = -9223372036854775807L;

         return this;

@@ -14151,11 +15335,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetUtf8String(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasUtf8String = true;

         result.utf8String_ = value;

         return this;

       }

       public Builder ClearUtf8String() {

+        PrepareBuilder();

         result.hasUtf8String = false;

         result.utf8String_ = (string) global::Google.ProtocolBuffers.TestProtos.TestExtremeDefaultValues.Descriptor.Fields[5].DefaultValue;

         return this;

@@ -14169,11 +15355,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetZeroFloat(value); }

       }

       public Builder SetZeroFloat(float value) {

+        PrepareBuilder();

         result.hasZeroFloat = true;

         result.zeroFloat_ = value;

         return this;

       }

       public Builder ClearZeroFloat() {

+        PrepareBuilder();

         result.hasZeroFloat = false;

         result.zeroFloat_ = 0F;

         return this;

@@ -14187,11 +15375,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetOneFloat(value); }

       }

       public Builder SetOneFloat(float value) {

+        PrepareBuilder();

         result.hasOneFloat = true;

         result.oneFloat_ = value;

         return this;

       }

       public Builder ClearOneFloat() {

+        PrepareBuilder();

         result.hasOneFloat = false;

         result.oneFloat_ = 1F;

         return this;

@@ -14205,11 +15395,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetSmallFloat(value); }

       }

       public Builder SetSmallFloat(float value) {

+        PrepareBuilder();

         result.hasSmallFloat = true;

         result.smallFloat_ = value;

         return this;

       }

       public Builder ClearSmallFloat() {

+        PrepareBuilder();

         result.hasSmallFloat = false;

         result.smallFloat_ = 1.5F;

         return this;

@@ -14223,11 +15415,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetNegativeOneFloat(value); }

       }

       public Builder SetNegativeOneFloat(float value) {

+        PrepareBuilder();

         result.hasNegativeOneFloat = true;

         result.negativeOneFloat_ = value;

         return this;

       }

       public Builder ClearNegativeOneFloat() {

+        PrepareBuilder();

         result.hasNegativeOneFloat = false;

         result.negativeOneFloat_ = -1F;

         return this;

@@ -14241,11 +15435,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetNegativeFloat(value); }

       }

       public Builder SetNegativeFloat(float value) {

+        PrepareBuilder();

         result.hasNegativeFloat = true;

         result.negativeFloat_ = value;

         return this;

       }

       public Builder ClearNegativeFloat() {

+        PrepareBuilder();

         result.hasNegativeFloat = false;

         result.negativeFloat_ = -1.5F;

         return this;

@@ -14259,11 +15455,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetLargeFloat(value); }

       }

       public Builder SetLargeFloat(float value) {

+        PrepareBuilder();

         result.hasLargeFloat = true;

         result.largeFloat_ = value;

         return this;

       }

       public Builder ClearLargeFloat() {

+        PrepareBuilder();

         result.hasLargeFloat = false;

         result.largeFloat_ = 2E+08F;

         return this;

@@ -14277,11 +15475,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetSmallNegativeFloat(value); }

       }

       public Builder SetSmallNegativeFloat(float value) {

+        PrepareBuilder();

         result.hasSmallNegativeFloat = true;

         result.smallNegativeFloat_ = value;

         return this;

       }

       public Builder ClearSmallNegativeFloat() {

+        PrepareBuilder();

         result.hasSmallNegativeFloat = false;

         result.smallNegativeFloat_ = -8E-28F;

         return this;

@@ -14295,11 +15495,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetInfDouble(value); }

       }

       public Builder SetInfDouble(double value) {

+        PrepareBuilder();

         result.hasInfDouble = true;

         result.infDouble_ = value;

         return this;

       }

       public Builder ClearInfDouble() {

+        PrepareBuilder();

         result.hasInfDouble = false;

         result.infDouble_ = double.PositiveInfinity;

         return this;

@@ -14313,11 +15515,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetNegInfDouble(value); }

       }

       public Builder SetNegInfDouble(double value) {

+        PrepareBuilder();

         result.hasNegInfDouble = true;

         result.negInfDouble_ = value;

         return this;

       }

       public Builder ClearNegInfDouble() {

+        PrepareBuilder();

         result.hasNegInfDouble = false;

         result.negInfDouble_ = double.NegativeInfinity;

         return this;

@@ -14331,11 +15535,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetNanDouble(value); }

       }

       public Builder SetNanDouble(double value) {

+        PrepareBuilder();

         result.hasNanDouble = true;

         result.nanDouble_ = value;

         return this;

       }

       public Builder ClearNanDouble() {

+        PrepareBuilder();

         result.hasNanDouble = false;

         result.nanDouble_ = double.NaN;

         return this;

@@ -14349,11 +15555,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetInfFloat(value); }

       }

       public Builder SetInfFloat(float value) {

+        PrepareBuilder();

         result.hasInfFloat = true;

         result.infFloat_ = value;

         return this;

       }

       public Builder ClearInfFloat() {

+        PrepareBuilder();

         result.hasInfFloat = false;

         result.infFloat_ = float.PositiveInfinity;

         return this;

@@ -14367,11 +15575,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetNegInfFloat(value); }

       }

       public Builder SetNegInfFloat(float value) {

+        PrepareBuilder();

         result.hasNegInfFloat = true;

         result.negInfFloat_ = value;

         return this;

       }

       public Builder ClearNegInfFloat() {

+        PrepareBuilder();

         result.hasNegInfFloat = false;

         result.negInfFloat_ = float.NegativeInfinity;

         return this;

@@ -14385,11 +15595,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetNanFloat(value); }

       }

       public Builder SetNanFloat(float value) {

+        PrepareBuilder();

         result.hasNanFloat = true;

         result.nanFloat_ = value;

         return this;

       }

       public Builder ClearNanFloat() {

+        PrepareBuilder();

         result.hasNanFloat = false;

         result.nanFloat_ = float.NaN;

         return this;

@@ -14502,7 +15714,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(OneString prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -14512,21 +15724,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new OneString();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(OneString cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      OneString result = new OneString();

+      bool builderIsReadOnly;

+      OneString result;

+      

+      private OneString PrepareBuilder() {

+        if (builderIsReadOnly) {

+          OneString original = result;

+          result = new OneString();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override OneString MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new OneString();

+        result = DefaultInstance ?? new OneString();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -14538,12 +15777,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OneString BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        OneString returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -14557,6 +15795,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(OneString other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.OneString.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasData) {

           Data = other.Data;

         }

@@ -14569,6 +15808,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -14625,11 +15865,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetData(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasData = true;

         result.data_ = value;

         return this;

       }

       public Builder ClearData() {

+        PrepareBuilder();

         result.hasData = false;

         result.data_ = "";

         return this;

@@ -14742,7 +15984,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(OneBytes prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -14752,21 +15994,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new OneBytes();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(OneBytes cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      OneBytes result = new OneBytes();

+      bool builderIsReadOnly;

+      OneBytes result;

+      

+      private OneBytes PrepareBuilder() {

+        if (builderIsReadOnly) {

+          OneBytes original = result;

+          result = new OneBytes();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override OneBytes MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new OneBytes();

+        result = DefaultInstance ?? new OneBytes();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -14778,12 +16047,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OneBytes BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        OneBytes returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -14797,6 +16065,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(OneBytes other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.OneBytes.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasData) {

           Data = other.Data;

         }

@@ -14809,6 +16078,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -14865,11 +16135,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetData(pb::ByteString value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasData = true;

         result.data_ = value;

         return this;

       }

       public Builder ClearData() {

+        PrepareBuilder();

         result.hasData = false;

         result.data_ = pb::ByteString.Empty;

         return this;

@@ -15339,7 +16611,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestPackedTypes prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -15349,21 +16621,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestPackedTypes();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestPackedTypes cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestPackedTypes result = new TestPackedTypes();

+      bool builderIsReadOnly;

+      TestPackedTypes result;

+      

+      private TestPackedTypes PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestPackedTypes original = result;

+          result = new TestPackedTypes();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestPackedTypes MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestPackedTypes();

+        result = DefaultInstance ?? new TestPackedTypes();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -15375,8 +16674,8 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestPackedTypes BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.packedInt32_.MakeReadOnly();

         result.packedInt64_.MakeReadOnly();

@@ -15392,9 +16691,8 @@ namespace Google.ProtocolBuffers.TestProtos {
         result.packedDouble_.MakeReadOnly();

         result.packedBool_.MakeReadOnly();

         result.packedEnum_.MakeReadOnly();

-        TestPackedTypes returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -15408,6 +16706,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestPackedTypes other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestPackedTypes.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.packedInt32_.Count != 0) {

           base.AddRange(other.packedInt32_, result.packedInt32_);

         }

@@ -15459,6 +16758,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -15582,7 +16882,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       

       public pbc::IPopsicleList<int> PackedInt32List {

-        get { return result.packedInt32_; }

+        get { return PrepareBuilder().packedInt32_; }

       }

       public int PackedInt32Count {

         get { return result.PackedInt32Count; }

@@ -15591,24 +16891,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedInt32(index);

       }

       public Builder SetPackedInt32(int index, int value) {

+        PrepareBuilder();

         result.packedInt32_[index] = value;

         return this;

       }

       public Builder AddPackedInt32(int value) {

+        PrepareBuilder();

         result.packedInt32_.Add(value);

         return this;

       }

       public Builder AddRangePackedInt32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedInt32_);

         return this;

       }

       public Builder ClearPackedInt32() {

+        PrepareBuilder();

         result.packedInt32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> PackedInt64List {

-        get { return result.packedInt64_; }

+        get { return PrepareBuilder().packedInt64_; }

       }

       public int PackedInt64Count {

         get { return result.PackedInt64Count; }

@@ -15617,25 +16921,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedInt64(index);

       }

       public Builder SetPackedInt64(int index, long value) {

+        PrepareBuilder();

         result.packedInt64_[index] = value;

         return this;

       }

       public Builder AddPackedInt64(long value) {

+        PrepareBuilder();

         result.packedInt64_.Add(value);

         return this;

       }

       public Builder AddRangePackedInt64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedInt64_);

         return this;

       }

       public Builder ClearPackedInt64() {

+        PrepareBuilder();

         result.packedInt64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> PackedUint32List {

-        get { return result.packedUint32_; }

+        get { return PrepareBuilder().packedUint32_; }

       }

       public int PackedUint32Count {

         get { return result.PackedUint32Count; }

@@ -15646,27 +16954,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetPackedUint32(int index, uint value) {

+        PrepareBuilder();

         result.packedUint32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddPackedUint32(uint value) {

+        PrepareBuilder();

         result.packedUint32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangePackedUint32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedUint32_);

         return this;

       }

       public Builder ClearPackedUint32() {

+        PrepareBuilder();

         result.packedUint32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> PackedUint64List {

-        get { return result.packedUint64_; }

+        get { return PrepareBuilder().packedUint64_; }

       }

       public int PackedUint64Count {

         get { return result.PackedUint64Count; }

@@ -15677,26 +16989,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetPackedUint64(int index, ulong value) {

+        PrepareBuilder();

         result.packedUint64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddPackedUint64(ulong value) {

+        PrepareBuilder();

         result.packedUint64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangePackedUint64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedUint64_);

         return this;

       }

       public Builder ClearPackedUint64() {

+        PrepareBuilder();

         result.packedUint64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> PackedSint32List {

-        get { return result.packedSint32_; }

+        get { return PrepareBuilder().packedSint32_; }

       }

       public int PackedSint32Count {

         get { return result.PackedSint32Count; }

@@ -15705,24 +17021,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedSint32(index);

       }

       public Builder SetPackedSint32(int index, int value) {

+        PrepareBuilder();

         result.packedSint32_[index] = value;

         return this;

       }

       public Builder AddPackedSint32(int value) {

+        PrepareBuilder();

         result.packedSint32_.Add(value);

         return this;

       }

       public Builder AddRangePackedSint32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedSint32_);

         return this;

       }

       public Builder ClearPackedSint32() {

+        PrepareBuilder();

         result.packedSint32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> PackedSint64List {

-        get { return result.packedSint64_; }

+        get { return PrepareBuilder().packedSint64_; }

       }

       public int PackedSint64Count {

         get { return result.PackedSint64Count; }

@@ -15731,25 +17051,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedSint64(index);

       }

       public Builder SetPackedSint64(int index, long value) {

+        PrepareBuilder();

         result.packedSint64_[index] = value;

         return this;

       }

       public Builder AddPackedSint64(long value) {

+        PrepareBuilder();

         result.packedSint64_.Add(value);

         return this;

       }

       public Builder AddRangePackedSint64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedSint64_);

         return this;

       }

       public Builder ClearPackedSint64() {

+        PrepareBuilder();

         result.packedSint64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> PackedFixed32List {

-        get { return result.packedFixed32_; }

+        get { return PrepareBuilder().packedFixed32_; }

       }

       public int PackedFixed32Count {

         get { return result.PackedFixed32Count; }

@@ -15760,27 +17084,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetPackedFixed32(int index, uint value) {

+        PrepareBuilder();

         result.packedFixed32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddPackedFixed32(uint value) {

+        PrepareBuilder();

         result.packedFixed32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangePackedFixed32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedFixed32_);

         return this;

       }

       public Builder ClearPackedFixed32() {

+        PrepareBuilder();

         result.packedFixed32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> PackedFixed64List {

-        get { return result.packedFixed64_; }

+        get { return PrepareBuilder().packedFixed64_; }

       }

       public int PackedFixed64Count {

         get { return result.PackedFixed64Count; }

@@ -15791,26 +17119,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetPackedFixed64(int index, ulong value) {

+        PrepareBuilder();

         result.packedFixed64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddPackedFixed64(ulong value) {

+        PrepareBuilder();

         result.packedFixed64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangePackedFixed64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedFixed64_);

         return this;

       }

       public Builder ClearPackedFixed64() {

+        PrepareBuilder();

         result.packedFixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> PackedSfixed32List {

-        get { return result.packedSfixed32_; }

+        get { return PrepareBuilder().packedSfixed32_; }

       }

       public int PackedSfixed32Count {

         get { return result.PackedSfixed32Count; }

@@ -15819,24 +17151,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedSfixed32(index);

       }

       public Builder SetPackedSfixed32(int index, int value) {

+        PrepareBuilder();

         result.packedSfixed32_[index] = value;

         return this;

       }

       public Builder AddPackedSfixed32(int value) {

+        PrepareBuilder();

         result.packedSfixed32_.Add(value);

         return this;

       }

       public Builder AddRangePackedSfixed32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedSfixed32_);

         return this;

       }

       public Builder ClearPackedSfixed32() {

+        PrepareBuilder();

         result.packedSfixed32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> PackedSfixed64List {

-        get { return result.packedSfixed64_; }

+        get { return PrepareBuilder().packedSfixed64_; }

       }

       public int PackedSfixed64Count {

         get { return result.PackedSfixed64Count; }

@@ -15845,24 +17181,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedSfixed64(index);

       }

       public Builder SetPackedSfixed64(int index, long value) {

+        PrepareBuilder();

         result.packedSfixed64_[index] = value;

         return this;

       }

       public Builder AddPackedSfixed64(long value) {

+        PrepareBuilder();

         result.packedSfixed64_.Add(value);

         return this;

       }

       public Builder AddRangePackedSfixed64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedSfixed64_);

         return this;

       }

       public Builder ClearPackedSfixed64() {

+        PrepareBuilder();

         result.packedSfixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<float> PackedFloatList {

-        get { return result.packedFloat_; }

+        get { return PrepareBuilder().packedFloat_; }

       }

       public int PackedFloatCount {

         get { return result.PackedFloatCount; }

@@ -15871,24 +17211,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedFloat(index);

       }

       public Builder SetPackedFloat(int index, float value) {

+        PrepareBuilder();

         result.packedFloat_[index] = value;

         return this;

       }

       public Builder AddPackedFloat(float value) {

+        PrepareBuilder();

         result.packedFloat_.Add(value);

         return this;

       }

       public Builder AddRangePackedFloat(scg::IEnumerable<float> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedFloat_);

         return this;

       }

       public Builder ClearPackedFloat() {

+        PrepareBuilder();

         result.packedFloat_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<double> PackedDoubleList {

-        get { return result.packedDouble_; }

+        get { return PrepareBuilder().packedDouble_; }

       }

       public int PackedDoubleCount {

         get { return result.PackedDoubleCount; }

@@ -15897,24 +17241,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedDouble(index);

       }

       public Builder SetPackedDouble(int index, double value) {

+        PrepareBuilder();

         result.packedDouble_[index] = value;

         return this;

       }

       public Builder AddPackedDouble(double value) {

+        PrepareBuilder();

         result.packedDouble_.Add(value);

         return this;

       }

       public Builder AddRangePackedDouble(scg::IEnumerable<double> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedDouble_);

         return this;

       }

       public Builder ClearPackedDouble() {

+        PrepareBuilder();

         result.packedDouble_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<bool> PackedBoolList {

-        get { return result.packedBool_; }

+        get { return PrepareBuilder().packedBool_; }

       }

       public int PackedBoolCount {

         get { return result.PackedBoolCount; }

@@ -15923,24 +17271,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedBool(index);

       }

       public Builder SetPackedBool(int index, bool value) {

+        PrepareBuilder();

         result.packedBool_[index] = value;

         return this;

       }

       public Builder AddPackedBool(bool value) {

+        PrepareBuilder();

         result.packedBool_.Add(value);

         return this;

       }

       public Builder AddRangePackedBool(scg::IEnumerable<bool> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedBool_);

         return this;

       }

       public Builder ClearPackedBool() {

+        PrepareBuilder();

         result.packedBool_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> PackedEnumList {

-        get { return result.packedEnum_; }

+        get { return PrepareBuilder().packedEnum_; }

       }

       public int PackedEnumCount {

         get { return result.PackedEnumCount; }

@@ -15949,18 +17301,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedEnum(index);

       }

       public Builder SetPackedEnum(int index, global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.packedEnum_[index] = value;

         return this;

       }

       public Builder AddPackedEnum(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.packedEnum_.Add(value);

         return this;

       }

       public Builder AddRangePackedEnum(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedEnum_);

         return this;

       }

       public Builder ClearPackedEnum() {

+        PrepareBuilder();

         result.packedEnum_.Clear();

         return this;

       }

@@ -16374,7 +17730,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestUnpackedTypes prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -16384,21 +17740,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestUnpackedTypes();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestUnpackedTypes cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestUnpackedTypes result;

+      

+      private TestUnpackedTypes PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestUnpackedTypes original = result;

+          result = new TestUnpackedTypes();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestUnpackedTypes result = new TestUnpackedTypes();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestUnpackedTypes MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestUnpackedTypes();

+        result = DefaultInstance ?? new TestUnpackedTypes();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -16410,8 +17793,8 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestUnpackedTypes BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.unpackedInt32_.MakeReadOnly();

         result.unpackedInt64_.MakeReadOnly();

@@ -16427,9 +17810,8 @@ namespace Google.ProtocolBuffers.TestProtos {
         result.unpackedDouble_.MakeReadOnly();

         result.unpackedBool_.MakeReadOnly();

         result.unpackedEnum_.MakeReadOnly();

-        TestUnpackedTypes returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -16443,6 +17825,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestUnpackedTypes other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestUnpackedTypes.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.unpackedInt32_.Count != 0) {

           base.AddRange(other.unpackedInt32_, result.unpackedInt32_);

         }

@@ -16494,6 +17877,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -16617,7 +18001,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       

       public pbc::IPopsicleList<int> UnpackedInt32List {

-        get { return result.unpackedInt32_; }

+        get { return PrepareBuilder().unpackedInt32_; }

       }

       public int UnpackedInt32Count {

         get { return result.UnpackedInt32Count; }

@@ -16626,24 +18010,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedInt32(index);

       }

       public Builder SetUnpackedInt32(int index, int value) {

+        PrepareBuilder();

         result.unpackedInt32_[index] = value;

         return this;

       }

       public Builder AddUnpackedInt32(int value) {

+        PrepareBuilder();

         result.unpackedInt32_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedInt32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedInt32_);

         return this;

       }

       public Builder ClearUnpackedInt32() {

+        PrepareBuilder();

         result.unpackedInt32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> UnpackedInt64List {

-        get { return result.unpackedInt64_; }

+        get { return PrepareBuilder().unpackedInt64_; }

       }

       public int UnpackedInt64Count {

         get { return result.UnpackedInt64Count; }

@@ -16652,25 +18040,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedInt64(index);

       }

       public Builder SetUnpackedInt64(int index, long value) {

+        PrepareBuilder();

         result.unpackedInt64_[index] = value;

         return this;

       }

       public Builder AddUnpackedInt64(long value) {

+        PrepareBuilder();

         result.unpackedInt64_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedInt64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedInt64_);

         return this;

       }

       public Builder ClearUnpackedInt64() {

+        PrepareBuilder();

         result.unpackedInt64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> UnpackedUint32List {

-        get { return result.unpackedUint32_; }

+        get { return PrepareBuilder().unpackedUint32_; }

       }

       public int UnpackedUint32Count {

         get { return result.UnpackedUint32Count; }

@@ -16681,27 +18073,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetUnpackedUint32(int index, uint value) {

+        PrepareBuilder();

         result.unpackedUint32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddUnpackedUint32(uint value) {

+        PrepareBuilder();

         result.unpackedUint32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeUnpackedUint32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedUint32_);

         return this;

       }

       public Builder ClearUnpackedUint32() {

+        PrepareBuilder();

         result.unpackedUint32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> UnpackedUint64List {

-        get { return result.unpackedUint64_; }

+        get { return PrepareBuilder().unpackedUint64_; }

       }

       public int UnpackedUint64Count {

         get { return result.UnpackedUint64Count; }

@@ -16712,26 +18108,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetUnpackedUint64(int index, ulong value) {

+        PrepareBuilder();

         result.unpackedUint64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddUnpackedUint64(ulong value) {

+        PrepareBuilder();

         result.unpackedUint64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeUnpackedUint64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedUint64_);

         return this;

       }

       public Builder ClearUnpackedUint64() {

+        PrepareBuilder();

         result.unpackedUint64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> UnpackedSint32List {

-        get { return result.unpackedSint32_; }

+        get { return PrepareBuilder().unpackedSint32_; }

       }

       public int UnpackedSint32Count {

         get { return result.UnpackedSint32Count; }

@@ -16740,24 +18140,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedSint32(index);

       }

       public Builder SetUnpackedSint32(int index, int value) {

+        PrepareBuilder();

         result.unpackedSint32_[index] = value;

         return this;

       }

       public Builder AddUnpackedSint32(int value) {

+        PrepareBuilder();

         result.unpackedSint32_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedSint32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedSint32_);

         return this;

       }

       public Builder ClearUnpackedSint32() {

+        PrepareBuilder();

         result.unpackedSint32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> UnpackedSint64List {

-        get { return result.unpackedSint64_; }

+        get { return PrepareBuilder().unpackedSint64_; }

       }

       public int UnpackedSint64Count {

         get { return result.UnpackedSint64Count; }

@@ -16766,25 +18170,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedSint64(index);

       }

       public Builder SetUnpackedSint64(int index, long value) {

+        PrepareBuilder();

         result.unpackedSint64_[index] = value;

         return this;

       }

       public Builder AddUnpackedSint64(long value) {

+        PrepareBuilder();

         result.unpackedSint64_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedSint64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedSint64_);

         return this;

       }

       public Builder ClearUnpackedSint64() {

+        PrepareBuilder();

         result.unpackedSint64_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> UnpackedFixed32List {

-        get { return result.unpackedFixed32_; }

+        get { return PrepareBuilder().unpackedFixed32_; }

       }

       public int UnpackedFixed32Count {

         get { return result.UnpackedFixed32Count; }

@@ -16795,27 +18203,31 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetUnpackedFixed32(int index, uint value) {

+        PrepareBuilder();

         result.unpackedFixed32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddUnpackedFixed32(uint value) {

+        PrepareBuilder();

         result.unpackedFixed32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeUnpackedFixed32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedFixed32_);

         return this;

       }

       public Builder ClearUnpackedFixed32() {

+        PrepareBuilder();

         result.unpackedFixed32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> UnpackedFixed64List {

-        get { return result.unpackedFixed64_; }

+        get { return PrepareBuilder().unpackedFixed64_; }

       }

       public int UnpackedFixed64Count {

         get { return result.UnpackedFixed64Count; }

@@ -16826,26 +18238,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetUnpackedFixed64(int index, ulong value) {

+        PrepareBuilder();

         result.unpackedFixed64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddUnpackedFixed64(ulong value) {

+        PrepareBuilder();

         result.unpackedFixed64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeUnpackedFixed64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedFixed64_);

         return this;

       }

       public Builder ClearUnpackedFixed64() {

+        PrepareBuilder();

         result.unpackedFixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> UnpackedSfixed32List {

-        get { return result.unpackedSfixed32_; }

+        get { return PrepareBuilder().unpackedSfixed32_; }

       }

       public int UnpackedSfixed32Count {

         get { return result.UnpackedSfixed32Count; }

@@ -16854,24 +18270,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedSfixed32(index);

       }

       public Builder SetUnpackedSfixed32(int index, int value) {

+        PrepareBuilder();

         result.unpackedSfixed32_[index] = value;

         return this;

       }

       public Builder AddUnpackedSfixed32(int value) {

+        PrepareBuilder();

         result.unpackedSfixed32_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedSfixed32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedSfixed32_);

         return this;

       }

       public Builder ClearUnpackedSfixed32() {

+        PrepareBuilder();

         result.unpackedSfixed32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> UnpackedSfixed64List {

-        get { return result.unpackedSfixed64_; }

+        get { return PrepareBuilder().unpackedSfixed64_; }

       }

       public int UnpackedSfixed64Count {

         get { return result.UnpackedSfixed64Count; }

@@ -16880,24 +18300,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedSfixed64(index);

       }

       public Builder SetUnpackedSfixed64(int index, long value) {

+        PrepareBuilder();

         result.unpackedSfixed64_[index] = value;

         return this;

       }

       public Builder AddUnpackedSfixed64(long value) {

+        PrepareBuilder();

         result.unpackedSfixed64_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedSfixed64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedSfixed64_);

         return this;

       }

       public Builder ClearUnpackedSfixed64() {

+        PrepareBuilder();

         result.unpackedSfixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<float> UnpackedFloatList {

-        get { return result.unpackedFloat_; }

+        get { return PrepareBuilder().unpackedFloat_; }

       }

       public int UnpackedFloatCount {

         get { return result.UnpackedFloatCount; }

@@ -16906,24 +18330,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedFloat(index);

       }

       public Builder SetUnpackedFloat(int index, float value) {

+        PrepareBuilder();

         result.unpackedFloat_[index] = value;

         return this;

       }

       public Builder AddUnpackedFloat(float value) {

+        PrepareBuilder();

         result.unpackedFloat_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedFloat(scg::IEnumerable<float> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedFloat_);

         return this;

       }

       public Builder ClearUnpackedFloat() {

+        PrepareBuilder();

         result.unpackedFloat_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<double> UnpackedDoubleList {

-        get { return result.unpackedDouble_; }

+        get { return PrepareBuilder().unpackedDouble_; }

       }

       public int UnpackedDoubleCount {

         get { return result.UnpackedDoubleCount; }

@@ -16932,24 +18360,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedDouble(index);

       }

       public Builder SetUnpackedDouble(int index, double value) {

+        PrepareBuilder();

         result.unpackedDouble_[index] = value;

         return this;

       }

       public Builder AddUnpackedDouble(double value) {

+        PrepareBuilder();

         result.unpackedDouble_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedDouble(scg::IEnumerable<double> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedDouble_);

         return this;

       }

       public Builder ClearUnpackedDouble() {

+        PrepareBuilder();

         result.unpackedDouble_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<bool> UnpackedBoolList {

-        get { return result.unpackedBool_; }

+        get { return PrepareBuilder().unpackedBool_; }

       }

       public int UnpackedBoolCount {

         get { return result.UnpackedBoolCount; }

@@ -16958,24 +18390,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedBool(index);

       }

       public Builder SetUnpackedBool(int index, bool value) {

+        PrepareBuilder();

         result.unpackedBool_[index] = value;

         return this;

       }

       public Builder AddUnpackedBool(bool value) {

+        PrepareBuilder();

         result.unpackedBool_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedBool(scg::IEnumerable<bool> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedBool_);

         return this;

       }

       public Builder ClearUnpackedBool() {

+        PrepareBuilder();

         result.unpackedBool_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> UnpackedEnumList {

-        get { return result.unpackedEnum_; }

+        get { return PrepareBuilder().unpackedEnum_; }

       }

       public int UnpackedEnumCount {

         get { return result.UnpackedEnumCount; }

@@ -16984,18 +18420,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetUnpackedEnum(index);

       }

       public Builder SetUnpackedEnum(int index, global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.unpackedEnum_[index] = value;

         return this;

       }

       public Builder AddUnpackedEnum(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.unpackedEnum_.Add(value);

         return this;

       }

       public Builder AddRangeUnpackedEnum(scg::IEnumerable<global::Google.ProtocolBuffers.TestProtos.ForeignEnum> values) {

+        PrepareBuilder();

         base.AddRange(values, result.unpackedEnum_);

         return this;

       }

       public Builder ClearUnpackedEnum() {

+        PrepareBuilder();

         result.unpackedEnum_.Clear();

         return this;

       }

@@ -17095,7 +18535,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestPackedExtensions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -17105,21 +18545,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestPackedExtensions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestPackedExtensions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestPackedExtensions result;

+      

+      private TestPackedExtensions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestPackedExtensions original = result;

+          result = new TestPackedExtensions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestPackedExtensions result = new TestPackedExtensions();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestPackedExtensions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestPackedExtensions();

+        result = DefaultInstance ?? new TestPackedExtensions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -17131,12 +18598,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestPackedExtensions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestPackedExtensions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -17150,6 +18616,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestPackedExtensions other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestPackedExtensions.DefaultInstance) return this;

+        PrepareBuilder();

           this.MergeExtensionFields(other);

         this.MergeUnknownFields(other.UnknownFields);

         return this;

@@ -17160,6 +18627,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -17298,7 +18766,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestUnpackedExtensions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -17308,21 +18776,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestUnpackedExtensions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestUnpackedExtensions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestUnpackedExtensions result;

+      

+      private TestUnpackedExtensions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestUnpackedExtensions original = result;

+          result = new TestUnpackedExtensions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestUnpackedExtensions result = new TestUnpackedExtensions();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestUnpackedExtensions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestUnpackedExtensions();

+        result = DefaultInstance ?? new TestUnpackedExtensions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -17334,12 +18829,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestUnpackedExtensions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        TestUnpackedExtensions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -17353,6 +18847,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestUnpackedExtensions other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestUnpackedExtensions.DefaultInstance) return this;

+        PrepareBuilder();

           this.MergeExtensionFields(other);

         this.MergeUnknownFields(other.UnknownFields);

         return this;

@@ -17363,6 +18858,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -17553,7 +19049,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         public override Builder ToBuilder() { return CreateBuilder(this); }

         public override Builder CreateBuilderForType() { return new Builder(); }

         public static Builder CreateBuilder(DynamicMessageType prototype) {

-          return (Builder) new Builder().MergeFrom(prototype);

+          return new Builder(prototype);

         }

         

         [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -17563,21 +19059,48 @@ namespace Google.ProtocolBuffers.TestProtos {
           protected override Builder ThisBuilder {

             get { return this; }

           }

-          public Builder() {}

+          public Builder() {

+            result = DefaultInstance ?? new DynamicMessageType();

+            builderIsReadOnly = result == DefaultInstance;

+          }

+          internal Builder(DynamicMessageType cloneFrom) {

+            result = cloneFrom;

+            builderIsReadOnly = true;

+          }

+          

+          bool builderIsReadOnly;

+          DynamicMessageType result;

           

-          DynamicMessageType result = new DynamicMessageType();

+          private DynamicMessageType PrepareBuilder() {

+            if (builderIsReadOnly) {

+              DynamicMessageType original = result;

+              result = new DynamicMessageType();

+              builderIsReadOnly = false;

+              MergeFrom(original);

+            }

+            return result;

+          }

+          

+          public override bool IsInitialized {

+            get { return result.IsInitialized; }

+          }

           

           protected override DynamicMessageType MessageBeingBuilt {

-            get { return result; }

+            get { return PrepareBuilder(); }

           }

           

           public override Builder Clear() {

-            result = new DynamicMessageType();

+            result = DefaultInstance ?? new DynamicMessageType();

+            builderIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            return new Builder().MergeFrom(result);

+            if (builderIsReadOnly) {

+              return new Builder(result);

+            } else {

+              return new Builder().MergeFrom(result);

+            }

           }

           

           public override pbd::MessageDescriptor DescriptorForType {

@@ -17589,12 +19112,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override DynamicMessageType BuildPartial() {

-            if (result == null) {

-              throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+            if (builderIsReadOnly) {

+              return result;

             }

-            DynamicMessageType returnMe = result;

-            result = null;

-            return returnMe;

+            builderIsReadOnly = true;

+            return result;

           }

           

           public override Builder MergeFrom(pb::IMessage other) {

@@ -17608,6 +19130,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           

           public override Builder MergeFrom(DynamicMessageType other) {

             if (other == global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.DefaultInstance) return this;

+            PrepareBuilder();

             if (other.HasDynamicField) {

               DynamicField = other.DynamicField;

             }

@@ -17620,6 +19143,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+            PrepareBuilder();

             pb::UnknownFieldSet.Builder unknownFields = null;

             uint tag;

             string field_name;

@@ -17675,11 +19199,13 @@ namespace Google.ProtocolBuffers.TestProtos {
             set { SetDynamicField(value); }

           }

           public Builder SetDynamicField(int value) {

+            PrepareBuilder();

             result.hasDynamicField = true;

             result.dynamicField_ = value;

             return this;

           }

           public Builder ClearDynamicField() {

+            PrepareBuilder();

             result.hasDynamicField = false;

             result.dynamicField_ = 0;

             return this;

@@ -17883,7 +19409,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestDynamicExtensions prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -17893,21 +19419,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestDynamicExtensions();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestDynamicExtensions cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      TestDynamicExtensions result = new TestDynamicExtensions();

+      bool builderIsReadOnly;

+      TestDynamicExtensions result;

+      

+      private TestDynamicExtensions PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestDynamicExtensions original = result;

+          result = new TestDynamicExtensions();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestDynamicExtensions MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestDynamicExtensions();

+        result = DefaultInstance ?? new TestDynamicExtensions();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -17919,14 +19472,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestDynamicExtensions BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.repeatedExtension_.MakeReadOnly();

         result.packedExtension_.MakeReadOnly();

-        TestDynamicExtensions returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -17940,6 +19492,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestDynamicExtensions other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.HasScalarExtension) {

           ScalarExtension = other.ScalarExtension;

         }

@@ -17970,6 +19523,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -18078,11 +19632,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetScalarExtension(uint value) {

+        PrepareBuilder();

         result.hasScalarExtension = true;

         result.scalarExtension_ = value;

         return this;

       }

       public Builder ClearScalarExtension() {

+        PrepareBuilder();

         result.hasScalarExtension = false;

         result.scalarExtension_ = 0;

         return this;

@@ -18096,11 +19652,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetEnumExtension(value); }

       }

       public Builder SetEnumExtension(global::Google.ProtocolBuffers.TestProtos.ForeignEnum value) {

+        PrepareBuilder();

         result.hasEnumExtension = true;

         result.enumExtension_ = value;

         return this;

       }

       public Builder ClearEnumExtension() {

+        PrepareBuilder();

         result.hasEnumExtension = false;

         result.enumExtension_ = global::Google.ProtocolBuffers.TestProtos.ForeignEnum.FOREIGN_FOO;

         return this;

@@ -18114,11 +19672,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         set { SetDynamicEnumExtension(value); }

       }

       public Builder SetDynamicEnumExtension(global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicEnumType value) {

+        PrepareBuilder();

         result.hasDynamicEnumExtension = true;

         result.dynamicEnumExtension_ = value;

         return this;

       }

       public Builder ClearDynamicEnumExtension() {

+        PrepareBuilder();

         result.hasDynamicEnumExtension = false;

         result.dynamicEnumExtension_ = global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicEnumType.DYNAMIC_FOO;

         return this;

@@ -18133,18 +19693,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetMessageExtension(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasMessageExtension = true;

         result.messageExtension_ = value;

         return this;

       }

       public Builder SetMessageExtension(global::Google.ProtocolBuffers.TestProtos.ForeignMessage.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasMessageExtension = true;

         result.messageExtension_ = builderForValue.Build();

         return this;

       }

       public Builder MergeMessageExtension(global::Google.ProtocolBuffers.TestProtos.ForeignMessage value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasMessageExtension &&

             result.messageExtension_ != global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance) {

             result.messageExtension_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.CreateBuilder(result.messageExtension_).MergeFrom(value).BuildPartial();

@@ -18155,6 +19718,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearMessageExtension() {

+        PrepareBuilder();

         result.hasMessageExtension = false;

         result.messageExtension_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

         return this;

@@ -18169,18 +19733,21 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetDynamicMessageExtension(global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.hasDynamicMessageExtension = true;

         result.dynamicMessageExtension_ = value;

         return this;

       }

       public Builder SetDynamicMessageExtension(global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.Builder builderForValue) {

         pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");

+        PrepareBuilder();

         result.hasDynamicMessageExtension = true;

         result.dynamicMessageExtension_ = builderForValue.Build();

         return this;

       }

       public Builder MergeDynamicMessageExtension(global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         if (result.hasDynamicMessageExtension &&

             result.dynamicMessageExtension_ != global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.DefaultInstance) {

             result.dynamicMessageExtension_ = global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.CreateBuilder(result.dynamicMessageExtension_).MergeFrom(value).BuildPartial();

@@ -18191,13 +19758,14 @@ namespace Google.ProtocolBuffers.TestProtos {
         return this;

       }

       public Builder ClearDynamicMessageExtension() {

+        PrepareBuilder();

         result.hasDynamicMessageExtension = false;

         result.dynamicMessageExtension_ = global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.DefaultInstance;

         return this;

       }

       

       public pbc::IPopsicleList<string> RepeatedExtensionList {

-        get { return result.repeatedExtension_; }

+        get { return PrepareBuilder().repeatedExtension_; }

       }

       public int RepeatedExtensionCount {

         get { return result.RepeatedExtensionCount; }

@@ -18207,25 +19775,29 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       public Builder SetRepeatedExtension(int index, string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedExtension_[index] = value;

         return this;

       }

       public Builder AddRepeatedExtension(string value) {

         pb::ThrowHelper.ThrowIfNull(value, "value");

+        PrepareBuilder();

         result.repeatedExtension_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedExtension(scg::IEnumerable<string> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedExtension_);

         return this;

       }

       public Builder ClearRepeatedExtension() {

+        PrepareBuilder();

         result.repeatedExtension_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> PackedExtensionList {

-        get { return result.packedExtension_; }

+        get { return PrepareBuilder().packedExtension_; }

       }

       public int PackedExtensionCount {

         get { return result.PackedExtensionCount; }

@@ -18234,18 +19806,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetPackedExtension(index);

       }

       public Builder SetPackedExtension(int index, int value) {

+        PrepareBuilder();

         result.packedExtension_[index] = value;

         return this;

       }

       public Builder AddPackedExtension(int value) {

+        PrepareBuilder();

         result.packedExtension_.Add(value);

         return this;

       }

       public Builder AddRangePackedExtension(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.packedExtension_);

         return this;

       }

       public Builder ClearPackedExtension() {

+        PrepareBuilder();

         result.packedExtension_.Clear();

         return this;

       }

@@ -18479,7 +20055,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(TestRepeatedScalarDifferentTagSizes prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -18489,21 +20065,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new TestRepeatedScalarDifferentTagSizes();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(TestRepeatedScalarDifferentTagSizes cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      TestRepeatedScalarDifferentTagSizes result;

+      

+      private TestRepeatedScalarDifferentTagSizes PrepareBuilder() {

+        if (builderIsReadOnly) {

+          TestRepeatedScalarDifferentTagSizes original = result;

+          result = new TestRepeatedScalarDifferentTagSizes();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      TestRepeatedScalarDifferentTagSizes result = new TestRepeatedScalarDifferentTagSizes();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override TestRepeatedScalarDifferentTagSizes MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new TestRepeatedScalarDifferentTagSizes();

+        result = DefaultInstance ?? new TestRepeatedScalarDifferentTagSizes();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -18515,8 +20118,8 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRepeatedScalarDifferentTagSizes BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

         result.repeatedFixed32_.MakeReadOnly();

         result.repeatedInt32_.MakeReadOnly();

@@ -18524,9 +20127,8 @@ namespace Google.ProtocolBuffers.TestProtos {
         result.repeatedInt64_.MakeReadOnly();

         result.repeatedFloat_.MakeReadOnly();

         result.repeatedUint64_.MakeReadOnly();

-        TestRepeatedScalarDifferentTagSizes returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -18540,6 +20142,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(TestRepeatedScalarDifferentTagSizes other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.TestRepeatedScalarDifferentTagSizes.DefaultInstance) return this;

+        PrepareBuilder();

         if (other.repeatedFixed32_.Count != 0) {

           base.AddRange(other.repeatedFixed32_, result.repeatedFixed32_);

         }

@@ -18567,6 +20170,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -18642,7 +20246,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<uint> RepeatedFixed32List {

-        get { return result.repeatedFixed32_; }

+        get { return PrepareBuilder().repeatedFixed32_; }

       }

       public int RepeatedFixed32Count {

         get { return result.RepeatedFixed32Count; }

@@ -18653,26 +20257,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedFixed32(int index, uint value) {

+        PrepareBuilder();

         result.repeatedFixed32_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedFixed32(uint value) {

+        PrepareBuilder();

         result.repeatedFixed32_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedFixed32(scg::IEnumerable<uint> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedFixed32_);

         return this;

       }

       public Builder ClearRepeatedFixed32() {

+        PrepareBuilder();

         result.repeatedFixed32_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<int> RepeatedInt32List {

-        get { return result.repeatedInt32_; }

+        get { return PrepareBuilder().repeatedInt32_; }

       }

       public int RepeatedInt32Count {

         get { return result.RepeatedInt32Count; }

@@ -18681,25 +20289,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedInt32(index);

       }

       public Builder SetRepeatedInt32(int index, int value) {

+        PrepareBuilder();

         result.repeatedInt32_[index] = value;

         return this;

       }

       public Builder AddRepeatedInt32(int value) {

+        PrepareBuilder();

         result.repeatedInt32_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedInt32(scg::IEnumerable<int> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedInt32_);

         return this;

       }

       public Builder ClearRepeatedInt32() {

+        PrepareBuilder();

         result.repeatedInt32_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> RepeatedFixed64List {

-        get { return result.repeatedFixed64_; }

+        get { return PrepareBuilder().repeatedFixed64_; }

       }

       public int RepeatedFixed64Count {

         get { return result.RepeatedFixed64Count; }

@@ -18710,26 +20322,30 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedFixed64(int index, ulong value) {

+        PrepareBuilder();

         result.repeatedFixed64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedFixed64(ulong value) {

+        PrepareBuilder();

         result.repeatedFixed64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedFixed64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedFixed64_);

         return this;

       }

       public Builder ClearRepeatedFixed64() {

+        PrepareBuilder();

         result.repeatedFixed64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<long> RepeatedInt64List {

-        get { return result.repeatedInt64_; }

+        get { return PrepareBuilder().repeatedInt64_; }

       }

       public int RepeatedInt64Count {

         get { return result.RepeatedInt64Count; }

@@ -18738,24 +20354,28 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedInt64(index);

       }

       public Builder SetRepeatedInt64(int index, long value) {

+        PrepareBuilder();

         result.repeatedInt64_[index] = value;

         return this;

       }

       public Builder AddRepeatedInt64(long value) {

+        PrepareBuilder();

         result.repeatedInt64_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedInt64(scg::IEnumerable<long> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedInt64_);

         return this;

       }

       public Builder ClearRepeatedInt64() {

+        PrepareBuilder();

         result.repeatedInt64_.Clear();

         return this;

       }

       

       public pbc::IPopsicleList<float> RepeatedFloatList {

-        get { return result.repeatedFloat_; }

+        get { return PrepareBuilder().repeatedFloat_; }

       }

       public int RepeatedFloatCount {

         get { return result.RepeatedFloatCount; }

@@ -18764,25 +20384,29 @@ namespace Google.ProtocolBuffers.TestProtos {
         return result.GetRepeatedFloat(index);

       }

       public Builder SetRepeatedFloat(int index, float value) {

+        PrepareBuilder();

         result.repeatedFloat_[index] = value;

         return this;

       }

       public Builder AddRepeatedFloat(float value) {

+        PrepareBuilder();

         result.repeatedFloat_.Add(value);

         return this;

       }

       public Builder AddRangeRepeatedFloat(scg::IEnumerable<float> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedFloat_);

         return this;

       }

       public Builder ClearRepeatedFloat() {

+        PrepareBuilder();

         result.repeatedFloat_.Clear();

         return this;

       }

       

       [global::System.CLSCompliant(false)]

       public pbc::IPopsicleList<ulong> RepeatedUint64List {

-        get { return result.repeatedUint64_; }

+        get { return PrepareBuilder().repeatedUint64_; }

       }

       public int RepeatedUint64Count {

         get { return result.RepeatedUint64Count; }

@@ -18793,20 +20417,24 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       [global::System.CLSCompliant(false)]

       public Builder SetRepeatedUint64(int index, ulong value) {

+        PrepareBuilder();

         result.repeatedUint64_[index] = value;

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRepeatedUint64(ulong value) {

+        PrepareBuilder();

         result.repeatedUint64_.Add(value);

         return this;

       }

       [global::System.CLSCompliant(false)]

       public Builder AddRangeRepeatedUint64(scg::IEnumerable<ulong> values) {

+        PrepareBuilder();

         base.AddRange(values, result.repeatedUint64_);

         return this;

       }

       public Builder ClearRepeatedUint64() {

+        PrepareBuilder();

         result.repeatedUint64_.Clear();

         return this;

       }

@@ -18902,7 +20530,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(FooRequest prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -18912,21 +20540,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new FooRequest();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(FooRequest cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      FooRequest result = new FooRequest();

+      bool builderIsReadOnly;

+      FooRequest result;

+      

+      private FooRequest PrepareBuilder() {

+        if (builderIsReadOnly) {

+          FooRequest original = result;

+          result = new FooRequest();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override FooRequest MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new FooRequest();

+        result = DefaultInstance ?? new FooRequest();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -18938,12 +20593,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override FooRequest BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        FooRequest returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -18957,6 +20611,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(FooRequest other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.FooRequest.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -18966,6 +20621,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -19100,7 +20756,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(FooResponse prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -19110,21 +20766,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new FooResponse();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(FooResponse cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      FooResponse result;

+      

+      private FooResponse PrepareBuilder() {

+        if (builderIsReadOnly) {

+          FooResponse original = result;

+          result = new FooResponse();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      FooResponse result = new FooResponse();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override FooResponse MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new FooResponse();

+        result = DefaultInstance ?? new FooResponse();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -19136,12 +20819,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override FooResponse BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        FooResponse returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -19155,6 +20837,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(FooResponse other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.FooResponse.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -19164,6 +20847,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -19298,7 +20982,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(BarRequest prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -19308,21 +20992,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new BarRequest();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(BarRequest cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

+      

+      bool builderIsReadOnly;

+      BarRequest result;

+      

+      private BarRequest PrepareBuilder() {

+        if (builderIsReadOnly) {

+          BarRequest original = result;

+          result = new BarRequest();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

       

-      BarRequest result = new BarRequest();

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override BarRequest MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new BarRequest();

+        result = DefaultInstance ?? new BarRequest();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -19334,12 +21045,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override BarRequest BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        BarRequest returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -19353,6 +21063,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(BarRequest other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.BarRequest.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -19362,6 +21073,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;

@@ -19496,7 +21208,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

     public static Builder CreateBuilder(BarResponse prototype) {

-      return (Builder) new Builder().MergeFrom(prototype);

+      return new Builder(prototype);

     }

     

     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

@@ -19506,21 +21218,48 @@ namespace Google.ProtocolBuffers.TestProtos {
       protected override Builder ThisBuilder {

         get { return this; }

       }

-      public Builder() {}

+      public Builder() {

+        result = DefaultInstance ?? new BarResponse();

+        builderIsReadOnly = result == DefaultInstance;

+      }

+      internal Builder(BarResponse cloneFrom) {

+        result = cloneFrom;

+        builderIsReadOnly = true;

+      }

       

-      BarResponse result = new BarResponse();

+      bool builderIsReadOnly;

+      BarResponse result;

+      

+      private BarResponse PrepareBuilder() {

+        if (builderIsReadOnly) {

+          BarResponse original = result;

+          result = new BarResponse();

+          builderIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

       

       protected override BarResponse MessageBeingBuilt {

-        get { return result; }

+        get { return PrepareBuilder(); }

       }

       

       public override Builder Clear() {

-        result = new BarResponse();

+        result = DefaultInstance ?? new BarResponse();

+        builderIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        return new Builder().MergeFrom(result);

+        if (builderIsReadOnly) {

+          return new Builder(result);

+        } else {

+          return new Builder().MergeFrom(result);

+        }

       }

       

       public override pbd::MessageDescriptor DescriptorForType {

@@ -19532,12 +21271,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override BarResponse BuildPartial() {

-        if (result == null) {

-          throw new global::System.InvalidOperationException("build() has already been called on this Builder");

+        if (builderIsReadOnly) {

+          return result;

         }

-        BarResponse returnMe = result;

-        result = null;

-        return returnMe;

+        builderIsReadOnly = true;

+        return result;

       }

       

       public override Builder MergeFrom(pb::IMessage other) {

@@ -19551,6 +21289,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       

       public override Builder MergeFrom(BarResponse other) {

         if (other == global::Google.ProtocolBuffers.TestProtos.BarResponse.DefaultInstance) return this;

+        PrepareBuilder();

         this.MergeUnknownFields(other.UnknownFields);

         return this;

       }

@@ -19560,6 +21299,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+        PrepareBuilder();

         pb::UnknownFieldSet.Builder unknownFields = null;

         uint tag;

         string field_name;