31 files changed, 4825 insertions, 3398 deletions

diff --git a/protos/extest/unittest_issues.proto b/protos/extest/unittest_issues.proto
index b725b3f1..5e0da0ae 100644
--- a/protos/extest/unittest_issues.proto
+++ b/protos/extest/unittest_issues.proto
@@ -86,6 +86,15 @@ message NumberField {
     optional int32 _01 = 1;

 }

 

+// Issue 28: Circular message dependencies result in null defaults for DefaultInstance

+

+message MyMessageAReferenceB {

+    required MyMessageBReferenceA value = 1;

+}

+

+message MyMessageBReferenceA {

+    required MyMessageAReferenceB value = 1;

+}

 

 // issue 19 - negative enum values

 

diff --git a/src/AddressBook/AddressBookProtos.cs b/src/AddressBook/AddressBookProtos.cs
index 1bad0888..7fc1c64d 100644
--- a/src/AddressBook/AddressBookProtos.cs
+++ b/src/AddressBook/AddressBookProtos.cs
@@ -69,7 +69,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class Person : pb::GeneratedMessage<Person, Person.Builder> {

     private Person() { }

-    private static readonly Person defaultInstance = new Builder().BuildPartial();

+    private static readonly Person defaultInstance = new Person().MakeReadOnly();

     private static readonly string[] _personFieldNames = new string[] { "email", "id", "name", "phone" };

     private static readonly uint[] _personFieldTags = new uint[] { 26, 16, 10, 34 };

     public static Person DefaultInstance {

@@ -77,7 +77,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
     }

     

     public override Person DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override Person ThisMessage {

@@ -110,7 +110,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class PhoneNumber : pb::GeneratedMessage<PhoneNumber, PhoneNumber.Builder> {

         private PhoneNumber() { }

-        private static readonly PhoneNumber defaultInstance = new Builder().BuildPartial();

+        private static readonly PhoneNumber defaultInstance = new PhoneNumber().MakeReadOnly();

         private static readonly string[] _phoneNumberFieldNames = new string[] { "number", "type" };

         private static readonly uint[] _phoneNumberFieldTags = new uint[] { 10, 16 };

         public static PhoneNumber DefaultInstance {

@@ -118,7 +118,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
         }

         

         public override PhoneNumber DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override PhoneNumber ThisMessage {

@@ -221,6 +221,10 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
         public static PhoneNumber ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private PhoneNumber MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -236,22 +240,22 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new PhoneNumber();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(PhoneNumber cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          PhoneNumber result;

+          private bool resultIsReadOnly;

+          private PhoneNumber result;

           

           private PhoneNumber PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               PhoneNumber original = result;

               result = new PhoneNumber();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -266,13 +270,13 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new PhoneNumber();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -288,11 +292,11 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
           }

           

           public override PhoneNumber BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -557,6 +561,11 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
     public static Person ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private Person MakeReadOnly() {

+      phone_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -572,22 +581,22 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new Person();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(Person cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      Person result;

+      private bool resultIsReadOnly;

+      private Person result;

       

       private Person PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           Person original = result;

           result = new Person();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -602,13 +611,13 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new Person();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -624,12 +633,11 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }

       

       public override Person BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.phone_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -841,7 +849,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class AddressBook : pb::GeneratedMessage<AddressBook, AddressBook.Builder> {

     private AddressBook() { }

-    private static readonly AddressBook defaultInstance = new Builder().BuildPartial();

+    private static readonly AddressBook defaultInstance = new AddressBook().MakeReadOnly();

     private static readonly string[] _addressBookFieldNames = new string[] { "person" };

     private static readonly uint[] _addressBookFieldTags = new uint[] { 10 };

     public static AddressBook DefaultInstance {

@@ -849,7 +857,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
     }

     

     public override AddressBook DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override AddressBook ThisMessage {

@@ -940,6 +948,11 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
     public static AddressBook ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private AddressBook MakeReadOnly() {

+      person_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -955,22 +968,22 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new AddressBook();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(AddressBook cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      AddressBook result;

+      private bool resultIsReadOnly;

+      private AddressBook result;

       

       private AddressBook PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           AddressBook original = result;

           result = new AddressBook();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -985,13 +998,13 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new AddressBook();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1007,12 +1020,11 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }

       

       public override AddressBook BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.person_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtoGen/MessageFieldGenerator.cs b/src/ProtoGen/MessageFieldGenerator.cs
index 8e2691ee..25b58a60 100644
--- a/src/ProtoGen/MessageFieldGenerator.cs
+++ b/src/ProtoGen/MessageFieldGenerator.cs
@@ -48,14 +48,14 @@ namespace Google.ProtocolBuffers.ProtoGen
         public void GenerateMembers(TextGenerator writer)

         {

             writer.WriteLine("private bool has{0};", PropertyName);

-            writer.WriteLine("private {0} {1}_ = {2};", TypeName, Name, DefaultValue);

+            writer.WriteLine("private {0} {1}_;", TypeName, Name);

             AddDeprecatedFlag(writer);

             writer.WriteLine("public bool Has{0} {{", PropertyName);

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

             writer.WriteLine("}");

             AddDeprecatedFlag(writer);

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

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

+            writer.WriteLine("  get {{ return {0}_ ?? {1}; }}", Name, DefaultValue);

             writer.WriteLine("}");

         }

 

@@ -104,7 +104,7 @@ namespace Google.ProtocolBuffers.ProtoGen
             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("  result.{0}_ = null;", Name);

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

         }

diff --git a/src/ProtoGen/MessageGenerator.cs b/src/ProtoGen/MessageGenerator.cs
index 89c4aaec..e9a037bf 100644
--- a/src/ProtoGen/MessageGenerator.cs
+++ b/src/ProtoGen/MessageGenerator.cs
@@ -183,7 +183,7 @@ namespace Google.ProtocolBuffers.ProtoGen
             writer.Indent();

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

             // Must call BuildPartial() to make sure all lists are made read-only

-            writer.WriteLine("private static readonly {0} defaultInstance = new Builder().BuildPartial();", ClassName);

+            writer.WriteLine("private static readonly {0} defaultInstance = new {0}().MakeReadOnly();", ClassName);

 

             if (OptimizeSpeed)

             {

@@ -204,7 +204,7 @@ namespace Google.ProtocolBuffers.ProtoGen
             writer.WriteLine("}");

             writer.WriteLine();

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

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

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

             writer.WriteLine("}");

             writer.WriteLine();

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

@@ -552,6 +552,17 @@ namespace Google.ProtocolBuffers.ProtoGen
 

         private void GenerateBuilder(TextGenerator writer)

         {

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

+            writer.Indent();

+            foreach (FieldDescriptor field in Descriptor.Fields)

+            {

+                CreateFieldGenerator(field).GenerateBuildingCode(writer);

+            }

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

+            writer.Outdent();

+            writer.WriteLine("}");

+            writer.WriteLine();

+

             writer.WriteLine("public static Builder CreateBuilder() { return new Builder(); }");

             writer.WriteLine("public override Builder ToBuilder() { return CreateBuilder(this); }");

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

@@ -594,23 +605,23 @@ namespace Google.ProtocolBuffers.ProtoGen
             //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("  result = DefaultInstance;");

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

             writer.WriteLine("}");

             //clone constructor

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

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

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

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

             writer.WriteLine("}");

             writer.WriteLine();

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

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

+            writer.WriteLine("private bool resultIsReadOnly;");

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

             writer.WriteLine();

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

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

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

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

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

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

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

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

             writer.WriteLine("  }");

             writer.WriteLine("  return result;");

@@ -626,13 +637,13 @@ namespace Google.ProtocolBuffers.ProtoGen
             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 = DefaultInstance ?? new {0}();", ClassName);

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

+            writer.WriteLine("  result = DefaultInstance;", ClassName);

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

             writer.WriteLine("  return this;");

             writer.WriteLine("}");

             writer.WriteLine();

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

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

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

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

             writer.WriteLine("  } else {");

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

@@ -653,15 +664,11 @@ namespace Google.ProtocolBuffers.ProtoGen
 

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

             writer.Indent();

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

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

             writer.WriteLine("  return result;");

             writer.WriteLine("}");

-            foreach (FieldDescriptor field in Descriptor.Fields)

-            {

-                CreateFieldGenerator(field).GenerateBuildingCode(writer);

-            }

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

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

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

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

             writer.Outdent();

             writer.WriteLine("}");

             writer.WriteLine();

diff --git a/src/ProtoGen/RepeatedEnumFieldGenerator.cs b/src/ProtoGen/RepeatedEnumFieldGenerator.cs
index aacee937..8c9f17b8 100644
--- a/src/ProtoGen/RepeatedEnumFieldGenerator.cs
+++ b/src/ProtoGen/RepeatedEnumFieldGenerator.cs
@@ -120,7 +120,7 @@ namespace Google.ProtocolBuffers.ProtoGen
 

         public void GenerateBuildingCode(TextGenerator writer)

         {

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

+            writer.WriteLine("{0}_.MakeReadOnly();", Name);

         }

 

         public void GenerateParsingCode(TextGenerator writer)

diff --git a/src/ProtoGen/RepeatedMessageFieldGenerator.cs b/src/ProtoGen/RepeatedMessageFieldGenerator.cs
index 6cc1e631..a9a0143c 100644
--- a/src/ProtoGen/RepeatedMessageFieldGenerator.cs
+++ b/src/ProtoGen/RepeatedMessageFieldGenerator.cs
@@ -134,7 +134,7 @@ namespace Google.ProtocolBuffers.ProtoGen
 

         public void GenerateBuildingCode(TextGenerator writer)

         {

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

+            writer.WriteLine("{0}_.MakeReadOnly();", Name);

         }

 

         public void GenerateParsingCode(TextGenerator writer)

diff --git a/src/ProtoGen/RepeatedPrimitiveFieldGenerator.cs b/src/ProtoGen/RepeatedPrimitiveFieldGenerator.cs
index 645dce5c..b795f3b6 100644
--- a/src/ProtoGen/RepeatedPrimitiveFieldGenerator.cs
+++ b/src/ProtoGen/RepeatedPrimitiveFieldGenerator.cs
@@ -122,7 +122,7 @@ namespace Google.ProtocolBuffers.ProtoGen
 

         public void GenerateBuildingCode(TextGenerator writer)

         {

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

+            writer.WriteLine("{0}_.MakeReadOnly();", Name);

         }

 

         public void GenerateParsingCode(TextGenerator writer)

diff --git a/src/ProtocolBuffers.Test/ReusableBuilderTest.cs b/src/ProtocolBuffers.Test/ReusableBuilderTest.cs
index 194313cb..58d643ed 100644
--- a/src/ProtocolBuffers.Test/ReusableBuilderTest.cs
+++ b/src/ProtocolBuffers.Test/ReusableBuilderTest.cs
@@ -1,16 +1,29 @@
 using System;

 using System.Collections.Generic;

+using System.ComponentModel;

 using System.Text;

 using Google.ProtocolBuffers.Collections;

 using NUnit.Framework;

 using Google.ProtocolBuffers.TestProtos;

 using Google.ProtocolBuffers.Serialization;

+using UnitTest.Issues.TestProtos;

 

 namespace Google.ProtocolBuffers

 {

     [TestFixture]

     public class ReusableBuilderTest

     {

+        [Test, Description("Issue 28: Circular message dependencies result in null defaults for DefaultInstance")]

+        public void EnsureStaticCicularReference()

+        {

+            MyMessageAReferenceB ab = MyMessageAReferenceB.DefaultInstance;

+            Assert.IsNotNull(ab);

+            Assert.IsNotNull(ab.Value);

+            MyMessageBReferenceA ba = MyMessageBReferenceA.DefaultInstance;

+            Assert.IsNotNull(ba);

+            Assert.IsNotNull(ba.Value);

+        }

+

         [Test]

         public void TestModifyDefaultInstance()

         {

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestCSharpOptionsProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestCSharpOptionsProtoFile.cs
index 94ced3d5..13696c2d 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestCSharpOptionsProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestCSharpOptionsProtoFile.cs
@@ -61,7 +61,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class OptionsMessage : pb::GeneratedMessage<OptionsMessage, OptionsMessage.Builder> {

     private OptionsMessage() { }

-    private static readonly OptionsMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly OptionsMessage defaultInstance = new OptionsMessage().MakeReadOnly();

     private static readonly string[] _optionsMessageFieldNames = new string[] { "customized", "normal", "options_message" };

     private static readonly uint[] _optionsMessageFieldTags = new uint[] { 26, 10, 18 };

     public static OptionsMessage DefaultInstance {

@@ -69,7 +69,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override OptionsMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override OptionsMessage ThisMessage {

@@ -187,6 +187,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static OptionsMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private OptionsMessage MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -202,22 +206,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new OptionsMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(OptionsMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      OptionsMessage result;

+      private bool resultIsReadOnly;

+      private OptionsMessage result;

       

       private OptionsMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           OptionsMessage original = result;

           result = new OptionsMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -232,13 +236,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new OptionsMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -254,11 +258,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OptionsMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestCustomOptionsProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestCustomOptionsProtoFile.cs
index a3265da7..93d7fb9c 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestCustomOptionsProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestCustomOptionsProtoFile.cs
@@ -401,7 +401,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestMessageWithCustomOptions : pb::GeneratedMessage<TestMessageWithCustomOptions, TestMessageWithCustomOptions.Builder> {

     private TestMessageWithCustomOptions() { }

-    private static readonly TestMessageWithCustomOptions defaultInstance = new Builder().BuildPartial();

+    private static readonly TestMessageWithCustomOptions defaultInstance = new TestMessageWithCustomOptions().MakeReadOnly();

     private static readonly string[] _testMessageWithCustomOptionsFieldNames = new string[] { "field1" };

     private static readonly uint[] _testMessageWithCustomOptionsFieldTags = new uint[] { 10 };

     public static TestMessageWithCustomOptions DefaultInstance {

@@ -409,7 +409,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestMessageWithCustomOptions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestMessageWithCustomOptions ThisMessage {

@@ -510,6 +510,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMessageWithCustomOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestMessageWithCustomOptions MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -525,22 +529,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestMessageWithCustomOptions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestMessageWithCustomOptions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestMessageWithCustomOptions result;

+      private bool resultIsReadOnly;

+      private TestMessageWithCustomOptions result;

       

       private TestMessageWithCustomOptions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestMessageWithCustomOptions original = result;

           result = new TestMessageWithCustomOptions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -555,13 +559,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestMessageWithCustomOptions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -577,11 +581,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMessageWithCustomOptions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -687,7 +691,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class CustomOptionFooRequest : pb::GeneratedMessage<CustomOptionFooRequest, CustomOptionFooRequest.Builder> {

     private CustomOptionFooRequest() { }

-    private static readonly CustomOptionFooRequest defaultInstance = new Builder().BuildPartial();

+    private static readonly CustomOptionFooRequest defaultInstance = new CustomOptionFooRequest().MakeReadOnly();

     private static readonly string[] _customOptionFooRequestFieldNames = new string[] {  };

     private static readonly uint[] _customOptionFooRequestFieldTags = new uint[] {  };

     public static CustomOptionFooRequest DefaultInstance {

@@ -695,7 +699,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override CustomOptionFooRequest DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override CustomOptionFooRequest ThisMessage {

@@ -765,6 +769,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static CustomOptionFooRequest ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private CustomOptionFooRequest MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -780,22 +788,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new CustomOptionFooRequest();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(CustomOptionFooRequest cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      CustomOptionFooRequest result;

+      private bool resultIsReadOnly;

+      private CustomOptionFooRequest result;

       

       private CustomOptionFooRequest PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           CustomOptionFooRequest original = result;

           result = new CustomOptionFooRequest();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -810,13 +818,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new CustomOptionFooRequest();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -832,11 +840,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override CustomOptionFooRequest BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -914,7 +922,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class CustomOptionFooResponse : pb::GeneratedMessage<CustomOptionFooResponse, CustomOptionFooResponse.Builder> {

     private CustomOptionFooResponse() { }

-    private static readonly CustomOptionFooResponse defaultInstance = new Builder().BuildPartial();

+    private static readonly CustomOptionFooResponse defaultInstance = new CustomOptionFooResponse().MakeReadOnly();

     private static readonly string[] _customOptionFooResponseFieldNames = new string[] {  };

     private static readonly uint[] _customOptionFooResponseFieldTags = new uint[] {  };

     public static CustomOptionFooResponse DefaultInstance {

@@ -922,7 +930,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override CustomOptionFooResponse DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override CustomOptionFooResponse ThisMessage {

@@ -992,6 +1000,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static CustomOptionFooResponse ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private CustomOptionFooResponse MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1007,22 +1019,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new CustomOptionFooResponse();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(CustomOptionFooResponse cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      CustomOptionFooResponse result;

+      private bool resultIsReadOnly;

+      private CustomOptionFooResponse result;

       

       private CustomOptionFooResponse PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           CustomOptionFooResponse original = result;

           result = new CustomOptionFooResponse();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1037,13 +1049,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new CustomOptionFooResponse();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1059,11 +1071,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override CustomOptionFooResponse BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -1141,7 +1153,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class DummyMessageContainingEnum : pb::GeneratedMessage<DummyMessageContainingEnum, DummyMessageContainingEnum.Builder> {

     private DummyMessageContainingEnum() { }

-    private static readonly DummyMessageContainingEnum defaultInstance = new Builder().BuildPartial();

+    private static readonly DummyMessageContainingEnum defaultInstance = new DummyMessageContainingEnum().MakeReadOnly();

     private static readonly string[] _dummyMessageContainingEnumFieldNames = new string[] {  };

     private static readonly uint[] _dummyMessageContainingEnumFieldTags = new uint[] {  };

     public static DummyMessageContainingEnum DefaultInstance {

@@ -1149,7 +1161,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override DummyMessageContainingEnum DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override DummyMessageContainingEnum ThisMessage {

@@ -1234,6 +1246,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static DummyMessageContainingEnum ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private DummyMessageContainingEnum MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1249,22 +1265,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new DummyMessageContainingEnum();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(DummyMessageContainingEnum cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      DummyMessageContainingEnum result;

+      private bool resultIsReadOnly;

+      private DummyMessageContainingEnum result;

       

       private DummyMessageContainingEnum PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           DummyMessageContainingEnum original = result;

           result = new DummyMessageContainingEnum();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1279,13 +1295,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new DummyMessageContainingEnum();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1301,11 +1317,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override DummyMessageContainingEnum BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -1383,7 +1399,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class DummyMessageInvalidAsOptionType : pb::GeneratedMessage<DummyMessageInvalidAsOptionType, DummyMessageInvalidAsOptionType.Builder> {

     private DummyMessageInvalidAsOptionType() { }

-    private static readonly DummyMessageInvalidAsOptionType defaultInstance = new Builder().BuildPartial();

+    private static readonly DummyMessageInvalidAsOptionType defaultInstance = new DummyMessageInvalidAsOptionType().MakeReadOnly();

     private static readonly string[] _dummyMessageInvalidAsOptionTypeFieldNames = new string[] {  };

     private static readonly uint[] _dummyMessageInvalidAsOptionTypeFieldTags = new uint[] {  };

     public static DummyMessageInvalidAsOptionType DefaultInstance {

@@ -1391,7 +1407,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override DummyMessageInvalidAsOptionType DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override DummyMessageInvalidAsOptionType ThisMessage {

@@ -1461,6 +1477,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static DummyMessageInvalidAsOptionType ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private DummyMessageInvalidAsOptionType MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1476,22 +1496,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new DummyMessageInvalidAsOptionType();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(DummyMessageInvalidAsOptionType cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      DummyMessageInvalidAsOptionType result;

+      private bool resultIsReadOnly;

+      private DummyMessageInvalidAsOptionType result;

       

       private DummyMessageInvalidAsOptionType PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           DummyMessageInvalidAsOptionType original = result;

           result = new DummyMessageInvalidAsOptionType();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1506,13 +1526,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new DummyMessageInvalidAsOptionType();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1528,11 +1548,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override DummyMessageInvalidAsOptionType BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -1610,7 +1630,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class CustomOptionMinIntegerValues : pb::GeneratedMessage<CustomOptionMinIntegerValues, CustomOptionMinIntegerValues.Builder> {

     private CustomOptionMinIntegerValues() { }

-    private static readonly CustomOptionMinIntegerValues defaultInstance = new Builder().BuildPartial();

+    private static readonly CustomOptionMinIntegerValues defaultInstance = new CustomOptionMinIntegerValues().MakeReadOnly();

     private static readonly string[] _customOptionMinIntegerValuesFieldNames = new string[] {  };

     private static readonly uint[] _customOptionMinIntegerValuesFieldTags = new uint[] {  };

     public static CustomOptionMinIntegerValues DefaultInstance {

@@ -1618,7 +1638,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override CustomOptionMinIntegerValues DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override CustomOptionMinIntegerValues ThisMessage {

@@ -1688,6 +1708,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static CustomOptionMinIntegerValues ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private CustomOptionMinIntegerValues MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1703,22 +1727,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new CustomOptionMinIntegerValues();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(CustomOptionMinIntegerValues cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      CustomOptionMinIntegerValues result;

+      private bool resultIsReadOnly;

+      private CustomOptionMinIntegerValues result;

       

       private CustomOptionMinIntegerValues PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           CustomOptionMinIntegerValues original = result;

           result = new CustomOptionMinIntegerValues();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1733,13 +1757,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new CustomOptionMinIntegerValues();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1755,11 +1779,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override CustomOptionMinIntegerValues BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -1837,7 +1861,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class CustomOptionMaxIntegerValues : pb::GeneratedMessage<CustomOptionMaxIntegerValues, CustomOptionMaxIntegerValues.Builder> {

     private CustomOptionMaxIntegerValues() { }

-    private static readonly CustomOptionMaxIntegerValues defaultInstance = new Builder().BuildPartial();

+    private static readonly CustomOptionMaxIntegerValues defaultInstance = new CustomOptionMaxIntegerValues().MakeReadOnly();

     private static readonly string[] _customOptionMaxIntegerValuesFieldNames = new string[] {  };

     private static readonly uint[] _customOptionMaxIntegerValuesFieldTags = new uint[] {  };

     public static CustomOptionMaxIntegerValues DefaultInstance {

@@ -1845,7 +1869,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override CustomOptionMaxIntegerValues DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override CustomOptionMaxIntegerValues ThisMessage {

@@ -1915,6 +1939,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static CustomOptionMaxIntegerValues ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private CustomOptionMaxIntegerValues MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1930,22 +1958,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new CustomOptionMaxIntegerValues();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(CustomOptionMaxIntegerValues cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      CustomOptionMaxIntegerValues result;

+      private bool resultIsReadOnly;

+      private CustomOptionMaxIntegerValues result;

       

       private CustomOptionMaxIntegerValues PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           CustomOptionMaxIntegerValues original = result;

           result = new CustomOptionMaxIntegerValues();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1960,13 +1988,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new CustomOptionMaxIntegerValues();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1982,11 +2010,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override CustomOptionMaxIntegerValues BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -2064,7 +2092,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class CustomOptionOtherValues : pb::GeneratedMessage<CustomOptionOtherValues, CustomOptionOtherValues.Builder> {

     private CustomOptionOtherValues() { }

-    private static readonly CustomOptionOtherValues defaultInstance = new Builder().BuildPartial();

+    private static readonly CustomOptionOtherValues defaultInstance = new CustomOptionOtherValues().MakeReadOnly();

     private static readonly string[] _customOptionOtherValuesFieldNames = new string[] {  };

     private static readonly uint[] _customOptionOtherValuesFieldTags = new uint[] {  };

     public static CustomOptionOtherValues DefaultInstance {

@@ -2072,7 +2100,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override CustomOptionOtherValues DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override CustomOptionOtherValues ThisMessage {

@@ -2142,6 +2170,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static CustomOptionOtherValues ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private CustomOptionOtherValues MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -2157,22 +2189,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new CustomOptionOtherValues();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(CustomOptionOtherValues cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      CustomOptionOtherValues result;

+      private bool resultIsReadOnly;

+      private CustomOptionOtherValues result;

       

       private CustomOptionOtherValues PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           CustomOptionOtherValues original = result;

           result = new CustomOptionOtherValues();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -2187,13 +2219,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new CustomOptionOtherValues();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -2209,11 +2241,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override CustomOptionOtherValues BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -2291,7 +2323,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class SettingRealsFromPositiveInts : pb::GeneratedMessage<SettingRealsFromPositiveInts, SettingRealsFromPositiveInts.Builder> {

     private SettingRealsFromPositiveInts() { }

-    private static readonly SettingRealsFromPositiveInts defaultInstance = new Builder().BuildPartial();

+    private static readonly SettingRealsFromPositiveInts defaultInstance = new SettingRealsFromPositiveInts().MakeReadOnly();

     private static readonly string[] _settingRealsFromPositiveIntsFieldNames = new string[] {  };

     private static readonly uint[] _settingRealsFromPositiveIntsFieldTags = new uint[] {  };

     public static SettingRealsFromPositiveInts DefaultInstance {

@@ -2299,7 +2331,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override SettingRealsFromPositiveInts DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override SettingRealsFromPositiveInts ThisMessage {

@@ -2369,6 +2401,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SettingRealsFromPositiveInts ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private SettingRealsFromPositiveInts MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -2384,22 +2420,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new SettingRealsFromPositiveInts();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(SettingRealsFromPositiveInts cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      SettingRealsFromPositiveInts result;

+      private bool resultIsReadOnly;

+      private SettingRealsFromPositiveInts result;

       

       private SettingRealsFromPositiveInts PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           SettingRealsFromPositiveInts original = result;

           result = new SettingRealsFromPositiveInts();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -2414,13 +2450,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new SettingRealsFromPositiveInts();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -2436,11 +2472,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SettingRealsFromPositiveInts BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -2518,7 +2554,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class SettingRealsFromNegativeInts : pb::GeneratedMessage<SettingRealsFromNegativeInts, SettingRealsFromNegativeInts.Builder> {

     private SettingRealsFromNegativeInts() { }

-    private static readonly SettingRealsFromNegativeInts defaultInstance = new Builder().BuildPartial();

+    private static readonly SettingRealsFromNegativeInts defaultInstance = new SettingRealsFromNegativeInts().MakeReadOnly();

     private static readonly string[] _settingRealsFromNegativeIntsFieldNames = new string[] {  };

     private static readonly uint[] _settingRealsFromNegativeIntsFieldTags = new uint[] {  };

     public static SettingRealsFromNegativeInts DefaultInstance {

@@ -2526,7 +2562,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override SettingRealsFromNegativeInts DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override SettingRealsFromNegativeInts ThisMessage {

@@ -2596,6 +2632,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SettingRealsFromNegativeInts ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private SettingRealsFromNegativeInts MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -2611,22 +2651,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new SettingRealsFromNegativeInts();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(SettingRealsFromNegativeInts cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      SettingRealsFromNegativeInts result;

+      private bool resultIsReadOnly;

+      private SettingRealsFromNegativeInts result;

       

       private SettingRealsFromNegativeInts PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           SettingRealsFromNegativeInts original = result;

           result = new SettingRealsFromNegativeInts();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -2641,13 +2681,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new SettingRealsFromNegativeInts();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -2663,11 +2703,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SettingRealsFromNegativeInts BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -2745,7 +2785,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class ComplexOptionType1 : pb::ExtendableMessage<ComplexOptionType1, ComplexOptionType1.Builder> {

     private ComplexOptionType1() { }

-    private static readonly ComplexOptionType1 defaultInstance = new Builder().BuildPartial();

+    private static readonly ComplexOptionType1 defaultInstance = new ComplexOptionType1().MakeReadOnly();

     private static readonly string[] _complexOptionType1FieldNames = new string[] { "foo", "foo2", "foo3" };

     private static readonly uint[] _complexOptionType1FieldTags = new uint[] { 8, 16, 24 };

     public static ComplexOptionType1 DefaultInstance {

@@ -2753,7 +2793,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override ComplexOptionType1 DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override ComplexOptionType1 ThisMessage {

@@ -2875,6 +2915,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static ComplexOptionType1 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private ComplexOptionType1 MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -2890,22 +2934,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new ComplexOptionType1();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(ComplexOptionType1 cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      ComplexOptionType1 result;

+      private bool resultIsReadOnly;

+      private ComplexOptionType1 result;

       

       private ComplexOptionType1 PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           ComplexOptionType1 original = result;

           result = new ComplexOptionType1();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -2920,13 +2964,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new ComplexOptionType1();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -2942,11 +2986,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ComplexOptionType1 BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -3106,7 +3150,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class ComplexOptionType2 : pb::ExtendableMessage<ComplexOptionType2, ComplexOptionType2.Builder> {

     private ComplexOptionType2() { }

-    private static readonly ComplexOptionType2 defaultInstance = new Builder().BuildPartial();

+    private static readonly ComplexOptionType2 defaultInstance = new ComplexOptionType2().MakeReadOnly();

     private static readonly string[] _complexOptionType2FieldNames = new string[] { "bar", "baz", "fred" };

     private static readonly uint[] _complexOptionType2FieldTags = new uint[] { 10, 16, 26 };

     public static ComplexOptionType2 DefaultInstance {

@@ -3114,7 +3158,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override ComplexOptionType2 DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override ComplexOptionType2 ThisMessage {

@@ -3139,7 +3183,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class ComplexOptionType4 : pb::GeneratedMessage<ComplexOptionType4, ComplexOptionType4.Builder> {

         private ComplexOptionType4() { }

-        private static readonly ComplexOptionType4 defaultInstance = new Builder().BuildPartial();

+        private static readonly ComplexOptionType4 defaultInstance = new ComplexOptionType4().MakeReadOnly();

         private static readonly string[] _complexOptionType4FieldNames = new string[] { "waldo" };

         private static readonly uint[] _complexOptionType4FieldTags = new uint[] { 8 };

         public static ComplexOptionType4 DefaultInstance {

@@ -3147,7 +3191,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override ComplexOptionType4 DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override ComplexOptionType4 ThisMessage {

@@ -3235,6 +3279,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static ComplexOptionType4 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private ComplexOptionType4 MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -3250,22 +3298,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new ComplexOptionType4();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(ComplexOptionType4 cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          ComplexOptionType4 result;

+          private bool resultIsReadOnly;

+          private ComplexOptionType4 result;

           

           private ComplexOptionType4 PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               ComplexOptionType4 original = result;

               result = new ComplexOptionType4();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -3280,13 +3328,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new ComplexOptionType4();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -3302,11 +3350,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override ComplexOptionType4 BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -3411,12 +3459,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int BarFieldNumber = 1;

     private bool hasBar;

-    private global::Google.ProtocolBuffers.TestProtos.ComplexOptionType1 bar_ = global::Google.ProtocolBuffers.TestProtos.ComplexOptionType1.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.ComplexOptionType1 bar_;

     public bool HasBar {

       get { return hasBar; }

     }

     public global::Google.ProtocolBuffers.TestProtos.ComplexOptionType1 Bar {

-      get { return bar_; }

+      get { return bar_ ?? global::Google.ProtocolBuffers.TestProtos.ComplexOptionType1.DefaultInstance; }

     }

     

     public const int BazFieldNumber = 2;

@@ -3431,12 +3479,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int FredFieldNumber = 3;

     private bool hasFred;

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

+    private global::Google.ProtocolBuffers.TestProtos.ComplexOptionType2.Types.ComplexOptionType4 fred_;

     public bool HasFred {

       get { return hasFred; }

     }

     public global::Google.ProtocolBuffers.TestProtos.ComplexOptionType2.Types.ComplexOptionType4 Fred {

-      get { return fred_; }

+      get { return fred_ ?? global::Google.ProtocolBuffers.TestProtos.ComplexOptionType2.Types.ComplexOptionType4.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -3519,6 +3567,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static ComplexOptionType2 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private ComplexOptionType2 MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -3534,22 +3586,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new ComplexOptionType2();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(ComplexOptionType2 cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      ComplexOptionType2 result;

+      private bool resultIsReadOnly;

+      private ComplexOptionType2 result;

       

       private ComplexOptionType2 PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           ComplexOptionType2 original = result;

           result = new ComplexOptionType2();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -3564,13 +3616,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new ComplexOptionType2();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -3586,11 +3638,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ComplexOptionType2 BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -3726,7 +3778,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearBar() {

         PrepareBuilder();

         result.hasBar = false;

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

+        result.bar_ = null;

         return this;

       }

       

@@ -3786,7 +3838,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearFred() {

         PrepareBuilder();

         result.hasFred = false;

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

+        result.fred_ = null;

         return this;

       }

     }

@@ -3800,7 +3852,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class ComplexOptionType3 : pb::GeneratedMessage<ComplexOptionType3, ComplexOptionType3.Builder> {

     private ComplexOptionType3() { }

-    private static readonly ComplexOptionType3 defaultInstance = new Builder().BuildPartial();

+    private static readonly ComplexOptionType3 defaultInstance = new ComplexOptionType3().MakeReadOnly();

     private static readonly string[] _complexOptionType3FieldNames = new string[] { "complexoptiontype5", "qux" };

     private static readonly uint[] _complexOptionType3FieldTags = new uint[] { 19, 8 };

     public static ComplexOptionType3 DefaultInstance {

@@ -3808,7 +3860,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override ComplexOptionType3 DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override ComplexOptionType3 ThisMessage {

@@ -3833,7 +3885,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class ComplexOptionType5 : pb::GeneratedMessage<ComplexOptionType5, ComplexOptionType5.Builder> {

         private ComplexOptionType5() { }

-        private static readonly ComplexOptionType5 defaultInstance = new Builder().BuildPartial();

+        private static readonly ComplexOptionType5 defaultInstance = new ComplexOptionType5().MakeReadOnly();

         private static readonly string[] _complexOptionType5FieldNames = new string[] { "plugh" };

         private static readonly uint[] _complexOptionType5FieldTags = new uint[] { 24 };

         public static ComplexOptionType5 DefaultInstance {

@@ -3841,7 +3893,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override ComplexOptionType5 DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override ComplexOptionType5 ThisMessage {

@@ -3927,6 +3979,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static ComplexOptionType5 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private ComplexOptionType5 MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -3942,22 +3998,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new ComplexOptionType5();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(ComplexOptionType5 cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          ComplexOptionType5 result;

+          private bool resultIsReadOnly;

+          private ComplexOptionType5 result;

           

           private ComplexOptionType5 PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               ComplexOptionType5 original = result;

               result = new ComplexOptionType5();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -3972,13 +4028,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new ComplexOptionType5();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -3994,11 +4050,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override ComplexOptionType5 BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -4113,12 +4169,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int ComplexOptionType5FieldNumber = 2;

     private bool hasComplexOptionType5;

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

+    private global::Google.ProtocolBuffers.TestProtos.ComplexOptionType3.Types.ComplexOptionType5 complexOptionType5_;

     public bool HasComplexOptionType5 {

       get { return hasComplexOptionType5; }

     }

     public global::Google.ProtocolBuffers.TestProtos.ComplexOptionType3.Types.ComplexOptionType5 ComplexOptionType5 {

-      get { return complexOptionType5_; }

+      get { return complexOptionType5_ ?? global::Google.ProtocolBuffers.TestProtos.ComplexOptionType3.Types.ComplexOptionType5.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -4188,6 +4244,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static ComplexOptionType3 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private ComplexOptionType3 MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -4203,22 +4263,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new ComplexOptionType3();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(ComplexOptionType3 cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      ComplexOptionType3 result;

+      private bool resultIsReadOnly;

+      private ComplexOptionType3 result;

       

       private ComplexOptionType3 PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           ComplexOptionType3 original = result;

           result = new ComplexOptionType3();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -4233,13 +4293,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new ComplexOptionType3();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -4255,11 +4315,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ComplexOptionType3 BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -4402,7 +4462,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearComplexOptionType5() {

         PrepareBuilder();

         result.hasComplexOptionType5 = false;

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

+        result.complexOptionType5_ = null;

         return this;

       }

     }

@@ -4416,7 +4476,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class ComplexOpt6 : pb::GeneratedMessage<ComplexOpt6, ComplexOpt6.Builder> {

     private ComplexOpt6() { }

-    private static readonly ComplexOpt6 defaultInstance = new Builder().BuildPartial();

+    private static readonly ComplexOpt6 defaultInstance = new ComplexOpt6().MakeReadOnly();

     private static readonly string[] _complexOpt6FieldNames = new string[] { "xyzzy" };

     private static readonly uint[] _complexOpt6FieldTags = new uint[] { 60751608 };

     public static ComplexOpt6 DefaultInstance {

@@ -4424,7 +4484,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override ComplexOpt6 DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override ComplexOpt6 ThisMessage {

@@ -4510,6 +4570,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static ComplexOpt6 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private ComplexOpt6 MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -4525,22 +4589,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new ComplexOpt6();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(ComplexOpt6 cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      ComplexOpt6 result;

+      private bool resultIsReadOnly;

+      private ComplexOpt6 result;

       

       private ComplexOpt6 PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           ComplexOpt6 original = result;

           result = new ComplexOpt6();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -4555,13 +4619,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new ComplexOpt6();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -4577,11 +4641,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ComplexOpt6 BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -4686,7 +4750,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class VariousComplexOptions : pb::GeneratedMessage<VariousComplexOptions, VariousComplexOptions.Builder> {

     private VariousComplexOptions() { }

-    private static readonly VariousComplexOptions defaultInstance = new Builder().BuildPartial();

+    private static readonly VariousComplexOptions defaultInstance = new VariousComplexOptions().MakeReadOnly();

     private static readonly string[] _variousComplexOptionsFieldNames = new string[] {  };

     private static readonly uint[] _variousComplexOptionsFieldTags = new uint[] {  };

     public static VariousComplexOptions DefaultInstance {

@@ -4694,7 +4758,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override VariousComplexOptions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override VariousComplexOptions ThisMessage {

@@ -4764,6 +4828,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static VariousComplexOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private VariousComplexOptions MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -4779,22 +4847,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new VariousComplexOptions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(VariousComplexOptions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      VariousComplexOptions result;

+      private bool resultIsReadOnly;

+      private VariousComplexOptions result;

       

       private VariousComplexOptions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           VariousComplexOptions original = result;

           result = new VariousComplexOptions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -4809,13 +4877,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new VariousComplexOptions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -4831,11 +4899,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override VariousComplexOptions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestEmbedOptimizeForProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestEmbedOptimizeForProtoFile.cs
index 9af43135..aba049d8 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestEmbedOptimizeForProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestEmbedOptimizeForProtoFile.cs
@@ -65,7 +65,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestEmbedOptimizedForSize : pb::GeneratedMessage<TestEmbedOptimizedForSize, TestEmbedOptimizedForSize.Builder> {

     private TestEmbedOptimizedForSize() { }

-    private static readonly TestEmbedOptimizedForSize defaultInstance = new Builder().BuildPartial();

+    private static readonly TestEmbedOptimizedForSize defaultInstance = new TestEmbedOptimizedForSize().MakeReadOnly();

     private static readonly string[] _testEmbedOptimizedForSizeFieldNames = new string[] { "optional_message", "repeated_message" };

     private static readonly uint[] _testEmbedOptimizedForSizeFieldTags = new uint[] { 10, 18 };

     public static TestEmbedOptimizedForSize DefaultInstance {

@@ -73,7 +73,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestEmbedOptimizedForSize DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestEmbedOptimizedForSize ThisMessage {

@@ -90,12 +90,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int OptionalMessageFieldNumber = 1;

     private bool hasOptionalMessage;

-    private global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize optionalMessage_ = global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize optionalMessage_;

     public bool HasOptionalMessage {

       get { return hasOptionalMessage; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize OptionalMessage {

-      get { return optionalMessage_; }

+      get { return optionalMessage_ ?? global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize.DefaultInstance; }

     }

     

     public const int RepeatedMessageFieldNumber = 2;

@@ -183,6 +183,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestEmbedOptimizedForSize ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestEmbedOptimizedForSize MakeReadOnly() {

+      repeatedMessage_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -198,22 +203,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestEmbedOptimizedForSize();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestEmbedOptimizedForSize cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestEmbedOptimizedForSize result;

+      private bool resultIsReadOnly;

+      private TestEmbedOptimizedForSize result;

       

       private TestEmbedOptimizedForSize PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestEmbedOptimizedForSize original = result;

           result = new TestEmbedOptimizedForSize();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -228,13 +233,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestEmbedOptimizedForSize();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -250,12 +255,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestEmbedOptimizedForSize BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.repeatedMessage_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -378,7 +382,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalMessage() {

         PrepareBuilder();

         result.hasOptionalMessage = false;

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

+        result.optionalMessage_ = null;

         return this;

       }

       

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestExtrasIssuesProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestExtrasIssuesProtoFile.cs
index 47841404..be2d202f 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestExtrasIssuesProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestExtrasIssuesProtoFile.cs
@@ -26,6 +26,10 @@ namespace UnitTest.Issues.TestProtos {
     internal static pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.AB, global::UnitTest.Issues.TestProtos.AB.Builder> internal__static_unittest_issues_AB__FieldAccessorTable;

     internal static pbd::MessageDescriptor internal__static_unittest_issues_NumberField__Descriptor;

     internal static pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.NumberField, global::UnitTest.Issues.TestProtos.NumberField.Builder> internal__static_unittest_issues_NumberField__FieldAccessorTable;

+    internal static pbd::MessageDescriptor internal__static_unittest_issues_MyMessageAReferenceB__Descriptor;

+    internal static pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.MyMessageAReferenceB, global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.Builder> internal__static_unittest_issues_MyMessageAReferenceB__FieldAccessorTable;

+    internal static pbd::MessageDescriptor internal__static_unittest_issues_MyMessageBReferenceA__Descriptor;

+    internal static pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.MyMessageBReferenceA, global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.Builder> internal__static_unittest_issues_MyMessageBReferenceA__FieldAccessorTable;

     internal static pbd::MessageDescriptor internal__static_unittest_issues_NegativeEnumMessage__Descriptor;

     internal static pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.NegativeEnumMessage, global::UnitTest.Issues.TestProtos.NegativeEnumMessage.Builder> internal__static_unittest_issues_NegativeEnumMessage__FieldAccessorTable;

     internal static pbd::MessageDescriptor internal__static_unittest_issues_DeprecatedChild__Descriptor;

@@ -44,22 +48,26 @@ namespace UnitTest.Issues.TestProtos {
           "ChxleHRlc3QvdW5pdHRlc3RfaXNzdWVzLnByb3RvEg91bml0dGVzdF9pc3N1" + 

           "ZXMaJGdvb2dsZS9wcm90b2J1Zi9jc2hhcnBfb3B0aW9ucy5wcm90byIPCgFB" + 

           "EgoKAl9BGAEgASgFIg8KAUISCgoCQl8YASABKAUiEQoCQUISCwoDYV9iGAEg" + 

-          "ASgFIhoKC051bWJlckZpZWxkEgsKA18wMRgBIAEoBSKsAQoTTmVnYXRpdmVF" + 

-          "bnVtTWVzc2FnZRIsCgV2YWx1ZRgBIAEoDjIdLnVuaXR0ZXN0X2lzc3Vlcy5O" + 

-          "ZWdhdGl2ZUVudW0SLQoGdmFsdWVzGAIgAygOMh0udW5pdHRlc3RfaXNzdWVz" + 

-          "Lk5lZ2F0aXZlRW51bRI4Cg1wYWNrZWRfdmFsdWVzGAMgAygOMh0udW5pdHRl" + 

-          "c3RfaXNzdWVzLk5lZ2F0aXZlRW51bUICEAEiEQoPRGVwcmVjYXRlZENoaWxk" + 

-          "IrkCChdEZXByZWNhdGVkRmllbGRzTWVzc2FnZRIaCg5QcmltaXRpdmVWYWx1" + 

-          "ZRgBIAEoBUICGAESGgoOUHJpbWl0aXZlQXJyYXkYAiADKAVCAhgBEjoKDE1l" + 

-          "c3NhZ2VWYWx1ZRgDIAEoCzIgLnVuaXR0ZXN0X2lzc3Vlcy5EZXByZWNhdGVk" + 

-          "Q2hpbGRCAhgBEjoKDE1lc3NhZ2VBcnJheRgEIAMoCzIgLnVuaXR0ZXN0X2lz" + 

-          "c3Vlcy5EZXByZWNhdGVkQ2hpbGRCAhgBEjYKCUVudW1WYWx1ZRgFIAEoDjIf" + 

-          "LnVuaXR0ZXN0X2lzc3Vlcy5EZXByZWNhdGVkRW51bUICGAESNgoJRW51bUFy" + 

-          "cmF5GAYgAygOMh8udW5pdHRlc3RfaXNzdWVzLkRlcHJlY2F0ZWRFbnVtQgIY" + 

-          "ASpHCgxOZWdhdGl2ZUVudW0SFgoJRml2ZUJlbG93EPv//////////wESFQoI" + 

-          "TWludXNPbmUQ////////////ARIICgRaZXJvEAAqGQoORGVwcmVjYXRlZEVu" + 

-          "dW0SBwoDb25lEAFCQEgBwj47ChpVbml0VGVzdC5Jc3N1ZXMuVGVzdFByb3Rv" + 

-          "cxIdVW5pdFRlc3RFeHRyYXNJc3N1ZXNQcm90b0ZpbGU=");

+          "ASgFIhoKC051bWJlckZpZWxkEgsKA18wMRgBIAEoBSJMChRNeU1lc3NhZ2VB" + 

+          "UmVmZXJlbmNlQhI0CgV2YWx1ZRgBIAIoCzIlLnVuaXR0ZXN0X2lzc3Vlcy5N" + 

+          "eU1lc3NhZ2VCUmVmZXJlbmNlQSJMChRNeU1lc3NhZ2VCUmVmZXJlbmNlQRI0" + 

+          "CgV2YWx1ZRgBIAIoCzIlLnVuaXR0ZXN0X2lzc3Vlcy5NeU1lc3NhZ2VBUmVm" + 

+          "ZXJlbmNlQiKsAQoTTmVnYXRpdmVFbnVtTWVzc2FnZRIsCgV2YWx1ZRgBIAEo" + 

+          "DjIdLnVuaXR0ZXN0X2lzc3Vlcy5OZWdhdGl2ZUVudW0SLQoGdmFsdWVzGAIg" + 

+          "AygOMh0udW5pdHRlc3RfaXNzdWVzLk5lZ2F0aXZlRW51bRI4Cg1wYWNrZWRf" + 

+          "dmFsdWVzGAMgAygOMh0udW5pdHRlc3RfaXNzdWVzLk5lZ2F0aXZlRW51bUIC" + 

+          "EAEiEQoPRGVwcmVjYXRlZENoaWxkIrkCChdEZXByZWNhdGVkRmllbGRzTWVz" + 

+          "c2FnZRIaCg5QcmltaXRpdmVWYWx1ZRgBIAEoBUICGAESGgoOUHJpbWl0aXZl" + 

+          "QXJyYXkYAiADKAVCAhgBEjoKDE1lc3NhZ2VWYWx1ZRgDIAEoCzIgLnVuaXR0" + 

+          "ZXN0X2lzc3Vlcy5EZXByZWNhdGVkQ2hpbGRCAhgBEjoKDE1lc3NhZ2VBcnJh" + 

+          "eRgEIAMoCzIgLnVuaXR0ZXN0X2lzc3Vlcy5EZXByZWNhdGVkQ2hpbGRCAhgB" + 

+          "EjYKCUVudW1WYWx1ZRgFIAEoDjIfLnVuaXR0ZXN0X2lzc3Vlcy5EZXByZWNh" + 

+          "dGVkRW51bUICGAESNgoJRW51bUFycmF5GAYgAygOMh8udW5pdHRlc3RfaXNz" + 

+          "dWVzLkRlcHJlY2F0ZWRFbnVtQgIYASpHCgxOZWdhdGl2ZUVudW0SFgoJRml2" + 

+          "ZUJlbG93EPv//////////wESFQoITWludXNPbmUQ////////////ARIICgRa" + 

+          "ZXJvEAAqGQoORGVwcmVjYXRlZEVudW0SBwoDb25lEAFCQEgBwj47ChpVbml0" + 

+          "VGVzdC5Jc3N1ZXMuVGVzdFByb3RvcxIdVW5pdFRlc3RFeHRyYXNJc3N1ZXNQ" + 

+          "cm90b0ZpbGU=");

       pbd::FileDescriptor.InternalDescriptorAssigner assigner = delegate(pbd::FileDescriptor root) {

         descriptor = root;

         internal__static_unittest_issues_A__Descriptor = Descriptor.MessageTypes[0];

@@ -78,15 +86,23 @@ namespace UnitTest.Issues.TestProtos {
         internal__static_unittest_issues_NumberField__FieldAccessorTable = 

             new pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.NumberField, global::UnitTest.Issues.TestProtos.NumberField.Builder>(internal__static_unittest_issues_NumberField__Descriptor,

                 new string[] { "_01", });

-        internal__static_unittest_issues_NegativeEnumMessage__Descriptor = Descriptor.MessageTypes[4];

+        internal__static_unittest_issues_MyMessageAReferenceB__Descriptor = Descriptor.MessageTypes[4];

+        internal__static_unittest_issues_MyMessageAReferenceB__FieldAccessorTable = 

+            new pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.MyMessageAReferenceB, global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.Builder>(internal__static_unittest_issues_MyMessageAReferenceB__Descriptor,

+                new string[] { "Value", });

+        internal__static_unittest_issues_MyMessageBReferenceA__Descriptor = Descriptor.MessageTypes[5];

+        internal__static_unittest_issues_MyMessageBReferenceA__FieldAccessorTable = 

+            new pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.MyMessageBReferenceA, global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.Builder>(internal__static_unittest_issues_MyMessageBReferenceA__Descriptor,

+                new string[] { "Value", });

+        internal__static_unittest_issues_NegativeEnumMessage__Descriptor = Descriptor.MessageTypes[6];

         internal__static_unittest_issues_NegativeEnumMessage__FieldAccessorTable = 

             new pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.NegativeEnumMessage, global::UnitTest.Issues.TestProtos.NegativeEnumMessage.Builder>(internal__static_unittest_issues_NegativeEnumMessage__Descriptor,

                 new string[] { "Value", "Values", "PackedValues", });

-        internal__static_unittest_issues_DeprecatedChild__Descriptor = Descriptor.MessageTypes[5];

+        internal__static_unittest_issues_DeprecatedChild__Descriptor = Descriptor.MessageTypes[7];

         internal__static_unittest_issues_DeprecatedChild__FieldAccessorTable = 

             new pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.DeprecatedChild, global::UnitTest.Issues.TestProtos.DeprecatedChild.Builder>(internal__static_unittest_issues_DeprecatedChild__Descriptor,

                 new string[] { });

-        internal__static_unittest_issues_DeprecatedFieldsMessage__Descriptor = Descriptor.MessageTypes[6];

+        internal__static_unittest_issues_DeprecatedFieldsMessage__Descriptor = Descriptor.MessageTypes[8];

         internal__static_unittest_issues_DeprecatedFieldsMessage__FieldAccessorTable = 

             new pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.DeprecatedFieldsMessage, global::UnitTest.Issues.TestProtos.DeprecatedFieldsMessage.Builder>(internal__static_unittest_issues_DeprecatedFieldsMessage__Descriptor,

                 new string[] { "PrimitiveValue", "PrimitiveArray", "MessageValue", "MessageArray", "EnumValue", "EnumArray", });

@@ -126,7 +142,7 @@ namespace UnitTest.Issues.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class A : pb::GeneratedMessage<A, A.Builder> {

     private A() { }

-    private static readonly A defaultInstance = new Builder().BuildPartial();

+    private static readonly A defaultInstance = new A().MakeReadOnly();

     private static readonly string[] _aFieldNames = new string[] { "_A" };

     private static readonly uint[] _aFieldTags = new uint[] { 8 };

     public static A DefaultInstance {

@@ -134,7 +150,7 @@ namespace UnitTest.Issues.TestProtos {
     }

     

     public override A DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override A ThisMessage {

@@ -220,6 +236,10 @@ namespace UnitTest.Issues.TestProtos {
     public static A ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private A MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -235,22 +255,22 @@ namespace UnitTest.Issues.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new A();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(A cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      A result;

+      private bool resultIsReadOnly;

+      private A result;

       

       private A PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           A original = result;

           result = new A();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -265,13 +285,13 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new A();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -287,11 +307,11 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override A BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -396,7 +416,7 @@ namespace UnitTest.Issues.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class B : pb::GeneratedMessage<B, B.Builder> {

     private B() { }

-    private static readonly B defaultInstance = new Builder().BuildPartial();

+    private static readonly B defaultInstance = new B().MakeReadOnly();

     private static readonly string[] _bFieldNames = new string[] { "B_" };

     private static readonly uint[] _bFieldTags = new uint[] { 8 };

     public static B DefaultInstance {

@@ -404,7 +424,7 @@ namespace UnitTest.Issues.TestProtos {
     }

     

     public override B DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override B ThisMessage {

@@ -490,6 +510,10 @@ namespace UnitTest.Issues.TestProtos {
     public static B ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private B MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -505,22 +529,22 @@ namespace UnitTest.Issues.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new B();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(B cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      B result;

+      private bool resultIsReadOnly;

+      private B result;

       

       private B PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           B original = result;

           result = new B();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -535,13 +559,13 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new B();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -557,11 +581,11 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override B BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -666,7 +690,7 @@ namespace UnitTest.Issues.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class AB : pb::GeneratedMessage<AB, AB.Builder> {

     private AB() { }

-    private static readonly AB defaultInstance = new Builder().BuildPartial();

+    private static readonly AB defaultInstance = new AB().MakeReadOnly();

     private static readonly string[] _aBFieldNames = new string[] { "a_b" };

     private static readonly uint[] _aBFieldTags = new uint[] { 8 };

     public static AB DefaultInstance {

@@ -674,7 +698,7 @@ namespace UnitTest.Issues.TestProtos {
     }

     

     public override AB DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override AB ThisMessage {

@@ -760,6 +784,10 @@ namespace UnitTest.Issues.TestProtos {
     public static AB ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private AB MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -775,22 +803,22 @@ namespace UnitTest.Issues.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new AB();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(AB cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      AB result;

+      private bool resultIsReadOnly;

+      private AB result;

       

       private AB PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           AB original = result;

           result = new AB();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -805,13 +833,13 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new AB();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -827,11 +855,11 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override AB BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -936,7 +964,7 @@ namespace UnitTest.Issues.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class NumberField : pb::GeneratedMessage<NumberField, NumberField.Builder> {

     private NumberField() { }

-    private static readonly NumberField defaultInstance = new Builder().BuildPartial();

+    private static readonly NumberField defaultInstance = new NumberField().MakeReadOnly();

     private static readonly string[] _numberFieldFieldNames = new string[] { "_01" };

     private static readonly uint[] _numberFieldFieldTags = new uint[] { 8 };

     public static NumberField DefaultInstance {

@@ -944,7 +972,7 @@ namespace UnitTest.Issues.TestProtos {
     }

     

     public override NumberField DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override NumberField ThisMessage {

@@ -1032,6 +1060,10 @@ namespace UnitTest.Issues.TestProtos {
     public static NumberField ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private NumberField MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1047,22 +1079,22 @@ namespace UnitTest.Issues.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new NumberField();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(NumberField cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      NumberField result;

+      private bool resultIsReadOnly;

+      private NumberField result;

       

       private NumberField PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           NumberField original = result;

           result = new NumberField();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1077,13 +1109,13 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new NumberField();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1099,11 +1131,11 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override NumberField BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -1208,9 +1240,611 @@ namespace UnitTest.Issues.TestProtos {
   [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]

   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]

   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

+  public sealed partial class MyMessageAReferenceB : pb::GeneratedMessage<MyMessageAReferenceB, MyMessageAReferenceB.Builder> {

+    private MyMessageAReferenceB() { }

+    private static readonly MyMessageAReferenceB defaultInstance = new MyMessageAReferenceB().MakeReadOnly();

+    private static readonly string[] _myMessageAReferenceBFieldNames = new string[] { "value" };

+    private static readonly uint[] _myMessageAReferenceBFieldTags = new uint[] { 10 };

+    public static MyMessageAReferenceB DefaultInstance {

+      get { return defaultInstance; }

+    }

+    

+    public override MyMessageAReferenceB DefaultInstanceForType {

+      get { return DefaultInstance; }

+    }

+    

+    protected override MyMessageAReferenceB ThisMessage {

+      get { return this; }

+    }

+    

+    public static pbd::MessageDescriptor Descriptor {

+      get { return global::UnitTest.Issues.TestProtos.UnitTestExtrasIssuesProtoFile.internal__static_unittest_issues_MyMessageAReferenceB__Descriptor; }

+    }

+    

+    protected override pb::FieldAccess.FieldAccessorTable<MyMessageAReferenceB, MyMessageAReferenceB.Builder> InternalFieldAccessors {

+      get { return global::UnitTest.Issues.TestProtos.UnitTestExtrasIssuesProtoFile.internal__static_unittest_issues_MyMessageAReferenceB__FieldAccessorTable; }

+    }

+    

+    public const int ValueFieldNumber = 1;

+    private bool hasValue;

+    private global::UnitTest.Issues.TestProtos.MyMessageBReferenceA value_;

+    public bool HasValue {

+      get { return hasValue; }

+    }

+    public global::UnitTest.Issues.TestProtos.MyMessageBReferenceA Value {

+      get { return value_ ?? global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.DefaultInstance; }

+    }

+    

+    public override bool IsInitialized {

+      get {

+        if (!hasValue) return false;

+        if (!Value.IsInitialized) return false;

+        return true;

+      }

+    }

+    

+    public override void WriteTo(pb::ICodedOutputStream output) {

+      int size = SerializedSize;

+      string[] field_names = _myMessageAReferenceBFieldNames;

+      if (hasValue) {

+        output.WriteMessage(1, field_names[0], Value);

+      }

+      UnknownFields.WriteTo(output);

+    }

+    

+    private int memoizedSerializedSize = -1;

+    public override int SerializedSize {

+      get {

+        int size = memoizedSerializedSize;

+        if (size != -1) return size;

+        

+        size = 0;

+        if (hasValue) {

+          size += pb::CodedOutputStream.ComputeMessageSize(1, Value);

+        }

+        size += UnknownFields.SerializedSize;

+        memoizedSerializedSize = size;

+        return size;

+      }

+    }

+    

+    public static MyMessageAReferenceB ParseFrom(pb::ByteString data) {

+      return ((Builder) CreateBuilder().MergeFrom(data)).BuildParsed();

+    }

+    public static MyMessageAReferenceB ParseFrom(pb::ByteString data, pb::ExtensionRegistry extensionRegistry) {

+      return ((Builder) CreateBuilder().MergeFrom(data, extensionRegistry)).BuildParsed();

+    }

+    public static MyMessageAReferenceB ParseFrom(byte[] data) {

+      return ((Builder) CreateBuilder().MergeFrom(data)).BuildParsed();

+    }

+    public static MyMessageAReferenceB ParseFrom(byte[] data, pb::ExtensionRegistry extensionRegistry) {

+      return ((Builder) CreateBuilder().MergeFrom(data, extensionRegistry)).BuildParsed();

+    }

+    public static MyMessageAReferenceB ParseFrom(global::System.IO.Stream input) {

+      return ((Builder) CreateBuilder().MergeFrom(input)).BuildParsed();

+    }

+    public static MyMessageAReferenceB ParseFrom(global::System.IO.Stream input, pb::ExtensionRegistry extensionRegistry) {

+      return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

+    }

+    public static MyMessageAReferenceB ParseDelimitedFrom(global::System.IO.Stream input) {

+      return CreateBuilder().MergeDelimitedFrom(input).BuildParsed();

+    }

+    public static MyMessageAReferenceB ParseDelimitedFrom(global::System.IO.Stream input, pb::ExtensionRegistry extensionRegistry) {

+      return CreateBuilder().MergeDelimitedFrom(input, extensionRegistry).BuildParsed();

+    }

+    public static MyMessageAReferenceB ParseFrom(pb::ICodedInputStream input) {

+      return ((Builder) CreateBuilder().MergeFrom(input)).BuildParsed();

+    }

+    public static MyMessageAReferenceB ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+      return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

+    }

+    private MyMessageAReferenceB MakeReadOnly() {

+      return this;

+    }

+    

+    public static Builder CreateBuilder() { return new Builder(); }

+    public override Builder ToBuilder() { return CreateBuilder(this); }

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

+    public static Builder CreateBuilder(MyMessageAReferenceB prototype) {

+      return new Builder(prototype);

+    }

+    

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

+    [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]

+    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

+    public sealed partial class Builder : pb::GeneratedBuilder<MyMessageAReferenceB, Builder> {

+      protected override Builder ThisBuilder {

+        get { return this; }

+      }

+      public Builder() {

+        result = DefaultInstance;

+        resultIsReadOnly = true;

+      }

+      internal Builder(MyMessageAReferenceB cloneFrom) {

+        result = cloneFrom;

+        resultIsReadOnly = true;

+      }

+      

+      private bool resultIsReadOnly;

+      private MyMessageAReferenceB result;

+      

+      private MyMessageAReferenceB PrepareBuilder() {

+        if (resultIsReadOnly) {

+          MyMessageAReferenceB original = result;

+          result = new MyMessageAReferenceB();

+          resultIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

+      

+      protected override MyMessageAReferenceB MessageBeingBuilt {

+        get { return PrepareBuilder(); }

+      }

+      

+      public override Builder Clear() {

+        result = DefaultInstance;

+        resultIsReadOnly = true;

+        return this;

+      }

+      

+      public override Builder Clone() {

+        if (resultIsReadOnly) {

+          return new Builder(result);

+        } else {

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

+        }

+      }

+      

+      public override pbd::MessageDescriptor DescriptorForType {

+        get { return global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.Descriptor; }

+      }

+      

+      public override MyMessageAReferenceB DefaultInstanceForType {

+        get { return global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.DefaultInstance; }

+      }

+      

+      public override MyMessageAReferenceB BuildPartial() {

+        if (resultIsReadOnly) {

+          return result;

+        }

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

+      }

+      

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

+        if (other is MyMessageAReferenceB) {

+          return MergeFrom((MyMessageAReferenceB) other);

+        } else {

+          base.MergeFrom(other);

+          return this;

+        }

+      }

+      

+      public override Builder MergeFrom(MyMessageAReferenceB other) {

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

+        PrepareBuilder();

+        if (other.HasValue) {

+          MergeValue(other.Value);

+        }

+        this.MergeUnknownFields(other.UnknownFields);

+        return this;

+      }

+      

+      public override Builder MergeFrom(pb::ICodedInputStream input) {

+        return MergeFrom(input, pb::ExtensionRegistry.Empty);

+      }

+      

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

+        PrepareBuilder();

+        pb::UnknownFieldSet.Builder unknownFields = null;

+        uint tag;

+        string field_name;

+        while (input.ReadTag(out tag, out field_name)) {

+          if(tag == 0 && field_name != null) {

+            int field_ordinal = global::System.Array.BinarySearch(_myMessageAReferenceBFieldNames, field_name, global::System.StringComparer.Ordinal);

+            if(field_ordinal >= 0)

+              tag = _myMessageAReferenceBFieldTags[field_ordinal];

+            else {

+              if (unknownFields == null) {

+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);

+              }

+              ParseUnknownField(input, unknownFields, extensionRegistry, tag, field_name);

+              continue;

+            }

+          }

+          switch (tag) {

+            case 0: {

+              throw pb::InvalidProtocolBufferException.InvalidTag();

+            }

+            default: {

+              if (pb::WireFormat.IsEndGroupTag(tag)) {

+                if (unknownFields != null) {

+                  this.UnknownFields = unknownFields.Build();

+                }

+                return this;

+              }

+              if (unknownFields == null) {

+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);

+              }

+              ParseUnknownField(input, unknownFields, extensionRegistry, tag, field_name);

+              break;

+            }

+            case 10: {

+              global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.Builder subBuilder = global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.CreateBuilder();

+              if (result.hasValue) {

+                subBuilder.MergeFrom(Value);

+              }

+              input.ReadMessage(subBuilder, extensionRegistry);

+              Value = subBuilder.BuildPartial();

+              break;

+            }

+          }

+        }

+        

+        if (unknownFields != null) {

+          this.UnknownFields = unknownFields.Build();

+        }

+        return this;

+      }

+      

+      

+      public bool HasValue {

+       get { return result.hasValue; }

+      }

+      public global::UnitTest.Issues.TestProtos.MyMessageBReferenceA Value {

+        get { return result.Value; }

+        set { SetValue(value); }

+      }

+      public Builder SetValue(global::UnitTest.Issues.TestProtos.MyMessageBReferenceA value) {

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

+        PrepareBuilder();

+        result.hasValue = true;

+        result.value_ = value;

+        return this;

+      }

+      public Builder SetValue(global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.Builder builderForValue) {

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

+        PrepareBuilder();

+        result.hasValue = true;

+        result.value_ = builderForValue.Build();

+        return this;

+      }

+      public Builder MergeValue(global::UnitTest.Issues.TestProtos.MyMessageBReferenceA value) {

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

+        PrepareBuilder();

+        if (result.hasValue &&

+            result.value_ != global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.DefaultInstance) {

+            result.value_ = global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.CreateBuilder(result.value_).MergeFrom(value).BuildPartial();

+        } else {

+          result.value_ = value;

+        }

+        result.hasValue = true;

+        return this;

+      }

+      public Builder ClearValue() {

+        PrepareBuilder();

+        result.hasValue = false;

+        result.value_ = null;

+        return this;

+      }

+    }

+    static MyMessageAReferenceB() {

+      object.ReferenceEquals(global::UnitTest.Issues.TestProtos.UnitTestExtrasIssuesProtoFile.Descriptor, null);

+    }

+  }

+  

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

+  [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]

+  [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

+  public sealed partial class MyMessageBReferenceA : pb::GeneratedMessage<MyMessageBReferenceA, MyMessageBReferenceA.Builder> {

+    private MyMessageBReferenceA() { }

+    private static readonly MyMessageBReferenceA defaultInstance = new MyMessageBReferenceA().MakeReadOnly();

+    private static readonly string[] _myMessageBReferenceAFieldNames = new string[] { "value" };

+    private static readonly uint[] _myMessageBReferenceAFieldTags = new uint[] { 10 };

+    public static MyMessageBReferenceA DefaultInstance {

+      get { return defaultInstance; }

+    }

+    

+    public override MyMessageBReferenceA DefaultInstanceForType {

+      get { return DefaultInstance; }

+    }

+    

+    protected override MyMessageBReferenceA ThisMessage {

+      get { return this; }

+    }

+    

+    public static pbd::MessageDescriptor Descriptor {

+      get { return global::UnitTest.Issues.TestProtos.UnitTestExtrasIssuesProtoFile.internal__static_unittest_issues_MyMessageBReferenceA__Descriptor; }

+    }

+    

+    protected override pb::FieldAccess.FieldAccessorTable<MyMessageBReferenceA, MyMessageBReferenceA.Builder> InternalFieldAccessors {

+      get { return global::UnitTest.Issues.TestProtos.UnitTestExtrasIssuesProtoFile.internal__static_unittest_issues_MyMessageBReferenceA__FieldAccessorTable; }

+    }

+    

+    public const int ValueFieldNumber = 1;

+    private bool hasValue;

+    private global::UnitTest.Issues.TestProtos.MyMessageAReferenceB value_;

+    public bool HasValue {

+      get { return hasValue; }

+    }

+    public global::UnitTest.Issues.TestProtos.MyMessageAReferenceB Value {

+      get { return value_ ?? global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.DefaultInstance; }

+    }

+    

+    public override bool IsInitialized {

+      get {

+        if (!hasValue) return false;

+        if (!Value.IsInitialized) return false;

+        return true;

+      }

+    }

+    

+    public override void WriteTo(pb::ICodedOutputStream output) {

+      int size = SerializedSize;

+      string[] field_names = _myMessageBReferenceAFieldNames;

+      if (hasValue) {

+        output.WriteMessage(1, field_names[0], Value);

+      }

+      UnknownFields.WriteTo(output);

+    }

+    

+    private int memoizedSerializedSize = -1;

+    public override int SerializedSize {

+      get {

+        int size = memoizedSerializedSize;

+        if (size != -1) return size;

+        

+        size = 0;

+        if (hasValue) {

+          size += pb::CodedOutputStream.ComputeMessageSize(1, Value);

+        }

+        size += UnknownFields.SerializedSize;

+        memoizedSerializedSize = size;

+        return size;

+      }

+    }

+    

+    public static MyMessageBReferenceA ParseFrom(pb::ByteString data) {

+      return ((Builder) CreateBuilder().MergeFrom(data)).BuildParsed();

+    }

+    public static MyMessageBReferenceA ParseFrom(pb::ByteString data, pb::ExtensionRegistry extensionRegistry) {

+      return ((Builder) CreateBuilder().MergeFrom(data, extensionRegistry)).BuildParsed();

+    }

+    public static MyMessageBReferenceA ParseFrom(byte[] data) {

+      return ((Builder) CreateBuilder().MergeFrom(data)).BuildParsed();

+    }

+    public static MyMessageBReferenceA ParseFrom(byte[] data, pb::ExtensionRegistry extensionRegistry) {

+      return ((Builder) CreateBuilder().MergeFrom(data, extensionRegistry)).BuildParsed();

+    }

+    public static MyMessageBReferenceA ParseFrom(global::System.IO.Stream input) {

+      return ((Builder) CreateBuilder().MergeFrom(input)).BuildParsed();

+    }

+    public static MyMessageBReferenceA ParseFrom(global::System.IO.Stream input, pb::ExtensionRegistry extensionRegistry) {

+      return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

+    }

+    public static MyMessageBReferenceA ParseDelimitedFrom(global::System.IO.Stream input) {

+      return CreateBuilder().MergeDelimitedFrom(input).BuildParsed();

+    }

+    public static MyMessageBReferenceA ParseDelimitedFrom(global::System.IO.Stream input, pb::ExtensionRegistry extensionRegistry) {

+      return CreateBuilder().MergeDelimitedFrom(input, extensionRegistry).BuildParsed();

+    }

+    public static MyMessageBReferenceA ParseFrom(pb::ICodedInputStream input) {

+      return ((Builder) CreateBuilder().MergeFrom(input)).BuildParsed();

+    }

+    public static MyMessageBReferenceA ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

+      return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

+    }

+    private MyMessageBReferenceA MakeReadOnly() {

+      return this;

+    }

+    

+    public static Builder CreateBuilder() { return new Builder(); }

+    public override Builder ToBuilder() { return CreateBuilder(this); }

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

+    public static Builder CreateBuilder(MyMessageBReferenceA prototype) {

+      return new Builder(prototype);

+    }

+    

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

+    [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]

+    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

+    public sealed partial class Builder : pb::GeneratedBuilder<MyMessageBReferenceA, Builder> {

+      protected override Builder ThisBuilder {

+        get { return this; }

+      }

+      public Builder() {

+        result = DefaultInstance;

+        resultIsReadOnly = true;

+      }

+      internal Builder(MyMessageBReferenceA cloneFrom) {

+        result = cloneFrom;

+        resultIsReadOnly = true;

+      }

+      

+      private bool resultIsReadOnly;

+      private MyMessageBReferenceA result;

+      

+      private MyMessageBReferenceA PrepareBuilder() {

+        if (resultIsReadOnly) {

+          MyMessageBReferenceA original = result;

+          result = new MyMessageBReferenceA();

+          resultIsReadOnly = false;

+          MergeFrom(original);

+        }

+        return result;

+      }

+      

+      public override bool IsInitialized {

+        get { return result.IsInitialized; }

+      }

+      

+      protected override MyMessageBReferenceA MessageBeingBuilt {

+        get { return PrepareBuilder(); }

+      }

+      

+      public override Builder Clear() {

+        result = DefaultInstance;

+        resultIsReadOnly = true;

+        return this;

+      }

+      

+      public override Builder Clone() {

+        if (resultIsReadOnly) {

+          return new Builder(result);

+        } else {

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

+        }

+      }

+      

+      public override pbd::MessageDescriptor DescriptorForType {

+        get { return global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.Descriptor; }

+      }

+      

+      public override MyMessageBReferenceA DefaultInstanceForType {

+        get { return global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.DefaultInstance; }

+      }

+      

+      public override MyMessageBReferenceA BuildPartial() {

+        if (resultIsReadOnly) {

+          return result;

+        }

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

+      }

+      

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

+        if (other is MyMessageBReferenceA) {

+          return MergeFrom((MyMessageBReferenceA) other);

+        } else {

+          base.MergeFrom(other);

+          return this;

+        }

+      }

+      

+      public override Builder MergeFrom(MyMessageBReferenceA other) {

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

+        PrepareBuilder();

+        if (other.HasValue) {

+          MergeValue(other.Value);

+        }

+        this.MergeUnknownFields(other.UnknownFields);

+        return this;

+      }

+      

+      public override Builder MergeFrom(pb::ICodedInputStream input) {

+        return MergeFrom(input, pb::ExtensionRegistry.Empty);

+      }

+      

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

+        PrepareBuilder();

+        pb::UnknownFieldSet.Builder unknownFields = null;

+        uint tag;

+        string field_name;

+        while (input.ReadTag(out tag, out field_name)) {

+          if(tag == 0 && field_name != null) {

+            int field_ordinal = global::System.Array.BinarySearch(_myMessageBReferenceAFieldNames, field_name, global::System.StringComparer.Ordinal);

+            if(field_ordinal >= 0)

+              tag = _myMessageBReferenceAFieldTags[field_ordinal];

+            else {

+              if (unknownFields == null) {

+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);

+              }

+              ParseUnknownField(input, unknownFields, extensionRegistry, tag, field_name);

+              continue;

+            }

+          }

+          switch (tag) {

+            case 0: {

+              throw pb::InvalidProtocolBufferException.InvalidTag();

+            }

+            default: {

+              if (pb::WireFormat.IsEndGroupTag(tag)) {

+                if (unknownFields != null) {

+                  this.UnknownFields = unknownFields.Build();

+                }

+                return this;

+              }

+              if (unknownFields == null) {

+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);

+              }

+              ParseUnknownField(input, unknownFields, extensionRegistry, tag, field_name);

+              break;

+            }

+            case 10: {

+              global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.Builder subBuilder = global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.CreateBuilder();

+              if (result.hasValue) {

+                subBuilder.MergeFrom(Value);

+              }

+              input.ReadMessage(subBuilder, extensionRegistry);

+              Value = subBuilder.BuildPartial();

+              break;

+            }

+          }

+        }

+        

+        if (unknownFields != null) {

+          this.UnknownFields = unknownFields.Build();

+        }

+        return this;

+      }

+      

+      

+      public bool HasValue {

+       get { return result.hasValue; }

+      }

+      public global::UnitTest.Issues.TestProtos.MyMessageAReferenceB Value {

+        get { return result.Value; }

+        set { SetValue(value); }

+      }

+      public Builder SetValue(global::UnitTest.Issues.TestProtos.MyMessageAReferenceB value) {

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

+        PrepareBuilder();

+        result.hasValue = true;

+        result.value_ = value;

+        return this;

+      }

+      public Builder SetValue(global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.Builder builderForValue) {

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

+        PrepareBuilder();

+        result.hasValue = true;

+        result.value_ = builderForValue.Build();

+        return this;

+      }

+      public Builder MergeValue(global::UnitTest.Issues.TestProtos.MyMessageAReferenceB value) {

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

+        PrepareBuilder();

+        if (result.hasValue &&

+            result.value_ != global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.DefaultInstance) {

+            result.value_ = global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.CreateBuilder(result.value_).MergeFrom(value).BuildPartial();

+        } else {

+          result.value_ = value;

+        }

+        result.hasValue = true;

+        return this;

+      }

+      public Builder ClearValue() {

+        PrepareBuilder();

+        result.hasValue = false;

+        result.value_ = null;

+        return this;

+      }

+    }

+    static MyMessageBReferenceA() {

+      object.ReferenceEquals(global::UnitTest.Issues.TestProtos.UnitTestExtrasIssuesProtoFile.Descriptor, null);

+    }

+  }

+  

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

+  [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]

+  [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class NegativeEnumMessage : pb::GeneratedMessage<NegativeEnumMessage, NegativeEnumMessage.Builder> {

     private NegativeEnumMessage() { }

-    private static readonly NegativeEnumMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly NegativeEnumMessage defaultInstance = new NegativeEnumMessage().MakeReadOnly();

     private static readonly string[] _negativeEnumMessageFieldNames = new string[] { "packed_values", "value", "values" };

     private static readonly uint[] _negativeEnumMessageFieldTags = new uint[] { 26, 8, 16 };

     public static NegativeEnumMessage DefaultInstance {

@@ -1218,7 +1852,7 @@ namespace UnitTest.Issues.TestProtos {
     }

     

     public override NegativeEnumMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override NegativeEnumMessage ThisMessage {

@@ -1357,6 +1991,12 @@ namespace UnitTest.Issues.TestProtos {
     public static NegativeEnumMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private NegativeEnumMessage MakeReadOnly() {

+      values_.MakeReadOnly();

+      packedValues_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1372,22 +2012,22 @@ namespace UnitTest.Issues.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new NegativeEnumMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(NegativeEnumMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      NegativeEnumMessage result;

+      private bool resultIsReadOnly;

+      private NegativeEnumMessage result;

       

       private NegativeEnumMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           NegativeEnumMessage original = result;

           result = new NegativeEnumMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1402,13 +2042,13 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new NegativeEnumMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1424,13 +2064,11 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override NegativeEnumMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.values_.MakeReadOnly();

-        result.packedValues_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -1637,7 +2275,7 @@ namespace UnitTest.Issues.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class DeprecatedChild : pb::GeneratedMessage<DeprecatedChild, DeprecatedChild.Builder> {

     private DeprecatedChild() { }

-    private static readonly DeprecatedChild defaultInstance = new Builder().BuildPartial();

+    private static readonly DeprecatedChild defaultInstance = new DeprecatedChild().MakeReadOnly();

     private static readonly string[] _deprecatedChildFieldNames = new string[] {  };

     private static readonly uint[] _deprecatedChildFieldTags = new uint[] {  };

     public static DeprecatedChild DefaultInstance {

@@ -1645,7 +2283,7 @@ namespace UnitTest.Issues.TestProtos {
     }

     

     public override DeprecatedChild DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override DeprecatedChild ThisMessage {

@@ -1715,6 +2353,10 @@ namespace UnitTest.Issues.TestProtos {
     public static DeprecatedChild ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private DeprecatedChild MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1730,22 +2372,22 @@ namespace UnitTest.Issues.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new DeprecatedChild();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(DeprecatedChild cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      DeprecatedChild result;

+      private bool resultIsReadOnly;

+      private DeprecatedChild result;

       

       private DeprecatedChild PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           DeprecatedChild original = result;

           result = new DeprecatedChild();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1760,13 +2402,13 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new DeprecatedChild();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1782,11 +2424,11 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override DeprecatedChild BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -1864,7 +2506,7 @@ namespace UnitTest.Issues.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class DeprecatedFieldsMessage : pb::GeneratedMessage<DeprecatedFieldsMessage, DeprecatedFieldsMessage.Builder> {

     private DeprecatedFieldsMessage() { }

-    private static readonly DeprecatedFieldsMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly DeprecatedFieldsMessage defaultInstance = new DeprecatedFieldsMessage().MakeReadOnly();

     private static readonly string[] _deprecatedFieldsMessageFieldNames = new string[] { "EnumArray", "EnumValue", "MessageArray", "MessageValue", "PrimitiveArray", "PrimitiveValue" };

     private static readonly uint[] _deprecatedFieldsMessageFieldTags = new uint[] { 48, 40, 34, 26, 16, 8 };

     public static DeprecatedFieldsMessage DefaultInstance {

@@ -1872,7 +2514,7 @@ namespace UnitTest.Issues.TestProtos {
     }

     

     public override DeprecatedFieldsMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override DeprecatedFieldsMessage ThisMessage {

@@ -1916,14 +2558,14 @@ namespace UnitTest.Issues.TestProtos {
     

     public const int MessageValueFieldNumber = 3;

     private bool hasMessageValue;

-    private global::UnitTest.Issues.TestProtos.DeprecatedChild messageValue_ = global::UnitTest.Issues.TestProtos.DeprecatedChild.DefaultInstance;

+    private global::UnitTest.Issues.TestProtos.DeprecatedChild messageValue_;

     [global::System.ObsoleteAttribute()]

     public bool HasMessageValue {

       get { return hasMessageValue; }

     }

     [global::System.ObsoleteAttribute()]

     public global::UnitTest.Issues.TestProtos.DeprecatedChild MessageValue {

-      get { return messageValue_; }

+      get { return messageValue_ ?? global::UnitTest.Issues.TestProtos.DeprecatedChild.DefaultInstance; }

     }

     

     public const int MessageArrayFieldNumber = 4;

@@ -2071,6 +2713,13 @@ namespace UnitTest.Issues.TestProtos {
     public static DeprecatedFieldsMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private DeprecatedFieldsMessage MakeReadOnly() {

+      primitiveArray_.MakeReadOnly();

+      messageArray_.MakeReadOnly();

+      enumArray_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -2086,22 +2735,22 @@ namespace UnitTest.Issues.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new DeprecatedFieldsMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(DeprecatedFieldsMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      DeprecatedFieldsMessage result;

+      private bool resultIsReadOnly;

+      private DeprecatedFieldsMessage result;

       

       private DeprecatedFieldsMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           DeprecatedFieldsMessage original = result;

           result = new DeprecatedFieldsMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -2116,13 +2765,13 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new DeprecatedFieldsMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -2138,14 +2787,11 @@ namespace UnitTest.Issues.TestProtos {
       }

       

       public override DeprecatedFieldsMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.primitiveArray_.MakeReadOnly();

-        result.messageArray_.MakeReadOnly();

-        result.enumArray_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -2382,7 +3028,7 @@ namespace UnitTest.Issues.TestProtos {
       public Builder ClearMessageValue() {

         PrepareBuilder();

         result.hasMessageValue = false;

-        result.messageValue_ = global::UnitTest.Issues.TestProtos.DeprecatedChild.DefaultInstance;

+        result.messageValue_ = null;

         return this;

       }

       

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSizeProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSizeProtoFile.cs
index 9ba1919e..a758ba81 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSizeProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSizeProtoFile.cs
@@ -138,13 +138,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class SizeMessage1 : pb::GeneratedMessage<SizeMessage1, SizeMessage1.Builder> {

     private SizeMessage1() { }

-    private static readonly SizeMessage1 defaultInstance = new Builder().BuildPartial();

+    private static readonly SizeMessage1 defaultInstance = new SizeMessage1().MakeReadOnly();

     public static SizeMessage1 DefaultInstance {

       get { return defaultInstance; }

     }

     

     public override SizeMessage1 DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override SizeMessage1 ThisMessage {

@@ -505,12 +505,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int Field15FieldNumber = 15;

     private bool hasField15;

-    private global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage field15_ = global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage field15_;

     public bool HasField15 {

       get { return hasField15; }

     }

     public global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage Field15 {

-      get { return field15_; }

+      get { return field15_ ?? global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage.DefaultInstance; }

     }

     

     public const int Field78FieldNumber = 78;

@@ -603,6 +603,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SizeMessage1 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private SizeMessage1 MakeReadOnly() {

+      field5_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -618,22 +623,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new SizeMessage1();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(SizeMessage1 cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      SizeMessage1 result;

+      private bool resultIsReadOnly;

+      private SizeMessage1 result;

       

       private SizeMessage1 PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           SizeMessage1 original = result;

           result = new SizeMessage1();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -648,13 +653,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new SizeMessage1();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -670,12 +675,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SizeMessage1 BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.field5_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

       

@@ -1417,7 +1421,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearField15() {

         PrepareBuilder();

         result.hasField15 = false;

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

+        result.field15_ = null;

         return this;

       }

       

@@ -1552,13 +1556,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class SizeMessage1SubMessage : pb::GeneratedMessage<SizeMessage1SubMessage, SizeMessage1SubMessage.Builder> {

     private SizeMessage1SubMessage() { }

-    private static readonly SizeMessage1SubMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly SizeMessage1SubMessage defaultInstance = new SizeMessage1SubMessage().MakeReadOnly();

     public static SizeMessage1SubMessage DefaultInstance {

       get { return defaultInstance; }

     }

     

     public override SizeMessage1SubMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override SizeMessage1SubMessage ThisMessage {

@@ -1807,6 +1811,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SizeMessage1SubMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private SizeMessage1SubMessage MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1822,22 +1830,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new SizeMessage1SubMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(SizeMessage1SubMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      SizeMessage1SubMessage result;

+      private bool resultIsReadOnly;

+      private SizeMessage1SubMessage result;

       

       private SizeMessage1SubMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           SizeMessage1SubMessage original = result;

           result = new SizeMessage1SubMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1852,13 +1860,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new SizeMessage1SubMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1874,11 +1882,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SizeMessage1SubMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

       

@@ -2302,13 +2310,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class SizeMessage2 : pb::GeneratedMessage<SizeMessage2, SizeMessage2.Builder> {

     private SizeMessage2() { }

-    private static readonly SizeMessage2 defaultInstance = new Builder().BuildPartial();

+    private static readonly SizeMessage2 defaultInstance = new SizeMessage2().MakeReadOnly();

     public static SizeMessage2 DefaultInstance {

       get { return defaultInstance; }

     }

     

     public override SizeMessage2 DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override SizeMessage2 ThisMessage {

@@ -2333,13 +2341,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class Group1 : pb::GeneratedMessage<Group1, Group1.Builder> {

         private Group1() { }

-        private static readonly Group1 defaultInstance = new Builder().BuildPartial();

+        private static readonly Group1 defaultInstance = new Group1().MakeReadOnly();

         public static Group1 DefaultInstance {

           get { return defaultInstance; }

         }

         

         public override Group1 DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override Group1 ThisMessage {

@@ -2513,12 +2521,12 @@ namespace Google.ProtocolBuffers.TestProtos {
         

         public const int Field31FieldNumber = 31;

         private bool hasField31;

-        private global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage field31_ = global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage.DefaultInstance;

+        private global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage field31_;

         public bool HasField31 {

           get { return hasField31; }

         }

         public global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage Field31 {

-          get { return field31_; }

+          get { return field31_ ?? global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage.DefaultInstance; }

         }

         

         public static Group1 ParseFrom(pb::ByteString data) {

@@ -2551,6 +2559,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Group1 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private Group1 MakeReadOnly() {

+          field14_.MakeReadOnly();

+          field22_.MakeReadOnly();

+          field73_.MakeReadOnly();

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -2566,22 +2581,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new Group1();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(Group1 cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          Group1 result;

+          private bool resultIsReadOnly;

+          private Group1 result;

           

           private Group1 PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               Group1 original = result;

               result = new Group1();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -2596,13 +2611,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new Group1();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -2618,14 +2633,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Group1 BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            result.field14_.MakeReadOnly();

-            result.field22_.MakeReadOnly();

-            result.field73_.MakeReadOnly();

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

           

@@ -3007,7 +3019,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           public Builder ClearField31() {

             PrepareBuilder();

             result.hasField31 = false;

-            result.field31_ = global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage.DefaultInstance;

+            result.field31_ = null;

             return this;

           }

         }

@@ -3357,6 +3369,14 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SizeMessage2 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private SizeMessage2 MakeReadOnly() {

+      group1_.MakeReadOnly();

+      field128_.MakeReadOnly();

+      field127_.MakeReadOnly();

+      field130_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -3372,22 +3392,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new SizeMessage2();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(SizeMessage2 cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      SizeMessage2 result;

+      private bool resultIsReadOnly;

+      private SizeMessage2 result;

       

       private SizeMessage2 PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           SizeMessage2 original = result;

           result = new SizeMessage2();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -3402,13 +3422,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new SizeMessage2();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -3424,15 +3444,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SizeMessage2 BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.group1_.MakeReadOnly();

-        result.field128_.MakeReadOnly();

-        result.field127_.MakeReadOnly();

-        result.field130_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

       

@@ -4107,13 +4123,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class SizeMessage2GroupedMessage : pb::GeneratedMessage<SizeMessage2GroupedMessage, SizeMessage2GroupedMessage.Builder> {

     private SizeMessage2GroupedMessage() { }

-    private static readonly SizeMessage2GroupedMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly SizeMessage2GroupedMessage defaultInstance = new SizeMessage2GroupedMessage().MakeReadOnly();

     public static SizeMessage2GroupedMessage DefaultInstance {

       get { return defaultInstance; }

     }

     

     public override SizeMessage2GroupedMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override SizeMessage2GroupedMessage ThisMessage {

@@ -4268,6 +4284,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SizeMessage2GroupedMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private SizeMessage2GroupedMessage MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -4283,22 +4303,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new SizeMessage2GroupedMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(SizeMessage2GroupedMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      SizeMessage2GroupedMessage result;

+      private bool resultIsReadOnly;

+      private SizeMessage2GroupedMessage result;

       

       private SizeMessage2GroupedMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           SizeMessage2GroupedMessage original = result;

           result = new SizeMessage2GroupedMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -4313,13 +4333,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new SizeMessage2GroupedMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -4335,11 +4355,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SizeMessage2GroupedMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

       

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSpeedProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSpeedProtoFile.cs
index 496fb436..c7e1a93b 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSpeedProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSpeedProtoFile.cs
@@ -138,7 +138,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class SpeedMessage1 : pb::GeneratedMessage<SpeedMessage1, SpeedMessage1.Builder> {

     private SpeedMessage1() { }

-    private static readonly SpeedMessage1 defaultInstance = new Builder().BuildPartial();

+    private static readonly SpeedMessage1 defaultInstance = new SpeedMessage1().MakeReadOnly();

     private static readonly string[] _speedMessage1FieldNames = new string[] { "field1", "field100", "field101", "field102", "field103", "field104", "field12", "field128", "field129", "field13", "field130", "field131", "field14", "field15", "field150", "field16", "field17", "field18", "field2", "field22", "field23", "field24", "field25", "field271", "field272", "field280", "field29", "field3", "field30", "field4", "field5", "field59", "field6", "field60", "field67", "field68", "field7", "field78", "field80", "field81", "field9" };

     private static readonly uint[] _speedMessage1FieldTags = new uint[] { 10, 800, 808, 818, 826, 832, 96, 1024, 1034, 104, 1040, 1048, 112, 122, 1200, 128, 136, 146, 16, 176, 184, 192, 200, 2168, 2176, 2240, 232, 24, 240, 34, 41, 472, 48, 480, 536, 544, 58, 624, 640, 648, 74 };

     public static SpeedMessage1 DefaultInstance {

@@ -146,7 +146,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override SpeedMessage1 DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override SpeedMessage1 ThisMessage {

@@ -507,12 +507,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int Field15FieldNumber = 15;

     private bool hasField15;

-    private global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage field15_ = global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage field15_;

     public bool HasField15 {

       get { return hasField15; }

     }

     public global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage Field15 {

-      get { return field15_; }

+      get { return field15_ ?? global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage.DefaultInstance; }

     }

     

     public const int Field78FieldNumber = 78;

@@ -882,6 +882,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SpeedMessage1 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private SpeedMessage1 MakeReadOnly() {

+      field5_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -897,22 +902,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new SpeedMessage1();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(SpeedMessage1 cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      SpeedMessage1 result;

+      private bool resultIsReadOnly;

+      private SpeedMessage1 result;

       

       private SpeedMessage1 PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           SpeedMessage1 original = result;

           result = new SpeedMessage1();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -927,13 +932,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new SpeedMessage1();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -949,12 +954,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SpeedMessage1 BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.field5_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -2053,7 +2057,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearField15() {

         PrepareBuilder();

         result.hasField15 = false;

-        result.field15_ = global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage.DefaultInstance;

+        result.field15_ = null;

         return this;

       }

       

@@ -2188,7 +2192,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class SpeedMessage1SubMessage : pb::GeneratedMessage<SpeedMessage1SubMessage, SpeedMessage1SubMessage.Builder> {

     private SpeedMessage1SubMessage() { }

-    private static readonly SpeedMessage1SubMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly SpeedMessage1SubMessage defaultInstance = new SpeedMessage1SubMessage().MakeReadOnly();

     private static readonly string[] _speedMessage1SubMessageFieldNames = new string[] { "field1", "field12", "field13", "field14", "field15", "field16", "field19", "field2", "field20", "field203", "field204", "field205", "field206", "field207", "field21", "field22", "field23", "field28", "field3", "field300" };

     private static readonly uint[] _speedMessage1SubMessageFieldTags = new uint[] { 8, 96, 104, 112, 122, 128, 152, 16, 160, 1629, 1632, 1642, 1648, 1656, 169, 176, 184, 224, 24, 2400 };

     public static SpeedMessage1SubMessage DefaultInstance {

@@ -2196,7 +2200,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override SpeedMessage1SubMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override SpeedMessage1SubMessage ThisMessage {

@@ -2590,6 +2594,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SpeedMessage1SubMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private SpeedMessage1SubMessage MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -2605,22 +2613,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new SpeedMessage1SubMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(SpeedMessage1SubMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      SpeedMessage1SubMessage result;

+      private bool resultIsReadOnly;

+      private SpeedMessage1SubMessage result;

       

       private SpeedMessage1SubMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           SpeedMessage1SubMessage original = result;

           result = new SpeedMessage1SubMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -2635,13 +2643,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new SpeedMessage1SubMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -2657,11 +2665,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SpeedMessage1SubMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -3289,7 +3297,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class SpeedMessage2 : pb::GeneratedMessage<SpeedMessage2, SpeedMessage2.Builder> {

     private SpeedMessage2() { }

-    private static readonly SpeedMessage2 defaultInstance = new Builder().BuildPartial();

+    private static readonly SpeedMessage2 defaultInstance = new SpeedMessage2().MakeReadOnly();

     private static readonly string[] _speedMessage2FieldNames = new string[] { "field1", "field109", "field127", "field128", "field129", "field130", "field131", "field2", "field205", "field206", "field21", "field210", "field211", "field212", "field213", "field216", "field217", "field218", "field220", "field221", "field222", "field25", "field3", "field30", "field4", "field6", "field63", "field71", "field75", "group1" };

     private static readonly uint[] _speedMessage2FieldTags = new uint[] { 10, 872, 1018, 1026, 1032, 1040, 1048, 18, 1640, 1648, 168, 1680, 1688, 1696, 1704, 1728, 1736, 1744, 1760, 1768, 1781, 205, 24, 240, 32, 50, 504, 568, 600, 83 };

     public static SpeedMessage2 DefaultInstance {

@@ -3297,7 +3305,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override SpeedMessage2 DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override SpeedMessage2 ThisMessage {

@@ -3322,7 +3330,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class Group1 : pb::GeneratedMessage<Group1, Group1.Builder> {

         private Group1() { }

-        private static readonly Group1 defaultInstance = new Builder().BuildPartial();

+        private static readonly Group1 defaultInstance = new Group1().MakeReadOnly();

         private static readonly string[] _group1FieldNames = new string[] { "field11", "field12", "field13", "field14", "field15", "field16", "field20", "field22", "field24", "field26", "field27", "field28", "field29", "field31", "field5", "field73" };

         private static readonly uint[] _group1FieldTags = new uint[] { 93, 98, 106, 114, 120, 130, 160, 178, 194, 213, 218, 224, 234, 250, 40, 584 };

         public static Group1 DefaultInstance {

@@ -3330,7 +3338,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override Group1 DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override Group1 ThisMessage {

@@ -3504,12 +3512,12 @@ namespace Google.ProtocolBuffers.TestProtos {
         

         public const int Field31FieldNumber = 31;

         private bool hasField31;

-        private global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage field31_ = global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage.DefaultInstance;

+        private global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage field31_;

         public bool HasField31 {

           get { return hasField31; }

         }

         public global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage Field31 {

-          get { return field31_; }

+          get { return field31_ ?? global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage.DefaultInstance; }

         }

         

         public override bool IsInitialized {

@@ -3680,6 +3688,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Group1 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private Group1 MakeReadOnly() {

+          field14_.MakeReadOnly();

+          field22_.MakeReadOnly();

+          field73_.MakeReadOnly();

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -3695,22 +3710,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new Group1();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(Group1 cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          Group1 result;

+          private bool resultIsReadOnly;

+          private Group1 result;

           

           private Group1 PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               Group1 original = result;

               result = new Group1();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -3725,13 +3740,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new Group1();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -3747,14 +3762,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Group1 BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            result.field14_.MakeReadOnly();

-            result.field22_.MakeReadOnly();

-            result.field73_.MakeReadOnly();

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -4318,7 +4330,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           public Builder ClearField31() {

             PrepareBuilder();

             result.hasField31 = false;

-            result.field31_ = global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage.DefaultInstance;

+            result.field31_ = null;

             return this;

           }

         }

@@ -4888,6 +4900,14 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SpeedMessage2 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private SpeedMessage2 MakeReadOnly() {

+      group1_.MakeReadOnly();

+      field128_.MakeReadOnly();

+      field127_.MakeReadOnly();

+      field130_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -4903,22 +4923,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new SpeedMessage2();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(SpeedMessage2 cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      SpeedMessage2 result;

+      private bool resultIsReadOnly;

+      private SpeedMessage2 result;

       

       private SpeedMessage2 PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           SpeedMessage2 original = result;

           result = new SpeedMessage2();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -4933,13 +4953,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new SpeedMessage2();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -4955,15 +4975,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SpeedMessage2 BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.group1_.MakeReadOnly();

-        result.field128_.MakeReadOnly();

-        result.field127_.MakeReadOnly();

-        result.field130_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -5913,7 +5929,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class SpeedMessage2GroupedMessage : pb::GeneratedMessage<SpeedMessage2GroupedMessage, SpeedMessage2GroupedMessage.Builder> {

     private SpeedMessage2GroupedMessage() { }

-    private static readonly SpeedMessage2GroupedMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly SpeedMessage2GroupedMessage defaultInstance = new SpeedMessage2GroupedMessage().MakeReadOnly();

     private static readonly string[] _speedMessage2GroupedMessageFieldNames = new string[] { "field1", "field10", "field11", "field2", "field3", "field4", "field5", "field6", "field7", "field8", "field9" };

     private static readonly uint[] _speedMessage2GroupedMessageFieldTags = new uint[] { 13, 85, 88, 21, 29, 32, 40, 48, 56, 69, 72 };

     public static SpeedMessage2GroupedMessage DefaultInstance {

@@ -5921,7 +5937,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override SpeedMessage2GroupedMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override SpeedMessage2GroupedMessage ThisMessage {

@@ -6167,6 +6183,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SpeedMessage2GroupedMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private SpeedMessage2GroupedMessage MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -6182,22 +6202,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new SpeedMessage2GroupedMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(SpeedMessage2GroupedMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      SpeedMessage2GroupedMessage result;

+      private bool resultIsReadOnly;

+      private SpeedMessage2GroupedMessage result;

       

       private SpeedMessage2GroupedMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           SpeedMessage2GroupedMessage original = result;

           result = new SpeedMessage2GroupedMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -6212,13 +6232,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new SpeedMessage2GroupedMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -6234,11 +6254,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SpeedMessage2GroupedMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestImportLiteProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestImportLiteProtoFile.cs
index 98672cf2..f31fdd89 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestImportLiteProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestImportLiteProtoFile.cs
@@ -44,7 +44,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class ImportMessageLite : pb::GeneratedMessageLite<ImportMessageLite, ImportMessageLite.Builder> {

     private ImportMessageLite() { }

-    private static readonly ImportMessageLite defaultInstance = new Builder().BuildPartial();

+    private static readonly ImportMessageLite defaultInstance = new ImportMessageLite().MakeReadOnly();

     private static readonly string[] _importMessageLiteFieldNames = new string[] { "d" };

     private static readonly uint[] _importMessageLiteFieldTags = new uint[] { 8 };

     public static ImportMessageLite DefaultInstance {

@@ -52,7 +52,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override ImportMessageLite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override ImportMessageLite ThisMessage {

@@ -147,6 +147,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static ImportMessageLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private ImportMessageLite MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -162,22 +166,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new ImportMessageLite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(ImportMessageLite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      ImportMessageLite result;

+      private bool resultIsReadOnly;

+      private ImportMessageLite result;

       

       private ImportMessageLite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           ImportMessageLite original = result;

           result = new ImportMessageLite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -192,13 +196,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new ImportMessageLite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -210,11 +214,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ImportMessageLite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestImportProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestImportProtoFile.cs
index 086dda1d..c12aef8e 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestImportProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestImportProtoFile.cs
@@ -72,7 +72,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class ImportMessage : pb::GeneratedMessage<ImportMessage, ImportMessage.Builder> {

     private ImportMessage() { }

-    private static readonly ImportMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly ImportMessage defaultInstance = new ImportMessage().MakeReadOnly();

     private static readonly string[] _importMessageFieldNames = new string[] { "d" };

     private static readonly uint[] _importMessageFieldTags = new uint[] { 8 };

     public static ImportMessage DefaultInstance {

@@ -80,7 +80,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override ImportMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override ImportMessage ThisMessage {

@@ -166,6 +166,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static ImportMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private ImportMessage MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -181,22 +185,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new ImportMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(ImportMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      ImportMessage result;

+      private bool resultIsReadOnly;

+      private ImportMessage result;

       

       private ImportMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           ImportMessage original = result;

           result = new ImportMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -211,13 +215,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new ImportMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -233,11 +237,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ImportMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestMessageSetProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestMessageSetProtoFile.cs
index 6a90612c..f9cec1af 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestMessageSetProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestMessageSetProtoFile.cs
@@ -104,7 +104,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestMessageSet : pb::ExtendableMessage<TestMessageSet, TestMessageSet.Builder> {

     private TestMessageSet() { }

-    private static readonly TestMessageSet defaultInstance = new Builder().BuildPartial();

+    private static readonly TestMessageSet defaultInstance = new TestMessageSet().MakeReadOnly();

     private static readonly string[] _testMessageSetFieldNames = new string[] {  };

     private static readonly uint[] _testMessageSetFieldTags = new uint[] {  };

     public static TestMessageSet DefaultInstance {

@@ -112,7 +112,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestMessageSet DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestMessageSet ThisMessage {

@@ -186,6 +186,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMessageSet ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestMessageSet MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -201,22 +205,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestMessageSet();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestMessageSet cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestMessageSet result;

+      private bool resultIsReadOnly;

+      private TestMessageSet result;

       

       private TestMessageSet PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestMessageSet original = result;

           result = new TestMessageSet();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -231,13 +235,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestMessageSet();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -253,11 +257,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMessageSet BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -336,7 +340,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestMessageSetContainer : pb::GeneratedMessage<TestMessageSetContainer, TestMessageSetContainer.Builder> {

     private TestMessageSetContainer() { }

-    private static readonly TestMessageSetContainer defaultInstance = new Builder().BuildPartial();

+    private static readonly TestMessageSetContainer defaultInstance = new TestMessageSetContainer().MakeReadOnly();

     private static readonly string[] _testMessageSetContainerFieldNames = new string[] { "message_set" };

     private static readonly uint[] _testMessageSetContainerFieldTags = new uint[] { 10 };

     public static TestMessageSetContainer DefaultInstance {

@@ -344,7 +348,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestMessageSetContainer DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestMessageSetContainer ThisMessage {

@@ -361,12 +365,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int MessageSetFieldNumber = 1;

     private bool hasMessageSet;

-    private global::Google.ProtocolBuffers.TestProtos.TestMessageSet messageSet_ = global::Google.ProtocolBuffers.TestProtos.TestMessageSet.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestMessageSet messageSet_;

     public bool HasMessageSet {

       get { return hasMessageSet; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestMessageSet MessageSet {

-      get { return messageSet_; }

+      get { return messageSet_ ?? global::Google.ProtocolBuffers.TestProtos.TestMessageSet.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -430,6 +434,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMessageSetContainer ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestMessageSetContainer MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -445,22 +453,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestMessageSetContainer();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestMessageSetContainer cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestMessageSetContainer result;

+      private bool resultIsReadOnly;

+      private TestMessageSetContainer result;

       

       private TestMessageSetContainer PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestMessageSetContainer original = result;

           result = new TestMessageSetContainer();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -475,13 +483,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestMessageSetContainer();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -497,11 +505,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMessageSetContainer BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -617,7 +625,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearMessageSet() {

         PrepareBuilder();

         result.hasMessageSet = false;

-        result.messageSet_ = global::Google.ProtocolBuffers.TestProtos.TestMessageSet.DefaultInstance;

+        result.messageSet_ = null;

         return this;

       }

     }

@@ -631,7 +639,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestMessageSetExtension1 : pb::GeneratedMessage<TestMessageSetExtension1, TestMessageSetExtension1.Builder> {

     private TestMessageSetExtension1() { }

-    private static readonly TestMessageSetExtension1 defaultInstance = new Builder().BuildPartial();

+    private static readonly TestMessageSetExtension1 defaultInstance = new TestMessageSetExtension1().MakeReadOnly();

     private static readonly string[] _testMessageSetExtension1FieldNames = new string[] { "i" };

     private static readonly uint[] _testMessageSetExtension1FieldTags = new uint[] { 120 };

     public static TestMessageSetExtension1 DefaultInstance {

@@ -639,7 +647,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestMessageSetExtension1 DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestMessageSetExtension1 ThisMessage {

@@ -727,6 +735,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMessageSetExtension1 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestMessageSetExtension1 MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -742,22 +754,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestMessageSetExtension1();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestMessageSetExtension1 cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestMessageSetExtension1 result;

+      private bool resultIsReadOnly;

+      private TestMessageSetExtension1 result;

       

       private TestMessageSetExtension1 PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestMessageSetExtension1 original = result;

           result = new TestMessageSetExtension1();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -772,13 +784,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestMessageSetExtension1();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -794,11 +806,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMessageSetExtension1 BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -903,7 +915,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestMessageSetExtension2 : pb::GeneratedMessage<TestMessageSetExtension2, TestMessageSetExtension2.Builder> {

     private TestMessageSetExtension2() { }

-    private static readonly TestMessageSetExtension2 defaultInstance = new Builder().BuildPartial();

+    private static readonly TestMessageSetExtension2 defaultInstance = new TestMessageSetExtension2().MakeReadOnly();

     private static readonly string[] _testMessageSetExtension2FieldNames = new string[] { "str" };

     private static readonly uint[] _testMessageSetExtension2FieldTags = new uint[] { 202 };

     public static TestMessageSetExtension2 DefaultInstance {

@@ -911,7 +923,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestMessageSetExtension2 DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestMessageSetExtension2 ThisMessage {

@@ -999,6 +1011,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMessageSetExtension2 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestMessageSetExtension2 MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1014,22 +1030,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestMessageSetExtension2();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestMessageSetExtension2 cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestMessageSetExtension2 result;

+      private bool resultIsReadOnly;

+      private TestMessageSetExtension2 result;

       

       private TestMessageSetExtension2 PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestMessageSetExtension2 original = result;

           result = new TestMessageSetExtension2();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1044,13 +1060,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestMessageSetExtension2();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1066,11 +1082,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMessageSetExtension2 BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -1176,7 +1192,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class RawMessageSet : pb::GeneratedMessage<RawMessageSet, RawMessageSet.Builder> {

     private RawMessageSet() { }

-    private static readonly RawMessageSet defaultInstance = new Builder().BuildPartial();

+    private static readonly RawMessageSet defaultInstance = new RawMessageSet().MakeReadOnly();

     private static readonly string[] _rawMessageSetFieldNames = new string[] { "item" };

     private static readonly uint[] _rawMessageSetFieldTags = new uint[] { 11 };

     public static RawMessageSet DefaultInstance {

@@ -1184,7 +1200,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override RawMessageSet DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override RawMessageSet ThisMessage {

@@ -1209,7 +1225,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class Item : pb::GeneratedMessage<Item, Item.Builder> {

         private Item() { }

-        private static readonly Item defaultInstance = new Builder().BuildPartial();

+        private static readonly Item defaultInstance = new Item().MakeReadOnly();

         private static readonly string[] _itemFieldNames = new string[] { "message", "type_id" };

         private static readonly uint[] _itemFieldTags = new uint[] { 26, 16 };

         public static Item DefaultInstance {

@@ -1217,7 +1233,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override Item DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override Item ThisMessage {

@@ -1321,6 +1337,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Item ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private Item MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1336,22 +1356,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new Item();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(Item cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          Item result;

+          private bool resultIsReadOnly;

+          private Item result;

           

           private Item PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               Item original = result;

               result = new Item();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -1366,13 +1386,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new Item();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -1388,11 +1408,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Item BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -1596,6 +1616,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static RawMessageSet ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private RawMessageSet MakeReadOnly() {

+      item_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1611,22 +1636,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new RawMessageSet();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(RawMessageSet cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      RawMessageSet result;

+      private bool resultIsReadOnly;

+      private RawMessageSet result;

       

       private RawMessageSet PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           RawMessageSet original = result;

           result = new RawMessageSet();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1641,13 +1666,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new RawMessageSet();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1663,12 +1688,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override RawMessageSet BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.item_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestNoGenericServicesProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestNoGenericServicesProtoFile.cs
index 2cd2696b..ba56b145 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestNoGenericServicesProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestNoGenericServicesProtoFile.cs
@@ -82,7 +82,7 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestMessage : pb::ExtendableMessage<TestMessage, TestMessage.Builder> {

     private TestMessage() { }

-    private static readonly TestMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly TestMessage defaultInstance = new TestMessage().MakeReadOnly();

     private static readonly string[] _testMessageFieldNames = new string[] { "a" };

     private static readonly uint[] _testMessageFieldTags = new uint[] { 8 };

     public static TestMessage DefaultInstance {

@@ -90,7 +90,7 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
     }

     

     public override TestMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestMessage ThisMessage {

@@ -180,6 +180,10 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
     public static TestMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestMessage MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -195,22 +199,22 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestMessage result;

+      private bool resultIsReadOnly;

+      private TestMessage result;

       

       private TestMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestMessage original = result;

           result = new TestMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -225,13 +229,13 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -247,11 +251,11 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
       }

       

       public override TestMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestOptimizeForProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestOptimizeForProtoFile.cs
index 10c464a4..92d965d4 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestOptimizeForProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestOptimizeForProtoFile.cs
@@ -86,13 +86,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestOptimizedForSize : pb::ExtendableMessage<TestOptimizedForSize, TestOptimizedForSize.Builder> {

     private TestOptimizedForSize() { }

-    private static readonly TestOptimizedForSize defaultInstance = new Builder().BuildPartial();

+    private static readonly TestOptimizedForSize defaultInstance = new TestOptimizedForSize().MakeReadOnly();

     public static TestOptimizedForSize DefaultInstance {

       get { return defaultInstance; }

     }

     

     public override TestOptimizedForSize DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestOptimizedForSize ThisMessage {

@@ -123,12 +123,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int MsgFieldNumber = 19;

     private bool hasMsg;

-    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage msg_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage msg_;

     public bool HasMsg {

       get { return hasMsg; }

     }

     public global::Google.ProtocolBuffers.TestProtos.ForeignMessage Msg {

-      get { return msg_; }

+      get { return msg_ ?? global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance; }

     }

     

     public static TestOptimizedForSize ParseFrom(pb::ByteString data) {

@@ -161,6 +161,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestOptimizedForSize ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestOptimizedForSize MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -176,22 +180,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestOptimizedForSize();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestOptimizedForSize cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestOptimizedForSize result;

+      private bool resultIsReadOnly;

+      private TestOptimizedForSize result;

       

       private TestOptimizedForSize PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestOptimizedForSize original = result;

           result = new TestOptimizedForSize();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -206,13 +210,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestOptimizedForSize();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -228,11 +232,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestOptimizedForSize BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

       

@@ -292,7 +296,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearMsg() {

         PrepareBuilder();

         result.hasMsg = false;

-        result.msg_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

+        result.msg_ = null;

         return this;

       }

     }

@@ -306,13 +310,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestRequiredOptimizedForSize : pb::GeneratedMessage<TestRequiredOptimizedForSize, TestRequiredOptimizedForSize.Builder> {

     private TestRequiredOptimizedForSize() { }

-    private static readonly TestRequiredOptimizedForSize defaultInstance = new Builder().BuildPartial();

+    private static readonly TestRequiredOptimizedForSize defaultInstance = new TestRequiredOptimizedForSize().MakeReadOnly();

     public static TestRequiredOptimizedForSize DefaultInstance {

       get { return defaultInstance; }

     }

     

     public override TestRequiredOptimizedForSize DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestRequiredOptimizedForSize ThisMessage {

@@ -367,6 +371,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestRequiredOptimizedForSize ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestRequiredOptimizedForSize MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -382,22 +390,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestRequiredOptimizedForSize();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestRequiredOptimizedForSize cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestRequiredOptimizedForSize result;

+      private bool resultIsReadOnly;

+      private TestRequiredOptimizedForSize result;

       

       private TestRequiredOptimizedForSize PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestRequiredOptimizedForSize original = result;

           result = new TestRequiredOptimizedForSize();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -412,13 +420,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestRequiredOptimizedForSize();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -434,11 +442,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRequiredOptimizedForSize BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

       

@@ -472,13 +480,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestOptionalOptimizedForSize : pb::GeneratedMessage<TestOptionalOptimizedForSize, TestOptionalOptimizedForSize.Builder> {

     private TestOptionalOptimizedForSize() { }

-    private static readonly TestOptionalOptimizedForSize defaultInstance = new Builder().BuildPartial();

+    private static readonly TestOptionalOptimizedForSize defaultInstance = new TestOptionalOptimizedForSize().MakeReadOnly();

     public static TestOptionalOptimizedForSize DefaultInstance {

       get { return defaultInstance; }

     }

     

     public override TestOptionalOptimizedForSize DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestOptionalOptimizedForSize ThisMessage {

@@ -495,12 +503,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int OFieldNumber = 1;

     private bool hasO;

-    private global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize o_ = global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize o_;

     public bool HasO {

       get { return hasO; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize O {

-      get { return o_; }

+      get { return o_ ?? global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize.DefaultInstance; }

     }

     

     public static TestOptionalOptimizedForSize ParseFrom(pb::ByteString data) {

@@ -533,6 +541,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestOptionalOptimizedForSize ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestOptionalOptimizedForSize MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -548,22 +560,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestOptionalOptimizedForSize();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestOptionalOptimizedForSize cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestOptionalOptimizedForSize result;

+      private bool resultIsReadOnly;

+      private TestOptionalOptimizedForSize result;

       

       private TestOptionalOptimizedForSize PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestOptionalOptimizedForSize original = result;

           result = new TestOptionalOptimizedForSize();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -578,13 +590,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestOptionalOptimizedForSize();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -600,11 +612,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestOptionalOptimizedForSize BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

       

@@ -644,7 +656,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearO() {

         PrepareBuilder();

         result.hasO = false;

-        result.o_ = global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize.DefaultInstance;

+        result.o_ = null;

         return this;

       }

     }

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestProtoFile.cs
index 547d712a..35064953 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestProtoFile.cs
@@ -1123,7 +1123,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestAllTypes : pb::GeneratedMessage<TestAllTypes, TestAllTypes.Builder> {

     private TestAllTypes() { }

-    private static readonly TestAllTypes defaultInstance = new Builder().BuildPartial();

+    private static readonly TestAllTypes defaultInstance = new TestAllTypes().MakeReadOnly();

     private static readonly string[] _testAllTypesFieldNames = new string[] { "default_bool", "default_bytes", "default_cord", "default_double", "default_fixed32", "default_fixed64", "default_float", "default_foreign_enum", "default_import_enum", "default_int32", "default_int64", "default_nested_enum", "default_sfixed32", "default_sfixed64", "default_sint32", "default_sint64", "default_string", "default_string_piece", "default_uint32", "default_uint64", "optional_bool", "optional_bytes", "optional_cord", "optional_double", "optional_fixed32", "optional_fixed64", "optional_float", "optional_foreign_enum", "optional_foreign_message", "optional_import_enum", "optional_import_message", "optional_int32", "optional_int64", "optional_nested_enum", "optional_nested_message", "optional_sfixed32", "optional_sfixed64", "optional_sint32", "optional_sint64", "optional_string", "optional_string_piece", "optional_uint32", "optional_uint64", "optionalgroup", "repeated_bool", "repeated_bytes", "repeated_cord", "repeated_double", "repeated_fixed32", "repeated_fixed64", "repeated_float", "repeated_foreign_enum", "repeated_foreign_message", "repeated_import_enum", "repeated_import_message", "repeated_int32", "repeated_int64", "repeated_nested_enum", "repeated_nested_message", "repeated_sfixed32", "repeated_sfixed64", "repeated_sint32", "repeated_sint64", "repeated_string", "repeated_string_piece", "repeated_uint32", "repeated_uint64", "repeatedgroup" };

     private static readonly uint[] _testAllTypesFieldTags = new uint[] { 584, 602, 682, 577, 541, 545, 573, 656, 664, 488, 496, 648, 557, 561, 520, 528, 594, 674, 504, 512, 104, 122, 202, 97, 61, 65, 93, 176, 154, 184, 162, 8, 16, 168, 146, 77, 81, 40, 48, 114, 194, 24, 32, 131, 344, 362, 442, 337, 301, 305, 333, 416, 394, 424, 402, 248, 256, 408, 386, 317, 321, 280, 288, 354, 434, 264, 272, 371 };

     public static TestAllTypes DefaultInstance {

@@ -1131,7 +1131,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestAllTypes DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestAllTypes ThisMessage {

@@ -1164,7 +1164,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class NestedMessage : pb::GeneratedMessage<NestedMessage, NestedMessage.Builder> {

         private NestedMessage() { }

-        private static readonly NestedMessage defaultInstance = new Builder().BuildPartial();

+        private static readonly NestedMessage defaultInstance = new NestedMessage().MakeReadOnly();

         private static readonly string[] _nestedMessageFieldNames = new string[] { "bb" };

         private static readonly uint[] _nestedMessageFieldTags = new uint[] { 8 };

         public static NestedMessage DefaultInstance {

@@ -1172,7 +1172,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override NestedMessage DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override NestedMessage ThisMessage {

@@ -1258,6 +1258,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static NestedMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private NestedMessage MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1273,22 +1277,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new NestedMessage();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(NestedMessage cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          NestedMessage result;

+          private bool resultIsReadOnly;

+          private NestedMessage result;

           

           private NestedMessage PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               NestedMessage original = result;

               result = new NestedMessage();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -1303,13 +1307,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new NestedMessage();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -1325,11 +1329,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override NestedMessage BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -1434,7 +1438,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class OptionalGroup : pb::GeneratedMessage<OptionalGroup, OptionalGroup.Builder> {

         private OptionalGroup() { }

-        private static readonly OptionalGroup defaultInstance = new Builder().BuildPartial();

+        private static readonly OptionalGroup defaultInstance = new OptionalGroup().MakeReadOnly();

         private static readonly string[] _optionalGroupFieldNames = new string[] { "a" };

         private static readonly uint[] _optionalGroupFieldTags = new uint[] { 136 };

         public static OptionalGroup DefaultInstance {

@@ -1442,7 +1446,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override OptionalGroup DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override OptionalGroup ThisMessage {

@@ -1528,6 +1532,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static OptionalGroup ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private OptionalGroup MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1543,22 +1551,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new OptionalGroup();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(OptionalGroup cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          OptionalGroup result;

+          private bool resultIsReadOnly;

+          private OptionalGroup result;

           

           private OptionalGroup PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               OptionalGroup original = result;

               result = new OptionalGroup();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -1573,13 +1581,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new OptionalGroup();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -1595,11 +1603,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override OptionalGroup BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -1704,7 +1712,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class RepeatedGroup : pb::GeneratedMessage<RepeatedGroup, RepeatedGroup.Builder> {

         private RepeatedGroup() { }

-        private static readonly RepeatedGroup defaultInstance = new Builder().BuildPartial();

+        private static readonly RepeatedGroup defaultInstance = new RepeatedGroup().MakeReadOnly();

         private static readonly string[] _repeatedGroupFieldNames = new string[] { "a" };

         private static readonly uint[] _repeatedGroupFieldTags = new uint[] { 376 };

         public static RepeatedGroup DefaultInstance {

@@ -1712,7 +1720,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override RepeatedGroup DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override RepeatedGroup ThisMessage {

@@ -1798,6 +1806,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static RepeatedGroup ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private RepeatedGroup MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1813,22 +1825,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new RepeatedGroup();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(RepeatedGroup cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          RepeatedGroup result;

+          private bool resultIsReadOnly;

+          private RepeatedGroup result;

           

           private RepeatedGroup PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               RepeatedGroup original = result;

               result = new RepeatedGroup();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -1843,13 +1855,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new RepeatedGroup();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -1865,11 +1877,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override RepeatedGroup BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -2128,42 +2140,42 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int OptionalGroupFieldNumber = 16;

     private bool hasOptionalGroup;

-    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup optionalGroup_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup optionalGroup_;

     public bool HasOptionalGroup {

       get { return hasOptionalGroup; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup OptionalGroup {

-      get { return optionalGroup_; }

+      get { return optionalGroup_ ?? global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.DefaultInstance; }

     }

     

     public const int OptionalNestedMessageFieldNumber = 18;

     private bool hasOptionalNestedMessage;

-    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage optionalNestedMessage_;

     public bool HasOptionalNestedMessage {

       get { return hasOptionalNestedMessage; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage OptionalNestedMessage {

-      get { return optionalNestedMessage_; }

+      get { return optionalNestedMessage_ ?? global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance; }

     }

     

     public const int OptionalForeignMessageFieldNumber = 19;

     private bool hasOptionalForeignMessage;

-    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage optionalForeignMessage_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage optionalForeignMessage_;

     public bool HasOptionalForeignMessage {

       get { return hasOptionalForeignMessage; }

     }

     public global::Google.ProtocolBuffers.TestProtos.ForeignMessage OptionalForeignMessage {

-      get { return optionalForeignMessage_; }

+      get { return optionalForeignMessage_ ?? global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance; }

     }

     

     public const int OptionalImportMessageFieldNumber = 20;

     private bool hasOptionalImportMessage;

-    private global::Google.ProtocolBuffers.TestProtos.ImportMessage optionalImportMessage_ = global::Google.ProtocolBuffers.TestProtos.ImportMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.ImportMessage optionalImportMessage_;

     public bool HasOptionalImportMessage {

       get { return hasOptionalImportMessage; }

     }

     public global::Google.ProtocolBuffers.TestProtos.ImportMessage OptionalImportMessage {

-      get { return optionalImportMessage_; }

+      get { return optionalImportMessage_ ?? global::Google.ProtocolBuffers.TestProtos.ImportMessage.DefaultInstance; }

     }

     

     public const int OptionalNestedEnumFieldNumber = 21;

@@ -3271,6 +3283,34 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestAllTypes ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestAllTypes MakeReadOnly() {

+      repeatedInt32_.MakeReadOnly();

+      repeatedInt64_.MakeReadOnly();

+      repeatedUint32_.MakeReadOnly();

+      repeatedUint64_.MakeReadOnly();

+      repeatedSint32_.MakeReadOnly();

+      repeatedSint64_.MakeReadOnly();

+      repeatedFixed32_.MakeReadOnly();

+      repeatedFixed64_.MakeReadOnly();

+      repeatedSfixed32_.MakeReadOnly();

+      repeatedSfixed64_.MakeReadOnly();

+      repeatedFloat_.MakeReadOnly();

+      repeatedDouble_.MakeReadOnly();

+      repeatedBool_.MakeReadOnly();

+      repeatedString_.MakeReadOnly();

+      repeatedBytes_.MakeReadOnly();

+      repeatedGroup_.MakeReadOnly();

+      repeatedNestedMessage_.MakeReadOnly();

+      repeatedForeignMessage_.MakeReadOnly();

+      repeatedImportMessage_.MakeReadOnly();

+      repeatedNestedEnum_.MakeReadOnly();

+      repeatedForeignEnum_.MakeReadOnly();

+      repeatedImportEnum_.MakeReadOnly();

+      repeatedStringPiece_.MakeReadOnly();

+      repeatedCord_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -3286,22 +3326,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestAllTypes();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestAllTypes cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestAllTypes result;

+      private bool resultIsReadOnly;

+      private TestAllTypes result;

       

       private TestAllTypes PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestAllTypes original = result;

           result = new TestAllTypes();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -3316,13 +3356,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestAllTypes();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -3338,35 +3378,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestAllTypes BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.repeatedInt32_.MakeReadOnly();

-        result.repeatedInt64_.MakeReadOnly();

-        result.repeatedUint32_.MakeReadOnly();

-        result.repeatedUint64_.MakeReadOnly();

-        result.repeatedSint32_.MakeReadOnly();

-        result.repeatedSint64_.MakeReadOnly();

-        result.repeatedFixed32_.MakeReadOnly();

-        result.repeatedFixed64_.MakeReadOnly();

-        result.repeatedSfixed32_.MakeReadOnly();

-        result.repeatedSfixed64_.MakeReadOnly();

-        result.repeatedFloat_.MakeReadOnly();

-        result.repeatedDouble_.MakeReadOnly();

-        result.repeatedBool_.MakeReadOnly();

-        result.repeatedString_.MakeReadOnly();

-        result.repeatedBytes_.MakeReadOnly();

-        result.repeatedGroup_.MakeReadOnly();

-        result.repeatedNestedMessage_.MakeReadOnly();

-        result.repeatedForeignMessage_.MakeReadOnly();

-        result.repeatedImportMessage_.MakeReadOnly();

-        result.repeatedNestedEnum_.MakeReadOnly();

-        result.repeatedForeignEnum_.MakeReadOnly();

-        result.repeatedImportEnum_.MakeReadOnly();

-        result.repeatedStringPiece_.MakeReadOnly();

-        result.repeatedCord_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -4367,7 +4383,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalGroup() {

         PrepareBuilder();

         result.hasOptionalGroup = false;

-        result.optionalGroup_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.DefaultInstance;

+        result.optionalGroup_ = null;

         return this;

       }

       

@@ -4407,7 +4423,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalNestedMessage() {

         PrepareBuilder();

         result.hasOptionalNestedMessage = false;

-        result.optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance;

+        result.optionalNestedMessage_ = null;

         return this;

       }

       

@@ -4447,7 +4463,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalForeignMessage() {

         PrepareBuilder();

         result.hasOptionalForeignMessage = false;

-        result.optionalForeignMessage_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

+        result.optionalForeignMessage_ = null;

         return this;

       }

       

@@ -4487,7 +4503,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalImportMessage() {

         PrepareBuilder();

         result.hasOptionalImportMessage = false;

-        result.optionalImportMessage_ = global::Google.ProtocolBuffers.TestProtos.ImportMessage.DefaultInstance;

+        result.optionalImportMessage_ = null;

         return this;

       }

       

@@ -5819,7 +5835,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestDeprecatedFields : pb::GeneratedMessage<TestDeprecatedFields, TestDeprecatedFields.Builder> {

     private TestDeprecatedFields() { }

-    private static readonly TestDeprecatedFields defaultInstance = new Builder().BuildPartial();

+    private static readonly TestDeprecatedFields defaultInstance = new TestDeprecatedFields().MakeReadOnly();

     private static readonly string[] _testDeprecatedFieldsFieldNames = new string[] { "deprecated_int32" };

     private static readonly uint[] _testDeprecatedFieldsFieldTags = new uint[] { 8 };

     public static TestDeprecatedFields DefaultInstance {

@@ -5827,7 +5843,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestDeprecatedFields DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestDeprecatedFields ThisMessage {

@@ -5915,6 +5931,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestDeprecatedFields ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestDeprecatedFields MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -5930,22 +5950,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestDeprecatedFields();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestDeprecatedFields cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestDeprecatedFields result;

+      private bool resultIsReadOnly;

+      private TestDeprecatedFields result;

       

       private TestDeprecatedFields PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestDeprecatedFields original = result;

           result = new TestDeprecatedFields();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -5960,13 +5980,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestDeprecatedFields();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -5982,11 +6002,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestDeprecatedFields BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -6095,7 +6115,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class ForeignMessage : pb::GeneratedMessage<ForeignMessage, ForeignMessage.Builder> {

     private ForeignMessage() { }

-    private static readonly ForeignMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly ForeignMessage defaultInstance = new ForeignMessage().MakeReadOnly();

     private static readonly string[] _foreignMessageFieldNames = new string[] { "c" };

     private static readonly uint[] _foreignMessageFieldTags = new uint[] { 8 };

     public static ForeignMessage DefaultInstance {

@@ -6103,7 +6123,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override ForeignMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override ForeignMessage ThisMessage {

@@ -6189,6 +6209,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static ForeignMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private ForeignMessage MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -6204,22 +6228,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new ForeignMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(ForeignMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      ForeignMessage result;

+      private bool resultIsReadOnly;

+      private ForeignMessage result;

       

       private ForeignMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           ForeignMessage original = result;

           result = new ForeignMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -6234,13 +6258,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new ForeignMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -6256,11 +6280,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ForeignMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -6365,7 +6389,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestAllExtensions : pb::ExtendableMessage<TestAllExtensions, TestAllExtensions.Builder> {

     private TestAllExtensions() { }

-    private static readonly TestAllExtensions defaultInstance = new Builder().BuildPartial();

+    private static readonly TestAllExtensions defaultInstance = new TestAllExtensions().MakeReadOnly();

     private static readonly string[] _testAllExtensionsFieldNames = new string[] {  };

     private static readonly uint[] _testAllExtensionsFieldTags = new uint[] {  };

     public static TestAllExtensions DefaultInstance {

@@ -6373,7 +6397,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestAllExtensions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestAllExtensions ThisMessage {

@@ -6447,6 +6471,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestAllExtensions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestAllExtensions MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -6462,22 +6490,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestAllExtensions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestAllExtensions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestAllExtensions result;

+      private bool resultIsReadOnly;

+      private TestAllExtensions result;

       

       private TestAllExtensions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestAllExtensions original = result;

           result = new TestAllExtensions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -6492,13 +6520,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestAllExtensions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -6514,11 +6542,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestAllExtensions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -6597,7 +6625,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class OptionalGroup_extension : pb::GeneratedMessage<OptionalGroup_extension, OptionalGroup_extension.Builder> {

     private OptionalGroup_extension() { }

-    private static readonly OptionalGroup_extension defaultInstance = new Builder().BuildPartial();

+    private static readonly OptionalGroup_extension defaultInstance = new OptionalGroup_extension().MakeReadOnly();

     private static readonly string[] _optionalGroupExtensionFieldNames = new string[] { "a" };

     private static readonly uint[] _optionalGroupExtensionFieldTags = new uint[] { 136 };

     public static OptionalGroup_extension DefaultInstance {

@@ -6605,7 +6633,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override OptionalGroup_extension DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override OptionalGroup_extension ThisMessage {

@@ -6691,6 +6719,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static OptionalGroup_extension ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private OptionalGroup_extension MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -6706,22 +6738,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new OptionalGroup_extension();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(OptionalGroup_extension cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      OptionalGroup_extension result;

+      private bool resultIsReadOnly;

+      private OptionalGroup_extension result;

       

       private OptionalGroup_extension PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           OptionalGroup_extension original = result;

           result = new OptionalGroup_extension();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -6736,13 +6768,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new OptionalGroup_extension();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -6758,11 +6790,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OptionalGroup_extension BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -6867,7 +6899,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class RepeatedGroup_extension : pb::GeneratedMessage<RepeatedGroup_extension, RepeatedGroup_extension.Builder> {

     private RepeatedGroup_extension() { }

-    private static readonly RepeatedGroup_extension defaultInstance = new Builder().BuildPartial();

+    private static readonly RepeatedGroup_extension defaultInstance = new RepeatedGroup_extension().MakeReadOnly();

     private static readonly string[] _repeatedGroupExtensionFieldNames = new string[] { "a" };

     private static readonly uint[] _repeatedGroupExtensionFieldTags = new uint[] { 376 };

     public static RepeatedGroup_extension DefaultInstance {

@@ -6875,7 +6907,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override RepeatedGroup_extension DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override RepeatedGroup_extension ThisMessage {

@@ -6961,6 +6993,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static RepeatedGroup_extension ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private RepeatedGroup_extension MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -6976,22 +7012,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new RepeatedGroup_extension();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(RepeatedGroup_extension cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      RepeatedGroup_extension result;

+      private bool resultIsReadOnly;

+      private RepeatedGroup_extension result;

       

       private RepeatedGroup_extension PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           RepeatedGroup_extension original = result;

           result = new RepeatedGroup_extension();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -7006,13 +7042,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new RepeatedGroup_extension();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -7028,11 +7064,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override RepeatedGroup_extension BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -7137,7 +7173,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestNestedExtension : pb::GeneratedMessage<TestNestedExtension, TestNestedExtension.Builder> {

     private TestNestedExtension() { }

-    private static readonly TestNestedExtension defaultInstance = new Builder().BuildPartial();

+    private static readonly TestNestedExtension defaultInstance = new TestNestedExtension().MakeReadOnly();

     private static readonly string[] _testNestedExtensionFieldNames = new string[] {  };

     private static readonly uint[] _testNestedExtensionFieldTags = new uint[] {  };

     public static TestNestedExtension DefaultInstance {

@@ -7145,7 +7181,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestNestedExtension DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestNestedExtension ThisMessage {

@@ -7217,6 +7253,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestNestedExtension ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestNestedExtension MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -7232,22 +7272,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestNestedExtension();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestNestedExtension cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestNestedExtension result;

+      private bool resultIsReadOnly;

+      private TestNestedExtension result;

       

       private TestNestedExtension PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestNestedExtension original = result;

           result = new TestNestedExtension();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -7262,13 +7302,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestNestedExtension();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -7284,11 +7324,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestNestedExtension BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -7366,7 +7406,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestRequired : pb::GeneratedMessage<TestRequired, TestRequired.Builder> {

     private TestRequired() { }

-    private static readonly TestRequired defaultInstance = new Builder().BuildPartial();

+    private static readonly TestRequired defaultInstance = new TestRequired().MakeReadOnly();

     private static readonly string[] _testRequiredFieldNames = new string[] { "a", "b", "c", "dummy10", "dummy11", "dummy12", "dummy13", "dummy14", "dummy15", "dummy16", "dummy17", "dummy18", "dummy19", "dummy2", "dummy20", "dummy21", "dummy22", "dummy23", "dummy24", "dummy25", "dummy26", "dummy27", "dummy28", "dummy29", "dummy30", "dummy31", "dummy32", "dummy4", "dummy5", "dummy6", "dummy7", "dummy8", "dummy9" };

     private static readonly uint[] _testRequiredFieldTags = new uint[] { 8, 24, 264, 80, 88, 96, 104, 112, 120, 128, 136, 144, 152, 16, 160, 168, 176, 184, 192, 200, 208, 216, 224, 232, 240, 248, 256, 32, 40, 48, 56, 64, 72 };

     public static TestRequired DefaultInstance {

@@ -7374,7 +7414,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestRequired DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestRequired ThisMessage {

@@ -7979,6 +8019,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestRequired ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestRequired MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -7994,22 +8038,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestRequired();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestRequired cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestRequired result;

+      private bool resultIsReadOnly;

+      private TestRequired result;

       

       private TestRequired PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestRequired original = result;

           result = new TestRequired();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -8024,13 +8068,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestRequired();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -8046,11 +8090,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRequired BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -9019,7 +9063,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestRequiredForeign : pb::GeneratedMessage<TestRequiredForeign, TestRequiredForeign.Builder> {

     private TestRequiredForeign() { }

-    private static readonly TestRequiredForeign defaultInstance = new Builder().BuildPartial();

+    private static readonly TestRequiredForeign defaultInstance = new TestRequiredForeign().MakeReadOnly();

     private static readonly string[] _testRequiredForeignFieldNames = new string[] { "dummy", "optional_message", "repeated_message" };

     private static readonly uint[] _testRequiredForeignFieldTags = new uint[] { 24, 10, 18 };

     public static TestRequiredForeign DefaultInstance {

@@ -9027,7 +9071,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestRequiredForeign DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestRequiredForeign ThisMessage {

@@ -9044,12 +9088,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int OptionalMessageFieldNumber = 1;

     private bool hasOptionalMessage;

-    private global::Google.ProtocolBuffers.TestProtos.TestRequired optionalMessage_ = global::Google.ProtocolBuffers.TestProtos.TestRequired.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestRequired optionalMessage_;

     public bool HasOptionalMessage {

       get { return hasOptionalMessage; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestRequired OptionalMessage {

-      get { return optionalMessage_; }

+      get { return optionalMessage_ ?? global::Google.ProtocolBuffers.TestProtos.TestRequired.DefaultInstance; }

     }

     

     public const int RepeatedMessageFieldNumber = 2;

@@ -9153,6 +9197,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestRequiredForeign ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestRequiredForeign MakeReadOnly() {

+      repeatedMessage_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -9168,22 +9217,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestRequiredForeign();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestRequiredForeign cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestRequiredForeign result;

+      private bool resultIsReadOnly;

+      private TestRequiredForeign result;

       

       private TestRequiredForeign PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestRequiredForeign original = result;

           result = new TestRequiredForeign();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -9198,13 +9247,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestRequiredForeign();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -9220,12 +9269,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRequiredForeign BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.repeatedMessage_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -9355,7 +9403,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalMessage() {

         PrepareBuilder();

         result.hasOptionalMessage = false;

-        result.optionalMessage_ = global::Google.ProtocolBuffers.TestProtos.TestRequired.DefaultInstance;

+        result.optionalMessage_ = null;

         return this;

       }

       

@@ -9433,7 +9481,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestForeignNested : pb::GeneratedMessage<TestForeignNested, TestForeignNested.Builder> {

     private TestForeignNested() { }

-    private static readonly TestForeignNested defaultInstance = new Builder().BuildPartial();

+    private static readonly TestForeignNested defaultInstance = new TestForeignNested().MakeReadOnly();

     private static readonly string[] _testForeignNestedFieldNames = new string[] { "foreign_nested" };

     private static readonly uint[] _testForeignNestedFieldTags = new uint[] { 10 };

     public static TestForeignNested DefaultInstance {

@@ -9441,7 +9489,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestForeignNested DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestForeignNested ThisMessage {

@@ -9458,12 +9506,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int ForeignNestedFieldNumber = 1;

     private bool hasForeignNested;

-    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage foreignNested_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage foreignNested_;

     public bool HasForeignNested {

       get { return hasForeignNested; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage ForeignNested {

-      get { return foreignNested_; }

+      get { return foreignNested_ ?? global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -9527,6 +9575,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestForeignNested ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestForeignNested MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -9542,22 +9594,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestForeignNested();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestForeignNested cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestForeignNested result;

+      private bool resultIsReadOnly;

+      private TestForeignNested result;

       

       private TestForeignNested PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestForeignNested original = result;

           result = new TestForeignNested();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -9572,13 +9624,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestForeignNested();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -9594,11 +9646,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestForeignNested BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -9714,7 +9766,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearForeignNested() {

         PrepareBuilder();

         result.hasForeignNested = false;

-        result.foreignNested_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance;

+        result.foreignNested_ = null;

         return this;

       }

     }

@@ -9728,7 +9780,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestEmptyMessage : pb::GeneratedMessage<TestEmptyMessage, TestEmptyMessage.Builder> {

     private TestEmptyMessage() { }

-    private static readonly TestEmptyMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly TestEmptyMessage defaultInstance = new TestEmptyMessage().MakeReadOnly();

     private static readonly string[] _testEmptyMessageFieldNames = new string[] {  };

     private static readonly uint[] _testEmptyMessageFieldTags = new uint[] {  };

     public static TestEmptyMessage DefaultInstance {

@@ -9736,7 +9788,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestEmptyMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestEmptyMessage ThisMessage {

@@ -9806,6 +9858,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestEmptyMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestEmptyMessage MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -9821,22 +9877,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestEmptyMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestEmptyMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestEmptyMessage result;

+      private bool resultIsReadOnly;

+      private TestEmptyMessage result;

       

       private TestEmptyMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestEmptyMessage original = result;

           result = new TestEmptyMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -9851,13 +9907,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestEmptyMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -9873,11 +9929,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestEmptyMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -9955,7 +10011,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestEmptyMessageWithExtensions : pb::ExtendableMessage<TestEmptyMessageWithExtensions, TestEmptyMessageWithExtensions.Builder> {

     private TestEmptyMessageWithExtensions() { }

-    private static readonly TestEmptyMessageWithExtensions defaultInstance = new Builder().BuildPartial();

+    private static readonly TestEmptyMessageWithExtensions defaultInstance = new TestEmptyMessageWithExtensions().MakeReadOnly();

     private static readonly string[] _testEmptyMessageWithExtensionsFieldNames = new string[] {  };

     private static readonly uint[] _testEmptyMessageWithExtensionsFieldTags = new uint[] {  };

     public static TestEmptyMessageWithExtensions DefaultInstance {

@@ -9963,7 +10019,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestEmptyMessageWithExtensions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestEmptyMessageWithExtensions ThisMessage {

@@ -10037,6 +10093,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestEmptyMessageWithExtensions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestEmptyMessageWithExtensions MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -10052,22 +10112,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestEmptyMessageWithExtensions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestEmptyMessageWithExtensions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestEmptyMessageWithExtensions result;

+      private bool resultIsReadOnly;

+      private TestEmptyMessageWithExtensions result;

       

       private TestEmptyMessageWithExtensions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestEmptyMessageWithExtensions original = result;

           result = new TestEmptyMessageWithExtensions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -10082,13 +10142,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestEmptyMessageWithExtensions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -10104,11 +10164,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestEmptyMessageWithExtensions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -10187,7 +10247,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestMultipleExtensionRanges : pb::ExtendableMessage<TestMultipleExtensionRanges, TestMultipleExtensionRanges.Builder> {

     private TestMultipleExtensionRanges() { }

-    private static readonly TestMultipleExtensionRanges defaultInstance = new Builder().BuildPartial();

+    private static readonly TestMultipleExtensionRanges defaultInstance = new TestMultipleExtensionRanges().MakeReadOnly();

     private static readonly string[] _testMultipleExtensionRangesFieldNames = new string[] {  };

     private static readonly uint[] _testMultipleExtensionRangesFieldTags = new uint[] {  };

     public static TestMultipleExtensionRanges DefaultInstance {

@@ -10195,7 +10255,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestMultipleExtensionRanges DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestMultipleExtensionRanges ThisMessage {

@@ -10271,6 +10331,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMultipleExtensionRanges ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestMultipleExtensionRanges MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -10286,22 +10350,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestMultipleExtensionRanges();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestMultipleExtensionRanges cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestMultipleExtensionRanges result;

+      private bool resultIsReadOnly;

+      private TestMultipleExtensionRanges result;

       

       private TestMultipleExtensionRanges PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestMultipleExtensionRanges original = result;

           result = new TestMultipleExtensionRanges();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -10316,13 +10380,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestMultipleExtensionRanges();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -10338,11 +10402,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMultipleExtensionRanges BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -10421,7 +10485,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestReallyLargeTagNumber : pb::GeneratedMessage<TestReallyLargeTagNumber, TestReallyLargeTagNumber.Builder> {

     private TestReallyLargeTagNumber() { }

-    private static readonly TestReallyLargeTagNumber defaultInstance = new Builder().BuildPartial();

+    private static readonly TestReallyLargeTagNumber defaultInstance = new TestReallyLargeTagNumber().MakeReadOnly();

     private static readonly string[] _testReallyLargeTagNumberFieldNames = new string[] { "a", "bb" };

     private static readonly uint[] _testReallyLargeTagNumberFieldTags = new uint[] { 8, 2147483640 };

     public static TestReallyLargeTagNumber DefaultInstance {

@@ -10429,7 +10493,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestReallyLargeTagNumber DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestReallyLargeTagNumber ThisMessage {

@@ -10531,6 +10595,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestReallyLargeTagNumber ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestReallyLargeTagNumber MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -10546,22 +10614,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestReallyLargeTagNumber();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestReallyLargeTagNumber cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestReallyLargeTagNumber result;

+      private bool resultIsReadOnly;

+      private TestReallyLargeTagNumber result;

       

       private TestReallyLargeTagNumber PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestReallyLargeTagNumber original = result;

           result = new TestReallyLargeTagNumber();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -10576,13 +10644,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestReallyLargeTagNumber();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -10598,11 +10666,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestReallyLargeTagNumber BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -10734,7 +10802,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestRecursiveMessage : pb::GeneratedMessage<TestRecursiveMessage, TestRecursiveMessage.Builder> {

     private TestRecursiveMessage() { }

-    private static readonly TestRecursiveMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly TestRecursiveMessage defaultInstance = new TestRecursiveMessage().MakeReadOnly();

     private static readonly string[] _testRecursiveMessageFieldNames = new string[] { "a", "i" };

     private static readonly uint[] _testRecursiveMessageFieldTags = new uint[] { 10, 16 };

     public static TestRecursiveMessage DefaultInstance {

@@ -10742,7 +10810,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestRecursiveMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestRecursiveMessage ThisMessage {

@@ -10759,12 +10827,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int AFieldNumber = 1;

     private bool hasA;

-    private global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage a_ = global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage a_;

     public bool HasA {

       get { return hasA; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage A {

-      get { return a_; }

+      get { return a_ ?? global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.DefaultInstance; }

     }

     

     public const int IFieldNumber = 2;

@@ -10844,6 +10912,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestRecursiveMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestRecursiveMessage MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -10859,22 +10931,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestRecursiveMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestRecursiveMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestRecursiveMessage result;

+      private bool resultIsReadOnly;

+      private TestRecursiveMessage result;

       

       private TestRecursiveMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestRecursiveMessage original = result;

           result = new TestRecursiveMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -10889,13 +10961,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestRecursiveMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -10911,11 +10983,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRecursiveMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -11038,7 +11110,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearA() {

         PrepareBuilder();

         result.hasA = false;

-        result.a_ = global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.DefaultInstance;

+        result.a_ = null;

         return this;

       }

       

@@ -11072,7 +11144,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestMutualRecursionA : pb::GeneratedMessage<TestMutualRecursionA, TestMutualRecursionA.Builder> {

     private TestMutualRecursionA() { }

-    private static readonly TestMutualRecursionA defaultInstance = new Builder().BuildPartial();

+    private static readonly TestMutualRecursionA defaultInstance = new TestMutualRecursionA().MakeReadOnly();

     private static readonly string[] _testMutualRecursionAFieldNames = new string[] { "bb" };

     private static readonly uint[] _testMutualRecursionAFieldTags = new uint[] { 10 };

     public static TestMutualRecursionA DefaultInstance {

@@ -11080,7 +11152,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestMutualRecursionA DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestMutualRecursionA ThisMessage {

@@ -11097,12 +11169,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int BbFieldNumber = 1;

     private bool hasBb;

-    private global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB bb_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB bb_;

     public bool HasBb {

       get { return hasBb; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB Bb {

-      get { return bb_; }

+      get { return bb_ ?? global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -11166,6 +11238,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMutualRecursionA ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestMutualRecursionA MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -11181,22 +11257,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestMutualRecursionA();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestMutualRecursionA cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestMutualRecursionA result;

+      private bool resultIsReadOnly;

+      private TestMutualRecursionA result;

       

       private TestMutualRecursionA PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestMutualRecursionA original = result;

           result = new TestMutualRecursionA();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -11211,13 +11287,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestMutualRecursionA();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -11233,11 +11309,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMutualRecursionA BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -11353,7 +11429,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearBb() {

         PrepareBuilder();

         result.hasBb = false;

-        result.bb_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.DefaultInstance;

+        result.bb_ = null;

         return this;

       }

     }

@@ -11367,7 +11443,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestMutualRecursionB : pb::GeneratedMessage<TestMutualRecursionB, TestMutualRecursionB.Builder> {

     private TestMutualRecursionB() { }

-    private static readonly TestMutualRecursionB defaultInstance = new Builder().BuildPartial();

+    private static readonly TestMutualRecursionB defaultInstance = new TestMutualRecursionB().MakeReadOnly();

     private static readonly string[] _testMutualRecursionBFieldNames = new string[] { "a", "optional_int32" };

     private static readonly uint[] _testMutualRecursionBFieldTags = new uint[] { 10, 16 };

     public static TestMutualRecursionB DefaultInstance {

@@ -11375,7 +11451,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestMutualRecursionB DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestMutualRecursionB ThisMessage {

@@ -11392,12 +11468,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int AFieldNumber = 1;

     private bool hasA;

-    private global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA a_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA a_;

     public bool HasA {

       get { return hasA; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA A {

-      get { return a_; }

+      get { return a_ ?? global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.DefaultInstance; }

     }

     

     public const int OptionalInt32FieldNumber = 2;

@@ -11477,6 +11553,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMutualRecursionB ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestMutualRecursionB MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -11492,22 +11572,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestMutualRecursionB();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestMutualRecursionB cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestMutualRecursionB result;

+      private bool resultIsReadOnly;

+      private TestMutualRecursionB result;

       

       private TestMutualRecursionB PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestMutualRecursionB original = result;

           result = new TestMutualRecursionB();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -11522,13 +11602,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestMutualRecursionB();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -11544,11 +11624,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMutualRecursionB BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -11671,7 +11751,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearA() {

         PrepareBuilder();

         result.hasA = false;

-        result.a_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.DefaultInstance;

+        result.a_ = null;

         return this;

       }

       

@@ -11705,7 +11785,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestDupFieldNumber : pb::GeneratedMessage<TestDupFieldNumber, TestDupFieldNumber.Builder> {

     private TestDupFieldNumber() { }

-    private static readonly TestDupFieldNumber defaultInstance = new Builder().BuildPartial();

+    private static readonly TestDupFieldNumber defaultInstance = new TestDupFieldNumber().MakeReadOnly();

     private static readonly string[] _testDupFieldNumberFieldNames = new string[] { "a", "bar", "foo" };

     private static readonly uint[] _testDupFieldNumberFieldTags = new uint[] { 8, 27, 19 };

     public static TestDupFieldNumber DefaultInstance {

@@ -11713,7 +11793,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestDupFieldNumber DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestDupFieldNumber ThisMessage {

@@ -11738,7 +11818,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class Foo : pb::GeneratedMessage<Foo, Foo.Builder> {

         private Foo() { }

-        private static readonly Foo defaultInstance = new Builder().BuildPartial();

+        private static readonly Foo defaultInstance = new Foo().MakeReadOnly();

         private static readonly string[] _fooFieldNames = new string[] { "a" };

         private static readonly uint[] _fooFieldTags = new uint[] { 8 };

         public static Foo DefaultInstance {

@@ -11746,7 +11826,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override Foo DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override Foo ThisMessage {

@@ -11832,6 +11912,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Foo ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private Foo MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -11847,22 +11931,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new Foo();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(Foo cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          Foo result;

+          private bool resultIsReadOnly;

+          private Foo result;

           

           private Foo PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               Foo original = result;

               result = new Foo();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -11877,13 +11961,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new Foo();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -11899,11 +11983,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Foo BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -12008,7 +12092,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class Bar : pb::GeneratedMessage<Bar, Bar.Builder> {

         private Bar() { }

-        private static readonly Bar defaultInstance = new Builder().BuildPartial();

+        private static readonly Bar defaultInstance = new Bar().MakeReadOnly();

         private static readonly string[] _barFieldNames = new string[] { "a" };

         private static readonly uint[] _barFieldTags = new uint[] { 8 };

         public static Bar DefaultInstance {

@@ -12016,7 +12100,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override Bar DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override Bar ThisMessage {

@@ -12102,6 +12186,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Bar ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private Bar MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -12117,22 +12205,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new Bar();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(Bar cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          Bar result;

+          private bool resultIsReadOnly;

+          private Bar result;

           

           private Bar PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               Bar original = result;

               result = new Bar();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -12147,13 +12235,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new Bar();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -12169,11 +12257,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Bar BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -12288,22 +12376,22 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int FooFieldNumber = 2;

     private bool hasFoo;

-    private global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo foo_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo foo_;

     public bool HasFoo {

       get { return hasFoo; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo Foo {

-      get { return foo_; }

+      get { return foo_ ?? global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.DefaultInstance; }

     }

     

     public const int BarFieldNumber = 3;

     private bool hasBar;

-    private global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar bar_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar bar_;

     public bool HasBar {

       get { return hasBar; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar Bar {

-      get { return bar_; }

+      get { return bar_ ?? global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -12379,6 +12467,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestDupFieldNumber ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestDupFieldNumber MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -12394,22 +12486,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestDupFieldNumber();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestDupFieldNumber cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestDupFieldNumber result;

+      private bool resultIsReadOnly;

+      private TestDupFieldNumber result;

       

       private TestDupFieldNumber PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestDupFieldNumber original = result;

           result = new TestDupFieldNumber();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -12424,13 +12516,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestDupFieldNumber();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -12446,11 +12538,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestDupFieldNumber BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -12605,7 +12697,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearFoo() {

         PrepareBuilder();

         result.hasFoo = false;

-        result.foo_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.DefaultInstance;

+        result.foo_ = null;

         return this;

       }

       

@@ -12645,7 +12737,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearBar() {

         PrepareBuilder();

         result.hasBar = false;

-        result.bar_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.DefaultInstance;

+        result.bar_ = null;

         return this;

       }

     }

@@ -12659,7 +12751,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestNestedMessageHasBits : pb::GeneratedMessage<TestNestedMessageHasBits, TestNestedMessageHasBits.Builder> {

     private TestNestedMessageHasBits() { }

-    private static readonly TestNestedMessageHasBits defaultInstance = new Builder().BuildPartial();

+    private static readonly TestNestedMessageHasBits defaultInstance = new TestNestedMessageHasBits().MakeReadOnly();

     private static readonly string[] _testNestedMessageHasBitsFieldNames = new string[] { "optional_nested_message" };

     private static readonly uint[] _testNestedMessageHasBitsFieldTags = new uint[] { 10 };

     public static TestNestedMessageHasBits DefaultInstance {

@@ -12667,7 +12759,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestNestedMessageHasBits DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestNestedMessageHasBits ThisMessage {

@@ -12692,7 +12784,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class NestedMessage : pb::GeneratedMessage<NestedMessage, NestedMessage.Builder> {

         private NestedMessage() { }

-        private static readonly NestedMessage defaultInstance = new Builder().BuildPartial();

+        private static readonly NestedMessage defaultInstance = new NestedMessage().MakeReadOnly();

         private static readonly string[] _nestedMessageFieldNames = new string[] { "nestedmessage_repeated_foreignmessage", "nestedmessage_repeated_int32" };

         private static readonly uint[] _nestedMessageFieldTags = new uint[] { 18, 8 };

         public static NestedMessage DefaultInstance {

@@ -12700,7 +12792,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override NestedMessage DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override NestedMessage ThisMessage {

@@ -12811,6 +12903,12 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static NestedMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private NestedMessage MakeReadOnly() {

+          nestedmessageRepeatedInt32_.MakeReadOnly();

+          nestedmessageRepeatedForeignmessage_.MakeReadOnly();

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -12826,22 +12924,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new NestedMessage();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(NestedMessage cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          NestedMessage result;

+          private bool resultIsReadOnly;

+          private NestedMessage result;

           

           private NestedMessage PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               NestedMessage original = result;

               result = new NestedMessage();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -12856,13 +12954,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new NestedMessage();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -12878,13 +12976,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override NestedMessage BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            result.nestedmessageRepeatedInt32_.MakeReadOnly();

-            result.nestedmessageRepeatedForeignmessage_.MakeReadOnly();

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -13051,12 +13147,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int OptionalNestedMessageFieldNumber = 1;

     private bool hasOptionalNestedMessage;

-    private global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage optionalNestedMessage_;

     public bool HasOptionalNestedMessage {

       get { return hasOptionalNestedMessage; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage OptionalNestedMessage {

-      get { return optionalNestedMessage_; }

+      get { return optionalNestedMessage_ ?? global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -13120,6 +13216,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestNestedMessageHasBits ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestNestedMessageHasBits MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -13135,22 +13235,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestNestedMessageHasBits();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestNestedMessageHasBits cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestNestedMessageHasBits result;

+      private bool resultIsReadOnly;

+      private TestNestedMessageHasBits result;

       

       private TestNestedMessageHasBits PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestNestedMessageHasBits original = result;

           result = new TestNestedMessageHasBits();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -13165,13 +13265,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestNestedMessageHasBits();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -13187,11 +13287,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestNestedMessageHasBits BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -13307,7 +13407,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalNestedMessage() {

         PrepareBuilder();

         result.hasOptionalNestedMessage = false;

-        result.optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.DefaultInstance;

+        result.optionalNestedMessage_ = null;

         return this;

       }

     }

@@ -13321,7 +13421,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestCamelCaseFieldNames : pb::GeneratedMessage<TestCamelCaseFieldNames, TestCamelCaseFieldNames.Builder> {

     private TestCamelCaseFieldNames() { }

-    private static readonly TestCamelCaseFieldNames defaultInstance = new Builder().BuildPartial();

+    private static readonly TestCamelCaseFieldNames defaultInstance = new TestCamelCaseFieldNames().MakeReadOnly();

     private static readonly string[] _testCamelCaseFieldNamesFieldNames = new string[] { "CordField", "EnumField", "MessageField", "PrimitiveField", "RepeatedCordField", "RepeatedEnumField", "RepeatedMessageField", "RepeatedPrimitiveField", "RepeatedStringField", "RepeatedStringPieceField", "StringField", "StringPieceField" };

     private static readonly uint[] _testCamelCaseFieldNamesFieldTags = new uint[] { 50, 24, 34, 8, 98, 72, 82, 56, 66, 90, 18, 42 };

     public static TestCamelCaseFieldNames DefaultInstance {

@@ -13329,7 +13429,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestCamelCaseFieldNames DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestCamelCaseFieldNames ThisMessage {

@@ -13376,12 +13476,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int MessageFieldFieldNumber = 4;

     private bool hasMessageField;

-    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage messageField_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage messageField_;

     public bool HasMessageField {

       get { return hasMessageField; }

     }

     public global::Google.ProtocolBuffers.TestProtos.ForeignMessage MessageField {

-      get { return messageField_; }

+      get { return messageField_ ?? global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance; }

     }

     

     public const int StringPieceFieldFieldNumber = 5;

@@ -13630,6 +13730,16 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestCamelCaseFieldNames ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestCamelCaseFieldNames MakeReadOnly() {

+      repeatedPrimitiveField_.MakeReadOnly();

+      repeatedStringField_.MakeReadOnly();

+      repeatedEnumField_.MakeReadOnly();

+      repeatedMessageField_.MakeReadOnly();

+      repeatedStringPieceField_.MakeReadOnly();

+      repeatedCordField_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -13645,22 +13755,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestCamelCaseFieldNames();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestCamelCaseFieldNames cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestCamelCaseFieldNames result;

+      private bool resultIsReadOnly;

+      private TestCamelCaseFieldNames result;

       

       private TestCamelCaseFieldNames PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestCamelCaseFieldNames original = result;

           result = new TestCamelCaseFieldNames();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -13675,13 +13785,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestCamelCaseFieldNames();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -13697,17 +13807,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestCamelCaseFieldNames BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.repeatedPrimitiveField_.MakeReadOnly();

-        result.repeatedStringField_.MakeReadOnly();

-        result.repeatedEnumField_.MakeReadOnly();

-        result.repeatedMessageField_.MakeReadOnly();

-        result.repeatedStringPieceField_.MakeReadOnly();

-        result.repeatedCordField_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -13980,7 +14084,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearMessageField() {

         PrepareBuilder();

         result.hasMessageField = false;

-        result.messageField_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

+        result.messageField_ = null;

         return this;

       }

       

@@ -14236,7 +14340,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestFieldOrderings : pb::ExtendableMessage<TestFieldOrderings, TestFieldOrderings.Builder> {

     private TestFieldOrderings() { }

-    private static readonly TestFieldOrderings defaultInstance = new Builder().BuildPartial();

+    private static readonly TestFieldOrderings defaultInstance = new TestFieldOrderings().MakeReadOnly();

     private static readonly string[] _testFieldOrderingsFieldNames = new string[] { "my_float", "my_int", "my_string" };

     private static readonly uint[] _testFieldOrderingsFieldTags = new uint[] { 813, 8, 90 };

     public static TestFieldOrderings DefaultInstance {

@@ -14244,7 +14348,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestFieldOrderings DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestFieldOrderings ThisMessage {

@@ -14367,6 +14471,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestFieldOrderings ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestFieldOrderings MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -14382,22 +14490,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestFieldOrderings();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestFieldOrderings cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestFieldOrderings result;

+      private bool resultIsReadOnly;

+      private TestFieldOrderings result;

       

       private TestFieldOrderings PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestFieldOrderings original = result;

           result = new TestFieldOrderings();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -14412,13 +14520,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestFieldOrderings();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -14434,11 +14542,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestFieldOrderings BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -14599,7 +14707,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestExtremeDefaultValues : pb::GeneratedMessage<TestExtremeDefaultValues, TestExtremeDefaultValues.Builder> {

     private TestExtremeDefaultValues() { }

-    private static readonly TestExtremeDefaultValues defaultInstance = new Builder().BuildPartial();

+    private static readonly TestExtremeDefaultValues defaultInstance = new TestExtremeDefaultValues().MakeReadOnly();

     private static readonly string[] _testExtremeDefaultValuesFieldNames = new string[] { "escaped_bytes", "inf_double", "inf_float", "large_float", "large_uint32", "large_uint64", "nan_double", "nan_float", "neg_inf_double", "neg_inf_float", "negative_float", "negative_one_float", "one_float", "small_float", "small_int32", "small_int64", "small_negative_float", "utf8_string", "zero_float" };

     private static readonly uint[] _testExtremeDefaultValuesFieldTags = new uint[] { 10, 113, 141, 101, 16, 24, 129, 157, 121, 149, 93, 85, 69, 77, 32, 40, 109, 50, 61 };

     public static TestExtremeDefaultValues DefaultInstance {

@@ -14607,7 +14715,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestExtremeDefaultValues DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestExtremeDefaultValues ThisMessage {

@@ -14983,6 +15091,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestExtremeDefaultValues ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestExtremeDefaultValues MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -14998,22 +15110,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestExtremeDefaultValues();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestExtremeDefaultValues cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestExtremeDefaultValues result;

+      private bool resultIsReadOnly;

+      private TestExtremeDefaultValues result;

       

       private TestExtremeDefaultValues PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestExtremeDefaultValues original = result;

           result = new TestExtremeDefaultValues();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -15028,13 +15140,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestExtremeDefaultValues();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -15050,11 +15162,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestExtremeDefaultValues BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -15651,7 +15763,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class OneString : pb::GeneratedMessage<OneString, OneString.Builder> {

     private OneString() { }

-    private static readonly OneString defaultInstance = new Builder().BuildPartial();

+    private static readonly OneString defaultInstance = new OneString().MakeReadOnly();

     private static readonly string[] _oneStringFieldNames = new string[] { "data" };

     private static readonly uint[] _oneStringFieldTags = new uint[] { 10 };

     public static OneString DefaultInstance {

@@ -15659,7 +15771,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override OneString DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override OneString ThisMessage {

@@ -15745,6 +15857,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static OneString ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private OneString MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -15760,22 +15876,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new OneString();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(OneString cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      OneString result;

+      private bool resultIsReadOnly;

+      private OneString result;

       

       private OneString PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           OneString original = result;

           result = new OneString();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -15790,13 +15906,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new OneString();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -15812,11 +15928,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OneString BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -15922,7 +16038,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class OneBytes : pb::GeneratedMessage<OneBytes, OneBytes.Builder> {

     private OneBytes() { }

-    private static readonly OneBytes defaultInstance = new Builder().BuildPartial();

+    private static readonly OneBytes defaultInstance = new OneBytes().MakeReadOnly();

     private static readonly string[] _oneBytesFieldNames = new string[] { "data" };

     private static readonly uint[] _oneBytesFieldTags = new uint[] { 10 };

     public static OneBytes DefaultInstance {

@@ -15930,7 +16046,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override OneBytes DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override OneBytes ThisMessage {

@@ -16016,6 +16132,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static OneBytes ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private OneBytes MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -16031,22 +16151,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new OneBytes();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(OneBytes cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      OneBytes result;

+      private bool resultIsReadOnly;

+      private OneBytes result;

       

       private OneBytes PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           OneBytes original = result;

           result = new OneBytes();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -16061,13 +16181,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new OneBytes();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -16083,11 +16203,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OneBytes BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -16193,7 +16313,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestPackedTypes : pb::GeneratedMessage<TestPackedTypes, TestPackedTypes.Builder> {

     private TestPackedTypes() { }

-    private static readonly TestPackedTypes defaultInstance = new Builder().BuildPartial();

+    private static readonly TestPackedTypes defaultInstance = new TestPackedTypes().MakeReadOnly();

     private static readonly string[] _testPackedTypesFieldNames = new string[] { "packed_bool", "packed_double", "packed_enum", "packed_fixed32", "packed_fixed64", "packed_float", "packed_int32", "packed_int64", "packed_sfixed32", "packed_sfixed64", "packed_sint32", "packed_sint64", "packed_uint32", "packed_uint64" };

     private static readonly uint[] _testPackedTypesFieldTags = new uint[] { 818, 810, 826, 770, 778, 802, 722, 730, 786, 794, 754, 762, 738, 746 };

     public static TestPackedTypes DefaultInstance {

@@ -16201,7 +16321,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestPackedTypes DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestPackedTypes ThisMessage {

@@ -16644,6 +16764,24 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestPackedTypes ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestPackedTypes MakeReadOnly() {

+      packedInt32_.MakeReadOnly();

+      packedInt64_.MakeReadOnly();

+      packedUint32_.MakeReadOnly();

+      packedUint64_.MakeReadOnly();

+      packedSint32_.MakeReadOnly();

+      packedSint64_.MakeReadOnly();

+      packedFixed32_.MakeReadOnly();

+      packedFixed64_.MakeReadOnly();

+      packedSfixed32_.MakeReadOnly();

+      packedSfixed64_.MakeReadOnly();

+      packedFloat_.MakeReadOnly();

+      packedDouble_.MakeReadOnly();

+      packedBool_.MakeReadOnly();

+      packedEnum_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -16659,22 +16797,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestPackedTypes();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestPackedTypes cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestPackedTypes result;

+      private bool resultIsReadOnly;

+      private TestPackedTypes result;

       

       private TestPackedTypes PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestPackedTypes original = result;

           result = new TestPackedTypes();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -16689,13 +16827,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestPackedTypes();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -16711,25 +16849,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestPackedTypes BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.packedInt32_.MakeReadOnly();

-        result.packedInt64_.MakeReadOnly();

-        result.packedUint32_.MakeReadOnly();

-        result.packedUint64_.MakeReadOnly();

-        result.packedSint32_.MakeReadOnly();

-        result.packedSint64_.MakeReadOnly();

-        result.packedFixed32_.MakeReadOnly();

-        result.packedFixed64_.MakeReadOnly();

-        result.packedSfixed32_.MakeReadOnly();

-        result.packedSfixed64_.MakeReadOnly();

-        result.packedFloat_.MakeReadOnly();

-        result.packedDouble_.MakeReadOnly();

-        result.packedBool_.MakeReadOnly();

-        result.packedEnum_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -17368,7 +17492,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestUnpackedTypes : pb::GeneratedMessage<TestUnpackedTypes, TestUnpackedTypes.Builder> {

     private TestUnpackedTypes() { }

-    private static readonly TestUnpackedTypes defaultInstance = new Builder().BuildPartial();

+    private static readonly TestUnpackedTypes defaultInstance = new TestUnpackedTypes().MakeReadOnly();

     private static readonly string[] _testUnpackedTypesFieldNames = new string[] { "unpacked_bool", "unpacked_double", "unpacked_enum", "unpacked_fixed32", "unpacked_fixed64", "unpacked_float", "unpacked_int32", "unpacked_int64", "unpacked_sfixed32", "unpacked_sfixed64", "unpacked_sint32", "unpacked_sint64", "unpacked_uint32", "unpacked_uint64" };

     private static readonly uint[] _testUnpackedTypesFieldTags = new uint[] { 816, 809, 824, 773, 777, 805, 720, 728, 789, 793, 752, 760, 736, 744 };

     public static TestUnpackedTypes DefaultInstance {

@@ -17376,7 +17500,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestUnpackedTypes DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestUnpackedTypes ThisMessage {

@@ -17764,6 +17888,24 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestUnpackedTypes ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestUnpackedTypes MakeReadOnly() {

+      unpackedInt32_.MakeReadOnly();

+      unpackedInt64_.MakeReadOnly();

+      unpackedUint32_.MakeReadOnly();

+      unpackedUint64_.MakeReadOnly();

+      unpackedSint32_.MakeReadOnly();

+      unpackedSint64_.MakeReadOnly();

+      unpackedFixed32_.MakeReadOnly();

+      unpackedFixed64_.MakeReadOnly();

+      unpackedSfixed32_.MakeReadOnly();

+      unpackedSfixed64_.MakeReadOnly();

+      unpackedFloat_.MakeReadOnly();

+      unpackedDouble_.MakeReadOnly();

+      unpackedBool_.MakeReadOnly();

+      unpackedEnum_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -17779,22 +17921,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestUnpackedTypes();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestUnpackedTypes cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestUnpackedTypes result;

+      private bool resultIsReadOnly;

+      private TestUnpackedTypes result;

       

       private TestUnpackedTypes PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestUnpackedTypes original = result;

           result = new TestUnpackedTypes();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -17809,13 +17951,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestUnpackedTypes();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -17831,25 +17973,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestUnpackedTypes BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.unpackedInt32_.MakeReadOnly();

-        result.unpackedInt64_.MakeReadOnly();

-        result.unpackedUint32_.MakeReadOnly();

-        result.unpackedUint64_.MakeReadOnly();

-        result.unpackedSint32_.MakeReadOnly();

-        result.unpackedSint64_.MakeReadOnly();

-        result.unpackedFixed32_.MakeReadOnly();

-        result.unpackedFixed64_.MakeReadOnly();

-        result.unpackedSfixed32_.MakeReadOnly();

-        result.unpackedSfixed64_.MakeReadOnly();

-        result.unpackedFloat_.MakeReadOnly();

-        result.unpackedDouble_.MakeReadOnly();

-        result.unpackedBool_.MakeReadOnly();

-        result.unpackedEnum_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -18488,7 +18616,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestPackedExtensions : pb::ExtendableMessage<TestPackedExtensions, TestPackedExtensions.Builder> {

     private TestPackedExtensions() { }

-    private static readonly TestPackedExtensions defaultInstance = new Builder().BuildPartial();

+    private static readonly TestPackedExtensions defaultInstance = new TestPackedExtensions().MakeReadOnly();

     private static readonly string[] _testPackedExtensionsFieldNames = new string[] {  };

     private static readonly uint[] _testPackedExtensionsFieldTags = new uint[] {  };

     public static TestPackedExtensions DefaultInstance {

@@ -18496,7 +18624,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestPackedExtensions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestPackedExtensions ThisMessage {

@@ -18570,6 +18698,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestPackedExtensions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestPackedExtensions MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -18585,22 +18717,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestPackedExtensions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestPackedExtensions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestPackedExtensions result;

+      private bool resultIsReadOnly;

+      private TestPackedExtensions result;

       

       private TestPackedExtensions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestPackedExtensions original = result;

           result = new TestPackedExtensions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -18615,13 +18747,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestPackedExtensions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -18637,11 +18769,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestPackedExtensions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -18720,7 +18852,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestUnpackedExtensions : pb::ExtendableMessage<TestUnpackedExtensions, TestUnpackedExtensions.Builder> {

     private TestUnpackedExtensions() { }

-    private static readonly TestUnpackedExtensions defaultInstance = new Builder().BuildPartial();

+    private static readonly TestUnpackedExtensions defaultInstance = new TestUnpackedExtensions().MakeReadOnly();

     private static readonly string[] _testUnpackedExtensionsFieldNames = new string[] {  };

     private static readonly uint[] _testUnpackedExtensionsFieldTags = new uint[] {  };

     public static TestUnpackedExtensions DefaultInstance {

@@ -18728,7 +18860,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestUnpackedExtensions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestUnpackedExtensions ThisMessage {

@@ -18802,6 +18934,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestUnpackedExtensions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestUnpackedExtensions MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -18817,22 +18953,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestUnpackedExtensions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestUnpackedExtensions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestUnpackedExtensions result;

+      private bool resultIsReadOnly;

+      private TestUnpackedExtensions result;

       

       private TestUnpackedExtensions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestUnpackedExtensions original = result;

           result = new TestUnpackedExtensions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -18847,13 +18983,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestUnpackedExtensions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -18869,11 +19005,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestUnpackedExtensions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -18952,7 +19088,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestDynamicExtensions : pb::GeneratedMessage<TestDynamicExtensions, TestDynamicExtensions.Builder> {

     private TestDynamicExtensions() { }

-    private static readonly TestDynamicExtensions defaultInstance = new Builder().BuildPartial();

+    private static readonly TestDynamicExtensions defaultInstance = new TestDynamicExtensions().MakeReadOnly();

     private static readonly string[] _testDynamicExtensionsFieldNames = new string[] { "dynamic_enum_extension", "dynamic_message_extension", "enum_extension", "message_extension", "packed_extension", "repeated_extension", "scalar_extension" };

     private static readonly uint[] _testDynamicExtensionsFieldTags = new uint[] { 16016, 16034, 16008, 16026, 16050, 16042, 16005 };

     public static TestDynamicExtensions DefaultInstance {

@@ -18960,7 +19096,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestDynamicExtensions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestDynamicExtensions ThisMessage {

@@ -18993,7 +19129,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class DynamicMessageType : pb::GeneratedMessage<DynamicMessageType, DynamicMessageType.Builder> {

         private DynamicMessageType() { }

-        private static readonly DynamicMessageType defaultInstance = new Builder().BuildPartial();

+        private static readonly DynamicMessageType defaultInstance = new DynamicMessageType().MakeReadOnly();

         private static readonly string[] _dynamicMessageTypeFieldNames = new string[] { "dynamic_field" };

         private static readonly uint[] _dynamicMessageTypeFieldTags = new uint[] { 16800 };

         public static DynamicMessageType DefaultInstance {

@@ -19001,7 +19137,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override DynamicMessageType DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override DynamicMessageType ThisMessage {

@@ -19087,6 +19223,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static DynamicMessageType ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private DynamicMessageType MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -19102,22 +19242,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new DynamicMessageType();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(DynamicMessageType cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          DynamicMessageType result;

+          private bool resultIsReadOnly;

+          private DynamicMessageType result;

           

           private DynamicMessageType PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               DynamicMessageType original = result;

               result = new DynamicMessageType();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -19132,13 +19272,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new DynamicMessageType();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -19154,11 +19294,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override DynamicMessageType BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -19294,22 +19434,22 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int MessageExtensionFieldNumber = 2003;

     private bool hasMessageExtension;

-    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage messageExtension_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage messageExtension_;

     public bool HasMessageExtension {

       get { return hasMessageExtension; }

     }

     public global::Google.ProtocolBuffers.TestProtos.ForeignMessage MessageExtension {

-      get { return messageExtension_; }

+      get { return messageExtension_ ?? global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance; }

     }

     

     public const int DynamicMessageExtensionFieldNumber = 2004;

     private bool hasDynamicMessageExtension;

-    private global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType dynamicMessageExtension_ = global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType dynamicMessageExtension_;

     public bool HasDynamicMessageExtension {

       get { return hasDynamicMessageExtension; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType DynamicMessageExtension {

-      get { return dynamicMessageExtension_; }

+      get { return dynamicMessageExtension_ ?? global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.DefaultInstance; }

     }

     

     public const int RepeatedExtensionFieldNumber = 2005;

@@ -19447,6 +19587,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestDynamicExtensions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestDynamicExtensions MakeReadOnly() {

+      repeatedExtension_.MakeReadOnly();

+      packedExtension_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -19462,22 +19608,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestDynamicExtensions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestDynamicExtensions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestDynamicExtensions result;

+      private bool resultIsReadOnly;

+      private TestDynamicExtensions result;

       

       private TestDynamicExtensions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestDynamicExtensions original = result;

           result = new TestDynamicExtensions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -19492,13 +19638,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestDynamicExtensions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -19514,13 +19660,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestDynamicExtensions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.repeatedExtension_.MakeReadOnly();

-        result.packedExtension_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -19762,7 +19906,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearMessageExtension() {

         PrepareBuilder();

         result.hasMessageExtension = false;

-        result.messageExtension_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

+        result.messageExtension_ = null;

         return this;

       }

       

@@ -19802,7 +19946,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearDynamicMessageExtension() {

         PrepareBuilder();

         result.hasDynamicMessageExtension = false;

-        result.dynamicMessageExtension_ = global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.DefaultInstance;

+        result.dynamicMessageExtension_ = null;

         return this;

       }

       

@@ -19878,7 +20022,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestRepeatedScalarDifferentTagSizes : pb::GeneratedMessage<TestRepeatedScalarDifferentTagSizes, TestRepeatedScalarDifferentTagSizes.Builder> {

     private TestRepeatedScalarDifferentTagSizes() { }

-    private static readonly TestRepeatedScalarDifferentTagSizes defaultInstance = new Builder().BuildPartial();

+    private static readonly TestRepeatedScalarDifferentTagSizes defaultInstance = new TestRepeatedScalarDifferentTagSizes().MakeReadOnly();

     private static readonly string[] _testRepeatedScalarDifferentTagSizesFieldNames = new string[] { "repeated_fixed32", "repeated_fixed64", "repeated_float", "repeated_int32", "repeated_int64", "repeated_uint64" };

     private static readonly uint[] _testRepeatedScalarDifferentTagSizesFieldTags = new uint[] { 101, 16369, 2097141, 104, 16376, 2097144 };

     public static TestRepeatedScalarDifferentTagSizes DefaultInstance {

@@ -19886,7 +20030,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestRepeatedScalarDifferentTagSizes DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestRepeatedScalarDifferentTagSizes ThisMessage {

@@ -20094,6 +20238,16 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestRepeatedScalarDifferentTagSizes ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestRepeatedScalarDifferentTagSizes MakeReadOnly() {

+      repeatedFixed32_.MakeReadOnly();

+      repeatedInt32_.MakeReadOnly();

+      repeatedFixed64_.MakeReadOnly();

+      repeatedInt64_.MakeReadOnly();

+      repeatedFloat_.MakeReadOnly();

+      repeatedUint64_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -20109,22 +20263,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestRepeatedScalarDifferentTagSizes();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestRepeatedScalarDifferentTagSizes cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestRepeatedScalarDifferentTagSizes result;

+      private bool resultIsReadOnly;

+      private TestRepeatedScalarDifferentTagSizes result;

       

       private TestRepeatedScalarDifferentTagSizes PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestRepeatedScalarDifferentTagSizes original = result;

           result = new TestRepeatedScalarDifferentTagSizes();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -20139,13 +20293,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestRepeatedScalarDifferentTagSizes();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -20161,17 +20315,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRepeatedScalarDifferentTagSizes BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.repeatedFixed32_.MakeReadOnly();

-        result.repeatedInt32_.MakeReadOnly();

-        result.repeatedFixed64_.MakeReadOnly();

-        result.repeatedInt64_.MakeReadOnly();

-        result.repeatedFloat_.MakeReadOnly();

-        result.repeatedUint64_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -20492,7 +20640,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class FooRequest : pb::GeneratedMessage<FooRequest, FooRequest.Builder> {

     private FooRequest() { }

-    private static readonly FooRequest defaultInstance = new Builder().BuildPartial();

+    private static readonly FooRequest defaultInstance = new FooRequest().MakeReadOnly();

     private static readonly string[] _fooRequestFieldNames = new string[] {  };

     private static readonly uint[] _fooRequestFieldTags = new uint[] {  };

     public static FooRequest DefaultInstance {

@@ -20500,7 +20648,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override FooRequest DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override FooRequest ThisMessage {

@@ -20570,6 +20718,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static FooRequest ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private FooRequest MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -20585,22 +20737,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new FooRequest();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(FooRequest cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      FooRequest result;

+      private bool resultIsReadOnly;

+      private FooRequest result;

       

       private FooRequest PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           FooRequest original = result;

           result = new FooRequest();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -20615,13 +20767,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new FooRequest();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -20637,11 +20789,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override FooRequest BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -20719,7 +20871,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class FooResponse : pb::GeneratedMessage<FooResponse, FooResponse.Builder> {

     private FooResponse() { }

-    private static readonly FooResponse defaultInstance = new Builder().BuildPartial();

+    private static readonly FooResponse defaultInstance = new FooResponse().MakeReadOnly();

     private static readonly string[] _fooResponseFieldNames = new string[] {  };

     private static readonly uint[] _fooResponseFieldTags = new uint[] {  };

     public static FooResponse DefaultInstance {

@@ -20727,7 +20879,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override FooResponse DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override FooResponse ThisMessage {

@@ -20797,6 +20949,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static FooResponse ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private FooResponse MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -20812,22 +20968,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new FooResponse();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(FooResponse cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      FooResponse result;

+      private bool resultIsReadOnly;

+      private FooResponse result;

       

       private FooResponse PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           FooResponse original = result;

           result = new FooResponse();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -20842,13 +20998,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new FooResponse();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -20864,11 +21020,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override FooResponse BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -20946,7 +21102,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class BarRequest : pb::GeneratedMessage<BarRequest, BarRequest.Builder> {

     private BarRequest() { }

-    private static readonly BarRequest defaultInstance = new Builder().BuildPartial();

+    private static readonly BarRequest defaultInstance = new BarRequest().MakeReadOnly();

     private static readonly string[] _barRequestFieldNames = new string[] {  };

     private static readonly uint[] _barRequestFieldTags = new uint[] {  };

     public static BarRequest DefaultInstance {

@@ -20954,7 +21110,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override BarRequest DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override BarRequest ThisMessage {

@@ -21024,6 +21180,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static BarRequest ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private BarRequest MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -21039,22 +21199,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new BarRequest();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(BarRequest cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      BarRequest result;

+      private bool resultIsReadOnly;

+      private BarRequest result;

       

       private BarRequest PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           BarRequest original = result;

           result = new BarRequest();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -21069,13 +21229,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new BarRequest();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -21091,11 +21251,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override BarRequest BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -21173,7 +21333,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class BarResponse : pb::GeneratedMessage<BarResponse, BarResponse.Builder> {

     private BarResponse() { }

-    private static readonly BarResponse defaultInstance = new Builder().BuildPartial();

+    private static readonly BarResponse defaultInstance = new BarResponse().MakeReadOnly();

     private static readonly string[] _barResponseFieldNames = new string[] {  };

     private static readonly uint[] _barResponseFieldTags = new uint[] {  };

     public static BarResponse DefaultInstance {

@@ -21181,7 +21341,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override BarResponse DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override BarResponse ThisMessage {

@@ -21251,6 +21411,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static BarResponse ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private BarResponse MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -21266,22 +21430,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new BarResponse();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(BarResponse cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      BarResponse result;

+      private bool resultIsReadOnly;

+      private BarResponse result;

       

       private BarResponse PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           BarResponse original = result;

           result = new BarResponse();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -21296,13 +21460,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new BarResponse();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -21318,11 +21482,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override BarResponse BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestRpcInterop.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestRpcInterop.cs
index cb805861..ce8f66d9 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestRpcInterop.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestRpcInterop.cs
@@ -84,7 +84,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class SearchRequest : pb::GeneratedMessage<SearchRequest, SearchRequest.Builder> {

     private SearchRequest() { }

-    private static readonly SearchRequest defaultInstance = new Builder().BuildPartial();

+    private static readonly SearchRequest defaultInstance = new SearchRequest().MakeReadOnly();

     private static readonly string[] _searchRequestFieldNames = new string[] { "Criteria" };

     private static readonly uint[] _searchRequestFieldTags = new uint[] { 10 };

     public static SearchRequest DefaultInstance {

@@ -92,7 +92,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override SearchRequest DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override SearchRequest ThisMessage {

@@ -185,6 +185,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SearchRequest ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private SearchRequest MakeReadOnly() {

+      criteria_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -200,22 +205,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new SearchRequest();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(SearchRequest cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      SearchRequest result;

+      private bool resultIsReadOnly;

+      private SearchRequest result;

       

       private SearchRequest PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           SearchRequest original = result;

           result = new SearchRequest();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -230,13 +235,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new SearchRequest();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -252,12 +257,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SearchRequest BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.criteria_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -374,7 +378,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class SearchResponse : pb::GeneratedMessage<SearchResponse, SearchResponse.Builder> {

     private SearchResponse() { }

-    private static readonly SearchResponse defaultInstance = new Builder().BuildPartial();

+    private static readonly SearchResponse defaultInstance = new SearchResponse().MakeReadOnly();

     private static readonly string[] _searchResponseFieldNames = new string[] { "results" };

     private static readonly uint[] _searchResponseFieldTags = new uint[] { 10 };

     public static SearchResponse DefaultInstance {

@@ -382,7 +386,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override SearchResponse DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override SearchResponse ThisMessage {

@@ -407,7 +411,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class ResultItem : pb::GeneratedMessage<ResultItem, ResultItem.Builder> {

         private ResultItem() { }

-        private static readonly ResultItem defaultInstance = new Builder().BuildPartial();

+        private static readonly ResultItem defaultInstance = new ResultItem().MakeReadOnly();

         private static readonly string[] _resultItemFieldNames = new string[] { "name", "url" };

         private static readonly uint[] _resultItemFieldTags = new uint[] { 18, 10 };

         public static ResultItem DefaultInstance {

@@ -415,7 +419,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override ResultItem DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override ResultItem ThisMessage {

@@ -518,6 +522,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static ResultItem ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private ResultItem MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -533,22 +541,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new ResultItem();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(ResultItem cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          ResultItem result;

+          private bool resultIsReadOnly;

+          private ResultItem result;

           

           private ResultItem PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               ResultItem original = result;

               result = new ResultItem();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -563,13 +571,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new ResultItem();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -585,11 +593,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override ResultItem BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -797,6 +805,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SearchResponse ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private SearchResponse MakeReadOnly() {

+      results_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -812,22 +825,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new SearchResponse();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(SearchResponse cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      SearchResponse result;

+      private bool resultIsReadOnly;

+      private SearchResponse result;

       

       private SearchResponse PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           SearchResponse original = result;

           result = new SearchResponse();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -842,13 +855,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new SearchResponse();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -864,12 +877,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override SearchResponse BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.results_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -998,7 +1010,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class RefineSearchRequest : pb::GeneratedMessage<RefineSearchRequest, RefineSearchRequest.Builder> {

     private RefineSearchRequest() { }

-    private static readonly RefineSearchRequest defaultInstance = new Builder().BuildPartial();

+    private static readonly RefineSearchRequest defaultInstance = new RefineSearchRequest().MakeReadOnly();

     private static readonly string[] _refineSearchRequestFieldNames = new string[] { "Criteria", "previous_results" };

     private static readonly uint[] _refineSearchRequestFieldTags = new uint[] { 10, 18 };

     public static RefineSearchRequest DefaultInstance {

@@ -1006,7 +1018,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override RefineSearchRequest DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override RefineSearchRequest ThisMessage {

@@ -1035,12 +1047,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int PreviousResultsFieldNumber = 2;

     private bool hasPreviousResults;

-    private global::Google.ProtocolBuffers.TestProtos.SearchResponse previousResults_ = global::Google.ProtocolBuffers.TestProtos.SearchResponse.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.SearchResponse previousResults_;

     public bool HasPreviousResults {

       get { return hasPreviousResults; }

     }

     public global::Google.ProtocolBuffers.TestProtos.SearchResponse PreviousResults {

-      get { return previousResults_; }

+      get { return previousResults_ ?? global::Google.ProtocolBuffers.TestProtos.SearchResponse.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -1117,6 +1129,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static RefineSearchRequest ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private RefineSearchRequest MakeReadOnly() {

+      criteria_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1132,22 +1149,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new RefineSearchRequest();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(RefineSearchRequest cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      RefineSearchRequest result;

+      private bool resultIsReadOnly;

+      private RefineSearchRequest result;

       

       private RefineSearchRequest PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           RefineSearchRequest original = result;

           result = new RefineSearchRequest();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1162,13 +1179,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new RefineSearchRequest();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1184,12 +1201,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override RefineSearchRequest BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.criteria_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -1344,7 +1360,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearPreviousResults() {

         PrepareBuilder();

         result.hasPreviousResults = false;

-        result.previousResults_ = global::Google.ProtocolBuffers.TestProtos.SearchResponse.DefaultInstance;

+        result.previousResults_ = null;

         return this;

       }

     }

diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestXmlSerializerTestProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestXmlSerializerTestProtoFile.cs
index 798f09ad..e63d8c3b 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestXmlSerializerTestProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestXmlSerializerTestProtoFile.cs
@@ -140,7 +140,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestXmlChild : pb::GeneratedMessage<TestXmlChild, TestXmlChild.Builder> {

     private TestXmlChild() { }

-    private static readonly TestXmlChild defaultInstance = new Builder().BuildPartial();

+    private static readonly TestXmlChild defaultInstance = new TestXmlChild().MakeReadOnly();

     private static readonly string[] _testXmlChildFieldNames = new string[] { "binary", "options" };

     private static readonly uint[] _testXmlChildFieldTags = new uint[] { 34, 24 };

     public static TestXmlChild DefaultInstance {

@@ -148,7 +148,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestXmlChild DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestXmlChild ThisMessage {

@@ -259,6 +259,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestXmlChild ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestXmlChild MakeReadOnly() {

+      options_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -275,22 +280,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestXmlChild();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestXmlChild cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestXmlChild result;

+      private bool resultIsReadOnly;

+      private TestXmlChild result;

       

       private TestXmlChild PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestXmlChild original = result;

           result = new TestXmlChild();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -305,13 +310,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestXmlChild();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -327,12 +332,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestXmlChild BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.options_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -486,7 +490,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestXmlNoFields : pb::GeneratedMessage<TestXmlNoFields, TestXmlNoFields.Builder> {

     private TestXmlNoFields() { }

-    private static readonly TestXmlNoFields defaultInstance = new Builder().BuildPartial();

+    private static readonly TestXmlNoFields defaultInstance = new TestXmlNoFields().MakeReadOnly();

     private static readonly string[] _testXmlNoFieldsFieldNames = new string[] {  };

     private static readonly uint[] _testXmlNoFieldsFieldTags = new uint[] {  };

     public static TestXmlNoFields DefaultInstance {

@@ -494,7 +498,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestXmlNoFields DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestXmlNoFields ThisMessage {

@@ -564,6 +568,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestXmlNoFields ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestXmlNoFields MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -580,22 +588,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestXmlNoFields();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestXmlNoFields cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestXmlNoFields result;

+      private bool resultIsReadOnly;

+      private TestXmlNoFields result;

       

       private TestXmlNoFields PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestXmlNoFields original = result;

           result = new TestXmlNoFields();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -610,13 +618,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestXmlNoFields();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -632,11 +640,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestXmlNoFields BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -715,7 +723,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestXmlRescursive : pb::GeneratedMessage<TestXmlRescursive, TestXmlRescursive.Builder> {

     private TestXmlRescursive() { }

-    private static readonly TestXmlRescursive defaultInstance = new Builder().BuildPartial();

+    private static readonly TestXmlRescursive defaultInstance = new TestXmlRescursive().MakeReadOnly();

     private static readonly string[] _testXmlRescursiveFieldNames = new string[] { "child" };

     private static readonly uint[] _testXmlRescursiveFieldTags = new uint[] { 10 };

     public static TestXmlRescursive DefaultInstance {

@@ -723,7 +731,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestXmlRescursive DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestXmlRescursive ThisMessage {

@@ -740,12 +748,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int ChildFieldNumber = 1;

     private bool hasChild;

-    private global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive child_ = global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive child_;

     public bool HasChild {

       get { return hasChild; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive Child {

-      get { return child_; }

+      get { return child_ ?? global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -809,6 +817,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestXmlRescursive ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestXmlRescursive MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -825,22 +837,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestXmlRescursive();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestXmlRescursive cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestXmlRescursive result;

+      private bool resultIsReadOnly;

+      private TestXmlRescursive result;

       

       private TestXmlRescursive PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestXmlRescursive original = result;

           result = new TestXmlRescursive();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -855,13 +867,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestXmlRescursive();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -877,11 +889,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestXmlRescursive BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -997,7 +1009,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearChild() {

         PrepareBuilder();

         result.hasChild = false;

-        result.child_ = global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive.DefaultInstance;

+        result.child_ = null;

         return this;

       }

     }

@@ -1012,7 +1024,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestXmlMessage : pb::ExtendableMessage<TestXmlMessage, TestXmlMessage.Builder> {

     private TestXmlMessage() { }

-    private static readonly TestXmlMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly TestXmlMessage defaultInstance = new TestXmlMessage().MakeReadOnly();

     private static readonly string[] _testXmlMessageFieldNames = new string[] { "child", "children", "number", "numbers", "text", "textlines", "valid" };

     private static readonly uint[] _testXmlMessageFieldTags = new uint[] { 10, 3211, 48, 16, 26, 5602, 40 };

     public static TestXmlMessage DefaultInstance {

@@ -1020,7 +1032,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestXmlMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestXmlMessage ThisMessage {

@@ -1046,7 +1058,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class Children : pb::GeneratedMessage<Children, Children.Builder> {

         private Children() { }

-        private static readonly Children defaultInstance = new Builder().BuildPartial();

+        private static readonly Children defaultInstance = new Children().MakeReadOnly();

         private static readonly string[] _childrenFieldNames = new string[] { "binary", "options" };

         private static readonly uint[] _childrenFieldTags = new uint[] { 34, 24 };

         public static Children DefaultInstance {

@@ -1054,7 +1066,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override Children DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override Children ThisMessage {

@@ -1165,6 +1177,11 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Children ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private Children MakeReadOnly() {

+          options_.MakeReadOnly();

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1181,22 +1198,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new Children();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(Children cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          Children result;

+          private bool resultIsReadOnly;

+          private Children result;

           

           private Children PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               Children original = result;

               result = new Children();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -1211,13 +1228,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new Children();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -1233,12 +1250,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Children BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            result.options_.MakeReadOnly();

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -1445,12 +1461,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int ChildFieldNumber = 1;

     private bool hasChild;

-    private global::Google.ProtocolBuffers.TestProtos.TestXmlChild child_ = global::Google.ProtocolBuffers.TestProtos.TestXmlChild.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestXmlChild child_;

     public bool HasChild {

       get { return hasChild; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestXmlChild Child {

-      get { return child_; }

+      get { return child_ ?? global::Google.ProtocolBuffers.TestProtos.TestXmlChild.DefaultInstance; }

     }

     

     public const int ChildrenFieldNumber = 401;

@@ -1576,6 +1592,13 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestXmlMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestXmlMessage MakeReadOnly() {

+      numbers_.MakeReadOnly();

+      textlines_.MakeReadOnly();

+      children_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1592,22 +1615,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestXmlMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestXmlMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestXmlMessage result;

+      private bool resultIsReadOnly;

+      private TestXmlMessage result;

       

       private TestXmlMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestXmlMessage original = result;

           result = new TestXmlMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1622,13 +1645,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestXmlMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1644,14 +1667,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestXmlMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.numbers_.MakeReadOnly();

-        result.textlines_.MakeReadOnly();

-        result.children_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -1934,7 +1954,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearChild() {

         PrepareBuilder();

         result.hasChild = false;

-        result.child_ = global::Google.ProtocolBuffers.TestProtos.TestXmlChild.DefaultInstance;

+        result.child_ = null;

         return this;

       }

       

@@ -1993,7 +2013,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestXmlExtension : pb::GeneratedMessage<TestXmlExtension, TestXmlExtension.Builder> {

     private TestXmlExtension() { }

-    private static readonly TestXmlExtension defaultInstance = new Builder().BuildPartial();

+    private static readonly TestXmlExtension defaultInstance = new TestXmlExtension().MakeReadOnly();

     private static readonly string[] _testXmlExtensionFieldNames = new string[] { "number" };

     private static readonly uint[] _testXmlExtensionFieldTags = new uint[] { 8 };

     public static TestXmlExtension DefaultInstance {

@@ -2001,7 +2021,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestXmlExtension DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestXmlExtension ThisMessage {

@@ -2088,6 +2108,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestXmlExtension ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestXmlExtension MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -2104,22 +2128,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestXmlExtension();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestXmlExtension cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestXmlExtension result;

+      private bool resultIsReadOnly;

+      private TestXmlExtension result;

       

       private TestXmlExtension PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestXmlExtension original = result;

           result = new TestXmlExtension();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -2134,13 +2158,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestXmlExtension();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -2156,11 +2180,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestXmlExtension BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtocolBuffers/DescriptorProtos/CSharpOptions.cs b/src/ProtocolBuffers/DescriptorProtos/CSharpOptions.cs
index 2bebb011..4eb9c05d 100644
--- a/src/ProtocolBuffers/DescriptorProtos/CSharpOptions.cs
+++ b/src/ProtocolBuffers/DescriptorProtos/CSharpOptions.cs
@@ -127,7 +127,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class CSharpFileOptions : pb::GeneratedMessage<CSharpFileOptions, CSharpFileOptions.Builder> {

     private CSharpFileOptions() { }

-    private static readonly CSharpFileOptions defaultInstance = new Builder().BuildPartial();

+    private static readonly CSharpFileOptions defaultInstance = new CSharpFileOptions().MakeReadOnly();

     private static readonly string[] _cSharpFileOptionsFieldNames = new string[] { "add_serializable", "cls_compliance", "code_contracts", "expand_namespace_directories", "file_extension", "ignore_google_protobuf", "multiple_files", "namespace", "nest_classes", "output_directory", "public_classes", "service_generator_type", "umbrella_classname", "umbrella_namespace" };

     private static readonly uint[] _cSharpFileOptionsFieldTags = new uint[] { 72, 64, 48, 56, 1770, 1792, 32, 10, 40, 1786, 24, 1800, 18, 1778 };

     public static CSharpFileOptions DefaultInstance {

@@ -135,7 +135,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override CSharpFileOptions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override CSharpFileOptions ThisMessage {

@@ -429,6 +429,10 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static CSharpFileOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private CSharpFileOptions MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -444,22 +448,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new CSharpFileOptions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(CSharpFileOptions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      CSharpFileOptions result;

+      private bool resultIsReadOnly;

+      private CSharpFileOptions result;

       

       private CSharpFileOptions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           CSharpFileOptions original = result;

           result = new CSharpFileOptions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -474,13 +478,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new CSharpFileOptions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -496,11 +500,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override CSharpFileOptions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -969,7 +973,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class CSharpFieldOptions : pb::GeneratedMessage<CSharpFieldOptions, CSharpFieldOptions.Builder> {

     private CSharpFieldOptions() { }

-    private static readonly CSharpFieldOptions defaultInstance = new Builder().BuildPartial();

+    private static readonly CSharpFieldOptions defaultInstance = new CSharpFieldOptions().MakeReadOnly();

     private static readonly string[] _cSharpFieldOptionsFieldNames = new string[] { "property_name" };

     private static readonly uint[] _cSharpFieldOptionsFieldTags = new uint[] { 10 };

     public static CSharpFieldOptions DefaultInstance {

@@ -977,7 +981,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override CSharpFieldOptions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override CSharpFieldOptions ThisMessage {

@@ -1063,6 +1067,10 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static CSharpFieldOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private CSharpFieldOptions MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1078,22 +1086,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new CSharpFieldOptions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(CSharpFieldOptions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      CSharpFieldOptions result;

+      private bool resultIsReadOnly;

+      private CSharpFieldOptions result;

       

       private CSharpFieldOptions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           CSharpFieldOptions original = result;

           result = new CSharpFieldOptions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1108,13 +1116,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new CSharpFieldOptions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1130,11 +1138,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override CSharpFieldOptions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -1240,7 +1248,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class CSharpServiceOptions : pb::GeneratedMessage<CSharpServiceOptions, CSharpServiceOptions.Builder> {

     private CSharpServiceOptions() { }

-    private static readonly CSharpServiceOptions defaultInstance = new Builder().BuildPartial();

+    private static readonly CSharpServiceOptions defaultInstance = new CSharpServiceOptions().MakeReadOnly();

     private static readonly string[] _cSharpServiceOptionsFieldNames = new string[] { "interface_id" };

     private static readonly uint[] _cSharpServiceOptionsFieldTags = new uint[] { 10 };

     public static CSharpServiceOptions DefaultInstance {

@@ -1248,7 +1256,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override CSharpServiceOptions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override CSharpServiceOptions ThisMessage {

@@ -1334,6 +1342,10 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static CSharpServiceOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private CSharpServiceOptions MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1349,22 +1361,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new CSharpServiceOptions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(CSharpServiceOptions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      CSharpServiceOptions result;

+      private bool resultIsReadOnly;

+      private CSharpServiceOptions result;

       

       private CSharpServiceOptions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           CSharpServiceOptions original = result;

           result = new CSharpServiceOptions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1379,13 +1391,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new CSharpServiceOptions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1401,11 +1413,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override CSharpServiceOptions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -1511,7 +1523,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class CSharpMethodOptions : pb::GeneratedMessage<CSharpMethodOptions, CSharpMethodOptions.Builder> {

     private CSharpMethodOptions() { }

-    private static readonly CSharpMethodOptions defaultInstance = new Builder().BuildPartial();

+    private static readonly CSharpMethodOptions defaultInstance = new CSharpMethodOptions().MakeReadOnly();

     private static readonly string[] _cSharpMethodOptionsFieldNames = new string[] { "dispatch_id" };

     private static readonly uint[] _cSharpMethodOptionsFieldTags = new uint[] { 8 };

     public static CSharpMethodOptions DefaultInstance {

@@ -1519,7 +1531,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override CSharpMethodOptions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override CSharpMethodOptions ThisMessage {

@@ -1605,6 +1617,10 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static CSharpMethodOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private CSharpMethodOptions MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1620,22 +1636,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new CSharpMethodOptions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(CSharpMethodOptions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      CSharpMethodOptions result;

+      private bool resultIsReadOnly;

+      private CSharpMethodOptions result;

       

       private CSharpMethodOptions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           CSharpMethodOptions original = result;

           result = new CSharpMethodOptions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1650,13 +1666,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new CSharpMethodOptions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1672,11 +1688,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override CSharpMethodOptions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtocolBuffers/DescriptorProtos/DescriptorProtoFile.cs b/src/ProtocolBuffers/DescriptorProtos/DescriptorProtoFile.cs
index de6e2ed8..01048ed3 100644
--- a/src/ProtocolBuffers/DescriptorProtos/DescriptorProtoFile.cs
+++ b/src/ProtocolBuffers/DescriptorProtos/DescriptorProtoFile.cs
@@ -234,7 +234,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class FileDescriptorSet : pb::GeneratedMessage<FileDescriptorSet, FileDescriptorSet.Builder> {

     private FileDescriptorSet() { }

-    private static readonly FileDescriptorSet defaultInstance = new Builder().BuildPartial();

+    private static readonly FileDescriptorSet defaultInstance = new FileDescriptorSet().MakeReadOnly();

     private static readonly string[] _fileDescriptorSetFieldNames = new string[] { "file" };

     private static readonly uint[] _fileDescriptorSetFieldTags = new uint[] { 10 };

     public static FileDescriptorSet DefaultInstance {

@@ -242,7 +242,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override FileDescriptorSet DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override FileDescriptorSet ThisMessage {

@@ -333,6 +333,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static FileDescriptorSet ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private FileDescriptorSet MakeReadOnly() {

+      file_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -348,22 +353,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new FileDescriptorSet();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(FileDescriptorSet cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      FileDescriptorSet result;

+      private bool resultIsReadOnly;

+      private FileDescriptorSet result;

       

       private FileDescriptorSet PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           FileDescriptorSet original = result;

           result = new FileDescriptorSet();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -378,13 +383,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new FileDescriptorSet();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -400,12 +405,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override FileDescriptorSet BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.file_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -534,7 +538,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class FileDescriptorProto : pb::GeneratedMessage<FileDescriptorProto, FileDescriptorProto.Builder> {

     private FileDescriptorProto() { }

-    private static readonly FileDescriptorProto defaultInstance = new Builder().BuildPartial();

+    private static readonly FileDescriptorProto defaultInstance = new FileDescriptorProto().MakeReadOnly();

     private static readonly string[] _fileDescriptorProtoFieldNames = new string[] { "dependency", "enum_type", "extension", "message_type", "name", "options", "package", "service" };

     private static readonly uint[] _fileDescriptorProtoFieldTags = new uint[] { 26, 42, 58, 34, 10, 66, 18, 50 };

     public static FileDescriptorProto DefaultInstance {

@@ -542,7 +546,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override FileDescriptorProto DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override FileDescriptorProto ThisMessage {

@@ -639,12 +643,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     

     public const int OptionsFieldNumber = 8;

     private bool hasOptions;

-    private global::Google.ProtocolBuffers.DescriptorProtos.FileOptions options_ = global::Google.ProtocolBuffers.DescriptorProtos.FileOptions.DefaultInstance;

+    private global::Google.ProtocolBuffers.DescriptorProtos.FileOptions options_;

     public bool HasOptions {

       get { return hasOptions; }

     }

     public global::Google.ProtocolBuffers.DescriptorProtos.FileOptions Options {

-      get { return options_; }

+      get { return options_ ?? global::Google.ProtocolBuffers.DescriptorProtos.FileOptions.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -770,6 +774,15 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static FileDescriptorProto ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private FileDescriptorProto MakeReadOnly() {

+      dependency_.MakeReadOnly();

+      messageType_.MakeReadOnly();

+      enumType_.MakeReadOnly();

+      service_.MakeReadOnly();

+      extension_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -785,22 +798,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new FileDescriptorProto();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(FileDescriptorProto cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      FileDescriptorProto result;

+      private bool resultIsReadOnly;

+      private FileDescriptorProto result;

       

       private FileDescriptorProto PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           FileDescriptorProto original = result;

           result = new FileDescriptorProto();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -815,13 +828,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new FileDescriptorProto();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -837,16 +850,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override FileDescriptorProto BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.dependency_.MakeReadOnly();

-        result.messageType_.MakeReadOnly();

-        result.enumType_.MakeReadOnly();

-        result.service_.MakeReadOnly();

-        result.extension_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -1261,7 +1269,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       public Builder ClearOptions() {

         PrepareBuilder();

         result.hasOptions = false;

-        result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.FileOptions.DefaultInstance;

+        result.options_ = null;

         return this;

       }

     }

@@ -1275,7 +1283,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class DescriptorProto : pb::GeneratedMessage<DescriptorProto, DescriptorProto.Builder> {

     private DescriptorProto() { }

-    private static readonly DescriptorProto defaultInstance = new Builder().BuildPartial();

+    private static readonly DescriptorProto defaultInstance = new DescriptorProto().MakeReadOnly();

     private static readonly string[] _descriptorProtoFieldNames = new string[] { "enum_type", "extension", "extension_range", "field", "name", "nested_type", "options" };

     private static readonly uint[] _descriptorProtoFieldTags = new uint[] { 34, 50, 42, 18, 10, 26, 58 };

     public static DescriptorProto DefaultInstance {

@@ -1283,7 +1291,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override DescriptorProto DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override DescriptorProto ThisMessage {

@@ -1308,7 +1316,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class ExtensionRange : pb::GeneratedMessage<ExtensionRange, ExtensionRange.Builder> {

         private ExtensionRange() { }

-        private static readonly ExtensionRange defaultInstance = new Builder().BuildPartial();

+        private static readonly ExtensionRange defaultInstance = new ExtensionRange().MakeReadOnly();

         private static readonly string[] _extensionRangeFieldNames = new string[] { "end", "start" };

         private static readonly uint[] _extensionRangeFieldTags = new uint[] { 16, 8 };

         public static ExtensionRange DefaultInstance {

@@ -1316,7 +1324,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         }

         

         public override ExtensionRange DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override ExtensionRange ThisMessage {

@@ -1418,6 +1426,10 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         public static ExtensionRange ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private ExtensionRange MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1433,22 +1445,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new ExtensionRange();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(ExtensionRange cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          ExtensionRange result;

+          private bool resultIsReadOnly;

+          private ExtensionRange result;

           

           private ExtensionRange PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               ExtensionRange original = result;

               result = new ExtensionRange();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -1463,13 +1475,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new ExtensionRange();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -1485,11 +1497,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
           }

           

           public override ExtensionRange BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -1691,12 +1703,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     

     public const int OptionsFieldNumber = 7;

     private bool hasOptions;

-    private global::Google.ProtocolBuffers.DescriptorProtos.MessageOptions options_ = global::Google.ProtocolBuffers.DescriptorProtos.MessageOptions.DefaultInstance;

+    private global::Google.ProtocolBuffers.DescriptorProtos.MessageOptions options_;

     public bool HasOptions {

       get { return hasOptions; }

     }

     public global::Google.ProtocolBuffers.DescriptorProtos.MessageOptions Options {

-      get { return options_; }

+      get { return options_ ?? global::Google.ProtocolBuffers.DescriptorProtos.MessageOptions.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -1811,6 +1823,15 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static DescriptorProto ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private DescriptorProto MakeReadOnly() {

+      field_.MakeReadOnly();

+      extension_.MakeReadOnly();

+      nestedType_.MakeReadOnly();

+      enumType_.MakeReadOnly();

+      extensionRange_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1826,22 +1847,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new DescriptorProto();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(DescriptorProto cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      DescriptorProto result;

+      private bool resultIsReadOnly;

+      private DescriptorProto result;

       

       private DescriptorProto PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           DescriptorProto original = result;

           result = new DescriptorProto();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1856,13 +1877,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new DescriptorProto();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1878,16 +1899,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override DescriptorProto BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.field_.MakeReadOnly();

-        result.extension_.MakeReadOnly();

-        result.nestedType_.MakeReadOnly();

-        result.enumType_.MakeReadOnly();

-        result.extensionRange_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -2286,7 +2302,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       public Builder ClearOptions() {

         PrepareBuilder();

         result.hasOptions = false;

-        result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.MessageOptions.DefaultInstance;

+        result.options_ = null;

         return this;

       }

     }

@@ -2300,7 +2316,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class FieldDescriptorProto : pb::GeneratedMessage<FieldDescriptorProto, FieldDescriptorProto.Builder> {

     private FieldDescriptorProto() { }

-    private static readonly FieldDescriptorProto defaultInstance = new Builder().BuildPartial();

+    private static readonly FieldDescriptorProto defaultInstance = new FieldDescriptorProto().MakeReadOnly();

     private static readonly string[] _fieldDescriptorProtoFieldNames = new string[] { "default_value", "extendee", "label", "name", "number", "options", "type", "type_name" };

     private static readonly uint[] _fieldDescriptorProtoFieldTags = new uint[] { 58, 18, 32, 10, 24, 66, 40, 50 };

     public static FieldDescriptorProto DefaultInstance {

@@ -2308,7 +2324,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override FieldDescriptorProto DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override FieldDescriptorProto ThisMessage {

@@ -2434,12 +2450,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     

     public const int OptionsFieldNumber = 8;

     private bool hasOptions;

-    private global::Google.ProtocolBuffers.DescriptorProtos.FieldOptions options_ = global::Google.ProtocolBuffers.DescriptorProtos.FieldOptions.DefaultInstance;

+    private global::Google.ProtocolBuffers.DescriptorProtos.FieldOptions options_;

     public bool HasOptions {

       get { return hasOptions; }

     }

     public global::Google.ProtocolBuffers.DescriptorProtos.FieldOptions Options {

-      get { return options_; }

+      get { return options_ ?? global::Google.ProtocolBuffers.DescriptorProtos.FieldOptions.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -2548,6 +2564,10 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static FieldDescriptorProto ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private FieldDescriptorProto MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -2563,22 +2583,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new FieldDescriptorProto();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(FieldDescriptorProto cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      FieldDescriptorProto result;

+      private bool resultIsReadOnly;

+      private FieldDescriptorProto result;

       

       private FieldDescriptorProto PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           FieldDescriptorProto original = result;

           result = new FieldDescriptorProto();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -2593,13 +2613,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new FieldDescriptorProto();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -2615,11 +2635,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override FieldDescriptorProto BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -2944,7 +2964,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       public Builder ClearOptions() {

         PrepareBuilder();

         result.hasOptions = false;

-        result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.FieldOptions.DefaultInstance;

+        result.options_ = null;

         return this;

       }

     }

@@ -2958,7 +2978,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class EnumDescriptorProto : pb::GeneratedMessage<EnumDescriptorProto, EnumDescriptorProto.Builder> {

     private EnumDescriptorProto() { }

-    private static readonly EnumDescriptorProto defaultInstance = new Builder().BuildPartial();

+    private static readonly EnumDescriptorProto defaultInstance = new EnumDescriptorProto().MakeReadOnly();

     private static readonly string[] _enumDescriptorProtoFieldNames = new string[] { "name", "options", "value" };

     private static readonly uint[] _enumDescriptorProtoFieldTags = new uint[] { 10, 26, 18 };

     public static EnumDescriptorProto DefaultInstance {

@@ -2966,7 +2986,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override EnumDescriptorProto DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override EnumDescriptorProto ThisMessage {

@@ -3005,12 +3025,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     

     public const int OptionsFieldNumber = 3;

     private bool hasOptions;

-    private global::Google.ProtocolBuffers.DescriptorProtos.EnumOptions options_ = global::Google.ProtocolBuffers.DescriptorProtos.EnumOptions.DefaultInstance;

+    private global::Google.ProtocolBuffers.DescriptorProtos.EnumOptions options_;

     public bool HasOptions {

       get { return hasOptions; }

     }

     public global::Google.ProtocolBuffers.DescriptorProtos.EnumOptions Options {

-      get { return options_; }

+      get { return options_ ?? global::Google.ProtocolBuffers.DescriptorProtos.EnumOptions.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -3092,6 +3112,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static EnumDescriptorProto ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private EnumDescriptorProto MakeReadOnly() {

+      value_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -3107,22 +3132,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new EnumDescriptorProto();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(EnumDescriptorProto cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      EnumDescriptorProto result;

+      private bool resultIsReadOnly;

+      private EnumDescriptorProto result;

       

       private EnumDescriptorProto PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           EnumDescriptorProto original = result;

           result = new EnumDescriptorProto();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -3137,13 +3162,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new EnumDescriptorProto();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -3159,12 +3184,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override EnumDescriptorProto BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.value_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -3359,7 +3383,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       public Builder ClearOptions() {

         PrepareBuilder();

         result.hasOptions = false;

-        result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.EnumOptions.DefaultInstance;

+        result.options_ = null;

         return this;

       }

     }

@@ -3373,7 +3397,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class EnumValueDescriptorProto : pb::GeneratedMessage<EnumValueDescriptorProto, EnumValueDescriptorProto.Builder> {

     private EnumValueDescriptorProto() { }

-    private static readonly EnumValueDescriptorProto defaultInstance = new Builder().BuildPartial();

+    private static readonly EnumValueDescriptorProto defaultInstance = new EnumValueDescriptorProto().MakeReadOnly();

     private static readonly string[] _enumValueDescriptorProtoFieldNames = new string[] { "name", "number", "options" };

     private static readonly uint[] _enumValueDescriptorProtoFieldTags = new uint[] { 10, 16, 26 };

     public static EnumValueDescriptorProto DefaultInstance {

@@ -3381,7 +3405,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override EnumValueDescriptorProto DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override EnumValueDescriptorProto ThisMessage {

@@ -3418,12 +3442,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     

     public const int OptionsFieldNumber = 3;

     private bool hasOptions;

-    private global::Google.ProtocolBuffers.DescriptorProtos.EnumValueOptions options_ = global::Google.ProtocolBuffers.DescriptorProtos.EnumValueOptions.DefaultInstance;

+    private global::Google.ProtocolBuffers.DescriptorProtos.EnumValueOptions options_;

     public bool HasOptions {

       get { return hasOptions; }

     }

     public global::Google.ProtocolBuffers.DescriptorProtos.EnumValueOptions Options {

-      get { return options_; }

+      get { return options_ ?? global::Google.ProtocolBuffers.DescriptorProtos.EnumValueOptions.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -3502,6 +3526,10 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static EnumValueDescriptorProto ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private EnumValueDescriptorProto MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -3517,22 +3545,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new EnumValueDescriptorProto();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(EnumValueDescriptorProto cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      EnumValueDescriptorProto result;

+      private bool resultIsReadOnly;

+      private EnumValueDescriptorProto result;

       

       private EnumValueDescriptorProto PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           EnumValueDescriptorProto original = result;

           result = new EnumValueDescriptorProto();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -3547,13 +3575,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new EnumValueDescriptorProto();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -3569,11 +3597,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override EnumValueDescriptorProto BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -3744,7 +3772,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       public Builder ClearOptions() {

         PrepareBuilder();

         result.hasOptions = false;

-        result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.EnumValueOptions.DefaultInstance;

+        result.options_ = null;

         return this;

       }

     }

@@ -3758,7 +3786,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class ServiceDescriptorProto : pb::GeneratedMessage<ServiceDescriptorProto, ServiceDescriptorProto.Builder> {

     private ServiceDescriptorProto() { }

-    private static readonly ServiceDescriptorProto defaultInstance = new Builder().BuildPartial();

+    private static readonly ServiceDescriptorProto defaultInstance = new ServiceDescriptorProto().MakeReadOnly();

     private static readonly string[] _serviceDescriptorProtoFieldNames = new string[] { "method", "name", "options" };

     private static readonly uint[] _serviceDescriptorProtoFieldTags = new uint[] { 18, 10, 26 };

     public static ServiceDescriptorProto DefaultInstance {

@@ -3766,7 +3794,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override ServiceDescriptorProto DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override ServiceDescriptorProto ThisMessage {

@@ -3805,12 +3833,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     

     public const int OptionsFieldNumber = 3;

     private bool hasOptions;

-    private global::Google.ProtocolBuffers.DescriptorProtos.ServiceOptions options_ = global::Google.ProtocolBuffers.DescriptorProtos.ServiceOptions.DefaultInstance;

+    private global::Google.ProtocolBuffers.DescriptorProtos.ServiceOptions options_;

     public bool HasOptions {

       get { return hasOptions; }

     }

     public global::Google.ProtocolBuffers.DescriptorProtos.ServiceOptions Options {

-      get { return options_; }

+      get { return options_ ?? global::Google.ProtocolBuffers.DescriptorProtos.ServiceOptions.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -3892,6 +3920,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static ServiceDescriptorProto ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private ServiceDescriptorProto MakeReadOnly() {

+      method_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -3907,22 +3940,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new ServiceDescriptorProto();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(ServiceDescriptorProto cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      ServiceDescriptorProto result;

+      private bool resultIsReadOnly;

+      private ServiceDescriptorProto result;

       

       private ServiceDescriptorProto PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           ServiceDescriptorProto original = result;

           result = new ServiceDescriptorProto();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -3937,13 +3970,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new ServiceDescriptorProto();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -3959,12 +3992,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override ServiceDescriptorProto BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.method_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -4159,7 +4191,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       public Builder ClearOptions() {

         PrepareBuilder();

         result.hasOptions = false;

-        result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.ServiceOptions.DefaultInstance;

+        result.options_ = null;

         return this;

       }

     }

@@ -4173,7 +4205,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class MethodDescriptorProto : pb::GeneratedMessage<MethodDescriptorProto, MethodDescriptorProto.Builder> {

     private MethodDescriptorProto() { }

-    private static readonly MethodDescriptorProto defaultInstance = new Builder().BuildPartial();

+    private static readonly MethodDescriptorProto defaultInstance = new MethodDescriptorProto().MakeReadOnly();

     private static readonly string[] _methodDescriptorProtoFieldNames = new string[] { "input_type", "name", "options", "output_type" };

     private static readonly uint[] _methodDescriptorProtoFieldTags = new uint[] { 18, 10, 34, 26 };

     public static MethodDescriptorProto DefaultInstance {

@@ -4181,7 +4213,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override MethodDescriptorProto DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override MethodDescriptorProto ThisMessage {

@@ -4228,12 +4260,12 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     

     public const int OptionsFieldNumber = 4;

     private bool hasOptions;

-    private global::Google.ProtocolBuffers.DescriptorProtos.MethodOptions options_ = global::Google.ProtocolBuffers.DescriptorProtos.MethodOptions.DefaultInstance;

+    private global::Google.ProtocolBuffers.DescriptorProtos.MethodOptions options_;

     public bool HasOptions {

       get { return hasOptions; }

     }

     public global::Google.ProtocolBuffers.DescriptorProtos.MethodOptions Options {

-      get { return options_; }

+      get { return options_ ?? global::Google.ProtocolBuffers.DescriptorProtos.MethodOptions.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -4318,6 +4350,10 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static MethodDescriptorProto ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private MethodDescriptorProto MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -4333,22 +4369,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new MethodDescriptorProto();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(MethodDescriptorProto cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      MethodDescriptorProto result;

+      private bool resultIsReadOnly;

+      private MethodDescriptorProto result;

       

       private MethodDescriptorProto PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           MethodDescriptorProto original = result;

           result = new MethodDescriptorProto();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -4363,13 +4399,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new MethodDescriptorProto();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -4385,11 +4421,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override MethodDescriptorProto BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -4589,7 +4625,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       public Builder ClearOptions() {

         PrepareBuilder();

         result.hasOptions = false;

-        result.options_ = global::Google.ProtocolBuffers.DescriptorProtos.MethodOptions.DefaultInstance;

+        result.options_ = null;

         return this;

       }

     }

@@ -4603,7 +4639,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class FileOptions : pb::ExtendableMessage<FileOptions, FileOptions.Builder> {

     private FileOptions() { }

-    private static readonly FileOptions defaultInstance = new Builder().BuildPartial();

+    private static readonly FileOptions defaultInstance = new FileOptions().MakeReadOnly();

     private static readonly string[] _fileOptionsFieldNames = new string[] { "cc_generic_services", "java_generic_services", "java_multiple_files", "java_outer_classname", "java_package", "optimize_for", "py_generic_services", "uninterpreted_option" };

     private static readonly uint[] _fileOptionsFieldTags = new uint[] { 128, 136, 80, 66, 10, 72, 144, 7994 };

     public static FileOptions DefaultInstance {

@@ -4611,7 +4647,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override FileOptions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override FileOptions ThisMessage {

@@ -4834,6 +4870,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static FileOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private FileOptions MakeReadOnly() {

+      uninterpretedOption_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -4849,22 +4890,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new FileOptions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(FileOptions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      FileOptions result;

+      private bool resultIsReadOnly;

+      private FileOptions result;

       

       private FileOptions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           FileOptions original = result;

           result = new FileOptions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -4879,13 +4920,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new FileOptions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -4901,12 +4942,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override FileOptions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.uninterpretedOption_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -5235,7 +5275,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class MessageOptions : pb::ExtendableMessage<MessageOptions, MessageOptions.Builder> {

     private MessageOptions() { }

-    private static readonly MessageOptions defaultInstance = new Builder().BuildPartial();

+    private static readonly MessageOptions defaultInstance = new MessageOptions().MakeReadOnly();

     private static readonly string[] _messageOptionsFieldNames = new string[] { "message_set_wire_format", "no_standard_descriptor_accessor", "uninterpreted_option" };

     private static readonly uint[] _messageOptionsFieldTags = new uint[] { 8, 16, 7994 };

     public static MessageOptions DefaultInstance {

@@ -5243,7 +5283,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override MessageOptions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override MessageOptions ThisMessage {

@@ -5370,6 +5410,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static MessageOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private MessageOptions MakeReadOnly() {

+      uninterpretedOption_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -5385,22 +5430,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new MessageOptions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(MessageOptions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      MessageOptions result;

+      private bool resultIsReadOnly;

+      private MessageOptions result;

       

       private MessageOptions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           MessageOptions original = result;

           result = new MessageOptions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -5415,13 +5460,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new MessageOptions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -5437,12 +5482,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override MessageOptions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.uninterpretedOption_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -5626,7 +5670,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class FieldOptions : pb::ExtendableMessage<FieldOptions, FieldOptions.Builder> {

     private FieldOptions() { }

-    private static readonly FieldOptions defaultInstance = new Builder().BuildPartial();

+    private static readonly FieldOptions defaultInstance = new FieldOptions().MakeReadOnly();

     private static readonly string[] _fieldOptionsFieldNames = new string[] { "ctype", "deprecated", "experimental_map_key", "packed", "uninterpreted_option" };

     private static readonly uint[] _fieldOptionsFieldTags = new uint[] { 8, 24, 74, 16, 7994 };

     public static FieldOptions DefaultInstance {

@@ -5634,7 +5678,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override FieldOptions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override FieldOptions ThisMessage {

@@ -5809,6 +5853,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static FieldOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private FieldOptions MakeReadOnly() {

+      uninterpretedOption_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -5824,22 +5873,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new FieldOptions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(FieldOptions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      FieldOptions result;

+      private bool resultIsReadOnly;

+      private FieldOptions result;

       

       private FieldOptions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           FieldOptions original = result;

           result = new FieldOptions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -5854,13 +5903,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new FieldOptions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -5876,12 +5925,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override FieldOptions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.uninterpretedOption_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -6128,7 +6176,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class EnumOptions : pb::ExtendableMessage<EnumOptions, EnumOptions.Builder> {

     private EnumOptions() { }

-    private static readonly EnumOptions defaultInstance = new Builder().BuildPartial();

+    private static readonly EnumOptions defaultInstance = new EnumOptions().MakeReadOnly();

     private static readonly string[] _enumOptionsFieldNames = new string[] { "uninterpreted_option" };

     private static readonly uint[] _enumOptionsFieldTags = new uint[] { 7994 };

     public static EnumOptions DefaultInstance {

@@ -6136,7 +6184,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override EnumOptions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override EnumOptions ThisMessage {

@@ -6231,6 +6279,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static EnumOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private EnumOptions MakeReadOnly() {

+      uninterpretedOption_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -6246,22 +6299,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new EnumOptions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(EnumOptions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      EnumOptions result;

+      private bool resultIsReadOnly;

+      private EnumOptions result;

       

       private EnumOptions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           EnumOptions original = result;

           result = new EnumOptions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -6276,13 +6329,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new EnumOptions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -6298,12 +6351,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override EnumOptions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.uninterpretedOption_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -6433,7 +6485,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class EnumValueOptions : pb::ExtendableMessage<EnumValueOptions, EnumValueOptions.Builder> {

     private EnumValueOptions() { }

-    private static readonly EnumValueOptions defaultInstance = new Builder().BuildPartial();

+    private static readonly EnumValueOptions defaultInstance = new EnumValueOptions().MakeReadOnly();

     private static readonly string[] _enumValueOptionsFieldNames = new string[] { "uninterpreted_option" };

     private static readonly uint[] _enumValueOptionsFieldTags = new uint[] { 7994 };

     public static EnumValueOptions DefaultInstance {

@@ -6441,7 +6493,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override EnumValueOptions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override EnumValueOptions ThisMessage {

@@ -6536,6 +6588,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static EnumValueOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private EnumValueOptions MakeReadOnly() {

+      uninterpretedOption_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -6551,22 +6608,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new EnumValueOptions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(EnumValueOptions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      EnumValueOptions result;

+      private bool resultIsReadOnly;

+      private EnumValueOptions result;

       

       private EnumValueOptions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           EnumValueOptions original = result;

           result = new EnumValueOptions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -6581,13 +6638,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new EnumValueOptions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -6603,12 +6660,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override EnumValueOptions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.uninterpretedOption_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -6738,7 +6794,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class ServiceOptions : pb::ExtendableMessage<ServiceOptions, ServiceOptions.Builder> {

     private ServiceOptions() { }

-    private static readonly ServiceOptions defaultInstance = new Builder().BuildPartial();

+    private static readonly ServiceOptions defaultInstance = new ServiceOptions().MakeReadOnly();

     private static readonly string[] _serviceOptionsFieldNames = new string[] { "uninterpreted_option" };

     private static readonly uint[] _serviceOptionsFieldTags = new uint[] { 7994 };

     public static ServiceOptions DefaultInstance {

@@ -6746,7 +6802,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override ServiceOptions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override ServiceOptions ThisMessage {

@@ -6841,6 +6897,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static ServiceOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private ServiceOptions MakeReadOnly() {

+      uninterpretedOption_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -6856,22 +6917,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new ServiceOptions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(ServiceOptions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      ServiceOptions result;

+      private bool resultIsReadOnly;

+      private ServiceOptions result;

       

       private ServiceOptions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           ServiceOptions original = result;

           result = new ServiceOptions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -6886,13 +6947,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new ServiceOptions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -6908,12 +6969,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override ServiceOptions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.uninterpretedOption_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -7043,7 +7103,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class MethodOptions : pb::ExtendableMessage<MethodOptions, MethodOptions.Builder> {

     private MethodOptions() { }

-    private static readonly MethodOptions defaultInstance = new Builder().BuildPartial();

+    private static readonly MethodOptions defaultInstance = new MethodOptions().MakeReadOnly();

     private static readonly string[] _methodOptionsFieldNames = new string[] { "uninterpreted_option" };

     private static readonly uint[] _methodOptionsFieldTags = new uint[] { 7994 };

     public static MethodOptions DefaultInstance {

@@ -7051,7 +7111,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override MethodOptions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override MethodOptions ThisMessage {

@@ -7146,6 +7206,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static MethodOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private MethodOptions MakeReadOnly() {

+      uninterpretedOption_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -7161,22 +7226,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new MethodOptions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(MethodOptions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      MethodOptions result;

+      private bool resultIsReadOnly;

+      private MethodOptions result;

       

       private MethodOptions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           MethodOptions original = result;

           result = new MethodOptions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -7191,13 +7256,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new MethodOptions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -7213,12 +7278,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override MethodOptions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.uninterpretedOption_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -7348,7 +7412,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class UninterpretedOption : pb::GeneratedMessage<UninterpretedOption, UninterpretedOption.Builder> {

     private UninterpretedOption() { }

-    private static readonly UninterpretedOption defaultInstance = new Builder().BuildPartial();

+    private static readonly UninterpretedOption defaultInstance = new UninterpretedOption().MakeReadOnly();

     private static readonly string[] _uninterpretedOptionFieldNames = new string[] { "double_value", "identifier_value", "name", "negative_int_value", "positive_int_value", "string_value" };

     private static readonly uint[] _uninterpretedOptionFieldTags = new uint[] { 49, 26, 18, 40, 32, 58 };

     public static UninterpretedOption DefaultInstance {

@@ -7356,7 +7420,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }

     

     public override UninterpretedOption DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override UninterpretedOption ThisMessage {

@@ -7381,7 +7445,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class NamePart : pb::GeneratedMessage<NamePart, NamePart.Builder> {

         private NamePart() { }

-        private static readonly NamePart defaultInstance = new Builder().BuildPartial();

+        private static readonly NamePart defaultInstance = new NamePart().MakeReadOnly();

         private static readonly string[] _namePartFieldNames = new string[] { "is_extension", "name_part" };

         private static readonly uint[] _namePartFieldTags = new uint[] { 16, 10 };

         public static NamePart DefaultInstance {

@@ -7389,7 +7453,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         }

         

         public override NamePart DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override NamePart ThisMessage {

@@ -7493,6 +7557,10 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         public static NamePart ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private NamePart MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -7508,22 +7576,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new NamePart();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(NamePart cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          NamePart result;

+          private bool resultIsReadOnly;

+          private NamePart result;

           

           private NamePart PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               NamePart original = result;

               result = new NamePart();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -7538,13 +7606,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new NamePart();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -7560,11 +7628,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
           }

           

           public override NamePart BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -7852,6 +7920,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static UninterpretedOption ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private UninterpretedOption MakeReadOnly() {

+      name_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -7867,22 +7940,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new UninterpretedOption();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(UninterpretedOption cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      UninterpretedOption result;

+      private bool resultIsReadOnly;

+      private UninterpretedOption result;

       

       private UninterpretedOption PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           UninterpretedOption original = result;

           result = new UninterpretedOption();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -7897,13 +7970,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new UninterpretedOption();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -7919,12 +7992,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }

       

       public override UninterpretedOption BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.name_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasFullProtoFile.cs b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasFullProtoFile.cs
index 260e6397..ba46dc6a 100644
--- a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasFullProtoFile.cs
+++ b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasFullProtoFile.cs
@@ -129,13 +129,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestInteropPerson : pb::ExtendableMessage<TestInteropPerson, TestInteropPerson.Builder> {

     private TestInteropPerson() { }

-    private static readonly TestInteropPerson defaultInstance = new Builder().BuildPartial();

+    private static readonly TestInteropPerson defaultInstance = new TestInteropPerson().MakeReadOnly();

     public static TestInteropPerson DefaultInstance {

       get { return defaultInstance; }

     }

     

     public override TestInteropPerson DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestInteropPerson ThisMessage {

@@ -168,13 +168,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class PhoneNumber : pb::GeneratedMessage<PhoneNumber, PhoneNumber.Builder> {

         private PhoneNumber() { }

-        private static readonly PhoneNumber defaultInstance = new Builder().BuildPartial();

+        private static readonly PhoneNumber defaultInstance = new PhoneNumber().MakeReadOnly();

         public static PhoneNumber DefaultInstance {

           get { return defaultInstance; }

         }

         

         public override PhoneNumber DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override PhoneNumber ThisMessage {

@@ -239,6 +239,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static PhoneNumber ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private PhoneNumber MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -254,22 +258,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new PhoneNumber();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(PhoneNumber cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          PhoneNumber result;

+          private bool resultIsReadOnly;

+          private PhoneNumber result;

           

           private PhoneNumber PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               PhoneNumber original = result;

               result = new PhoneNumber();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -284,13 +288,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new PhoneNumber();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -306,11 +310,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override PhoneNumber BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

           

@@ -365,13 +369,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class Addresses : pb::GeneratedMessage<Addresses, Addresses.Builder> {

         private Addresses() { }

-        private static readonly Addresses defaultInstance = new Builder().BuildPartial();

+        private static readonly Addresses defaultInstance = new Addresses().MakeReadOnly();

         public static Addresses DefaultInstance {

           get { return defaultInstance; }

         }

         

         public override Addresses DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override Addresses ThisMessage {

@@ -467,6 +471,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Addresses ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private Addresses MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -482,22 +490,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new Addresses();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(Addresses cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          Addresses result;

+          private bool resultIsReadOnly;

+          private Addresses result;

           

           private Addresses PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               Addresses original = result;

               result = new Addresses();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -512,13 +520,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new Addresses();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -534,11 +542,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Addresses BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

           

@@ -752,6 +760,13 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestInteropPerson ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestInteropPerson MakeReadOnly() {

+      codes_.MakeReadOnly();

+      phone_.MakeReadOnly();

+      addresses_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -767,22 +782,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestInteropPerson();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestInteropPerson cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestInteropPerson result;

+      private bool resultIsReadOnly;

+      private TestInteropPerson result;

       

       private TestInteropPerson PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestInteropPerson original = result;

           result = new TestInteropPerson();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -797,13 +812,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestInteropPerson();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -819,14 +834,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestInteropPerson BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.codes_.MakeReadOnly();

-        result.phone_.MakeReadOnly();

-        result.addresses_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

       

@@ -1020,13 +1032,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestInteropEmployeeId : pb::GeneratedMessage<TestInteropEmployeeId, TestInteropEmployeeId.Builder> {

     private TestInteropEmployeeId() { }

-    private static readonly TestInteropEmployeeId defaultInstance = new Builder().BuildPartial();

+    private static readonly TestInteropEmployeeId defaultInstance = new TestInteropEmployeeId().MakeReadOnly();

     public static TestInteropEmployeeId DefaultInstance {

       get { return defaultInstance; }

     }

     

     public override TestInteropEmployeeId DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestInteropEmployeeId ThisMessage {

@@ -1081,6 +1093,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestInteropEmployeeId ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestInteropEmployeeId MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1096,22 +1112,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestInteropEmployeeId();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestInteropEmployeeId cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestInteropEmployeeId result;

+      private bool resultIsReadOnly;

+      private TestInteropEmployeeId result;

       

       private TestInteropEmployeeId PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestInteropEmployeeId original = result;

           result = new TestInteropEmployeeId();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1126,13 +1142,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestInteropEmployeeId();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1148,11 +1164,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestInteropEmployeeId BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

       

@@ -1187,13 +1203,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestMissingFieldsA : pb::GeneratedMessage<TestMissingFieldsA, TestMissingFieldsA.Builder> {

     private TestMissingFieldsA() { }

-    private static readonly TestMissingFieldsA defaultInstance = new Builder().BuildPartial();

+    private static readonly TestMissingFieldsA defaultInstance = new TestMissingFieldsA().MakeReadOnly();

     public static TestMissingFieldsA DefaultInstance {

       get { return defaultInstance; }

     }

     

     public override TestMissingFieldsA DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestMissingFieldsA ThisMessage {

@@ -1218,13 +1234,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class SubA : pb::GeneratedMessage<SubA, SubA.Builder> {

         private SubA() { }

-        private static readonly SubA defaultInstance = new Builder().BuildPartial();

+        private static readonly SubA defaultInstance = new SubA().MakeReadOnly();

         public static SubA DefaultInstance {

           get { return defaultInstance; }

         }

         

         public override SubA DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override SubA ThisMessage {

@@ -1291,6 +1307,11 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static SubA ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private SubA MakeReadOnly() {

+          values_.MakeReadOnly();

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1306,22 +1327,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new SubA();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(SubA cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          SubA result;

+          private bool resultIsReadOnly;

+          private SubA result;

           

           private SubA PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               SubA original = result;

               result = new SubA();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -1336,13 +1357,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new SubA();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -1358,12 +1379,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override SubA BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            result.values_.MakeReadOnly();

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

           

@@ -1459,12 +1479,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int TestAFieldNumber = 11;

     private bool hasTestA;

-    private global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA testA_ = global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA testA_;

     public bool HasTestA {

       get { return hasTestA; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA TestA {

-      get { return testA_; }

+      get { return testA_ ?? global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA.DefaultInstance; }

     }

     

     public static TestMissingFieldsA ParseFrom(pb::ByteString data) {

@@ -1497,6 +1517,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMissingFieldsA ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestMissingFieldsA MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1512,22 +1536,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestMissingFieldsA();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestMissingFieldsA cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestMissingFieldsA result;

+      private bool resultIsReadOnly;

+      private TestMissingFieldsA result;

       

       private TestMissingFieldsA PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestMissingFieldsA original = result;

           result = new TestMissingFieldsA();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1542,13 +1566,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestMissingFieldsA();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1564,11 +1588,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMissingFieldsA BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

       

@@ -1670,7 +1694,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearTestA() {

         PrepareBuilder();

         result.hasTestA = false;

-        result.testA_ = global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA.DefaultInstance;

+        result.testA_ = null;

         return this;

       }

     }

@@ -1684,13 +1708,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestMissingFieldsB : pb::GeneratedMessage<TestMissingFieldsB, TestMissingFieldsB.Builder> {

     private TestMissingFieldsB() { }

-    private static readonly TestMissingFieldsB defaultInstance = new Builder().BuildPartial();

+    private static readonly TestMissingFieldsB defaultInstance = new TestMissingFieldsB().MakeReadOnly();

     public static TestMissingFieldsB DefaultInstance {

       get { return defaultInstance; }

     }

     

     public override TestMissingFieldsB DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestMissingFieldsB ThisMessage {

@@ -1715,13 +1739,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class SubB : pb::GeneratedMessage<SubB, SubB.Builder> {

         private SubB() { }

-        private static readonly SubB defaultInstance = new Builder().BuildPartial();

+        private static readonly SubB defaultInstance = new SubB().MakeReadOnly();

         public static SubB DefaultInstance {

           get { return defaultInstance; }

         }

         

         public override SubB DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override SubB ThisMessage {

@@ -1778,6 +1802,11 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static SubB ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private SubB MakeReadOnly() {

+          values_.MakeReadOnly();

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1793,22 +1822,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new SubB();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(SubB cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          SubB result;

+          private bool resultIsReadOnly;

+          private SubB result;

           

           private SubB PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               SubB original = result;

               result = new SubB();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -1823,13 +1852,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new SubB();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -1845,12 +1874,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override SubB BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            result.values_.MakeReadOnly();

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

           

@@ -1926,12 +1954,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int TestBFieldNumber = 12;

     private bool hasTestB;

-    private global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB testB_ = global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB testB_;

     public bool HasTestB {

       get { return hasTestB; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB TestB {

-      get { return testB_; }

+      get { return testB_ ?? global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB.DefaultInstance; }

     }

     

     public static TestMissingFieldsB ParseFrom(pb::ByteString data) {

@@ -1964,6 +1992,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMissingFieldsB ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestMissingFieldsB MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1979,22 +2011,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestMissingFieldsB();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestMissingFieldsB cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestMissingFieldsB result;

+      private bool resultIsReadOnly;

+      private TestMissingFieldsB result;

       

       private TestMissingFieldsB PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestMissingFieldsB original = result;

           result = new TestMissingFieldsB();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -2009,13 +2041,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestMissingFieldsB();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -2031,11 +2063,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMissingFieldsB BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

       

@@ -2137,7 +2169,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearTestB() {

         PrepareBuilder();

         result.hasTestB = false;

-        result.testB_ = global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB.DefaultInstance;

+        result.testB_ = null;

         return this;

       }

     }

diff --git a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasLiteProtoFile.cs b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasLiteProtoFile.cs
index 550bc012..e3df2328 100644
--- a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasLiteProtoFile.cs
+++ b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasLiteProtoFile.cs
@@ -61,7 +61,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestRequiredLite : pb::GeneratedMessageLite<TestRequiredLite, TestRequiredLite.Builder> {

     private TestRequiredLite() { }

-    private static readonly TestRequiredLite defaultInstance = new Builder().BuildPartial();

+    private static readonly TestRequiredLite defaultInstance = new TestRequiredLite().MakeReadOnly();

     private static readonly string[] _testRequiredLiteFieldNames = new string[] { "d", "en" };

     private static readonly uint[] _testRequiredLiteFieldTags = new uint[] { 8, 16 };

     public static TestRequiredLite DefaultInstance {

@@ -69,7 +69,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestRequiredLite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestRequiredLite ThisMessage {

@@ -185,6 +185,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestRequiredLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestRequiredLite MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -201,22 +205,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestRequiredLite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestRequiredLite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestRequiredLite result;

+      private bool resultIsReadOnly;

+      private TestRequiredLite result;

       

       private TestRequiredLite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestRequiredLite original = result;

           result = new TestRequiredLite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -231,13 +235,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestRequiredLite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -249,11 +253,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRequiredLite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -376,7 +380,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestInteropPersonLite : pb::ExtendableMessageLite<TestInteropPersonLite, TestInteropPersonLite.Builder> {

     private TestInteropPersonLite() { }

-    private static readonly TestInteropPersonLite defaultInstance = new Builder().BuildPartial();

+    private static readonly TestInteropPersonLite defaultInstance = new TestInteropPersonLite().MakeReadOnly();

     private static readonly string[] _testInteropPersonLiteFieldNames = new string[] { "addresses", "codes", "email", "id", "name", "phone" };

     private static readonly uint[] _testInteropPersonLiteFieldTags = new uint[] { 43, 82, 26, 16, 10, 34 };

     public static TestInteropPersonLite DefaultInstance {

@@ -384,7 +388,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestInteropPersonLite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestInteropPersonLite ThisMessage {

@@ -410,7 +414,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class PhoneNumber : pb::GeneratedMessageLite<PhoneNumber, PhoneNumber.Builder> {

         private PhoneNumber() { }

-        private static readonly PhoneNumber defaultInstance = new Builder().BuildPartial();

+        private static readonly PhoneNumber defaultInstance = new PhoneNumber().MakeReadOnly();

         private static readonly string[] _phoneNumberFieldNames = new string[] { "number", "type" };

         private static readonly uint[] _phoneNumberFieldTags = new uint[] { 10, 16 };

         public static PhoneNumber DefaultInstance {

@@ -418,7 +422,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override PhoneNumber DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override PhoneNumber ThisMessage {

@@ -533,6 +537,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static PhoneNumber ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private PhoneNumber MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -549,22 +557,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new PhoneNumber();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(PhoneNumber cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          PhoneNumber result;

+          private bool resultIsReadOnly;

+          private PhoneNumber result;

           

           private PhoneNumber PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               PhoneNumber original = result;

               result = new PhoneNumber();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -579,13 +587,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new PhoneNumber();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -597,11 +605,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override PhoneNumber BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -725,7 +733,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class Addresses : pb::GeneratedMessageLite<Addresses, Addresses.Builder> {

         private Addresses() { }

-        private static readonly Addresses defaultInstance = new Builder().BuildPartial();

+        private static readonly Addresses defaultInstance = new Addresses().MakeReadOnly();

         private static readonly string[] _addressesFieldNames = new string[] { "address", "address2", "city", "state", "zip" };

         private static readonly uint[] _addressesFieldTags = new uint[] { 10, 18, 26, 34, 45 };

         public static Addresses DefaultInstance {

@@ -733,7 +741,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override Addresses DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override Addresses ThisMessage {

@@ -909,6 +917,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Addresses ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private Addresses MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -925,22 +937,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new Addresses();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(Addresses cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          Addresses result;

+          private bool resultIsReadOnly;

+          private Addresses result;

           

           private Addresses PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               Addresses original = result;

               result = new Addresses();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -955,13 +967,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new Addresses();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -973,11 +985,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Addresses BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -1399,6 +1411,13 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestInteropPersonLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestInteropPersonLite MakeReadOnly() {

+      codes_.MakeReadOnly();

+      phone_.MakeReadOnly();

+      addresses_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1415,22 +1434,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestInteropPersonLite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestInteropPersonLite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestInteropPersonLite result;

+      private bool resultIsReadOnly;

+      private TestInteropPersonLite result;

       

       private TestInteropPersonLite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestInteropPersonLite original = result;

           result = new TestInteropPersonLite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1445,13 +1464,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestInteropPersonLite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1463,14 +1482,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestInteropPersonLite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.codes_.MakeReadOnly();

-        result.phone_.MakeReadOnly();

-        result.addresses_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -1759,7 +1775,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestInteropEmployeeIdLite : pb::GeneratedMessageLite<TestInteropEmployeeIdLite, TestInteropEmployeeIdLite.Builder> {

     private TestInteropEmployeeIdLite() { }

-    private static readonly TestInteropEmployeeIdLite defaultInstance = new Builder().BuildPartial();

+    private static readonly TestInteropEmployeeIdLite defaultInstance = new TestInteropEmployeeIdLite().MakeReadOnly();

     private static readonly string[] _testInteropEmployeeIdLiteFieldNames = new string[] { "number" };

     private static readonly uint[] _testInteropEmployeeIdLiteFieldTags = new uint[] { 10 };

     public static TestInteropEmployeeIdLite DefaultInstance {

@@ -1767,7 +1783,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestInteropEmployeeIdLite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestInteropEmployeeIdLite ThisMessage {

@@ -1863,6 +1879,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestInteropEmployeeIdLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestInteropEmployeeIdLite MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1879,22 +1899,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestInteropEmployeeIdLite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestInteropEmployeeIdLite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestInteropEmployeeIdLite result;

+      private bool resultIsReadOnly;

+      private TestInteropEmployeeIdLite result;

       

       private TestInteropEmployeeIdLite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestInteropEmployeeIdLite original = result;

           result = new TestInteropEmployeeIdLite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -1909,13 +1929,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestInteropEmployeeIdLite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -1927,11 +1947,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestInteropEmployeeIdLite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportLiteProtoFile.cs b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportLiteProtoFile.cs
index 98672cf2..f31fdd89 100644
--- a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportLiteProtoFile.cs
+++ b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportLiteProtoFile.cs
@@ -44,7 +44,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class ImportMessageLite : pb::GeneratedMessageLite<ImportMessageLite, ImportMessageLite.Builder> {

     private ImportMessageLite() { }

-    private static readonly ImportMessageLite defaultInstance = new Builder().BuildPartial();

+    private static readonly ImportMessageLite defaultInstance = new ImportMessageLite().MakeReadOnly();

     private static readonly string[] _importMessageLiteFieldNames = new string[] { "d" };

     private static readonly uint[] _importMessageLiteFieldTags = new uint[] { 8 };

     public static ImportMessageLite DefaultInstance {

@@ -52,7 +52,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override ImportMessageLite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override ImportMessageLite ThisMessage {

@@ -147,6 +147,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static ImportMessageLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private ImportMessageLite MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -162,22 +166,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new ImportMessageLite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(ImportMessageLite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      ImportMessageLite result;

+      private bool resultIsReadOnly;

+      private ImportMessageLite result;

       

       private ImportMessageLite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           ImportMessageLite original = result;

           result = new ImportMessageLite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -192,13 +196,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new ImportMessageLite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -210,11 +214,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ImportMessageLite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportProtoFile.cs b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportProtoFile.cs
index 086dda1d..c12aef8e 100644
--- a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportProtoFile.cs
+++ b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportProtoFile.cs
@@ -72,7 +72,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class ImportMessage : pb::GeneratedMessage<ImportMessage, ImportMessage.Builder> {

     private ImportMessage() { }

-    private static readonly ImportMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly ImportMessage defaultInstance = new ImportMessage().MakeReadOnly();

     private static readonly string[] _importMessageFieldNames = new string[] { "d" };

     private static readonly uint[] _importMessageFieldTags = new uint[] { 8 };

     public static ImportMessage DefaultInstance {

@@ -80,7 +80,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override ImportMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override ImportMessage ThisMessage {

@@ -166,6 +166,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static ImportMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private ImportMessage MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -181,22 +185,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new ImportMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(ImportMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      ImportMessage result;

+      private bool resultIsReadOnly;

+      private ImportMessage result;

       

       private ImportMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           ImportMessage original = result;

           result = new ImportMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -211,13 +215,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new ImportMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -233,11 +237,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ImportMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteImportNonLiteProtoFile.cs b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteImportNonLiteProtoFile.cs
index 2b059eb2..0e7b5070 100644
--- a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteImportNonLiteProtoFile.cs
+++ b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteImportNonLiteProtoFile.cs
@@ -33,7 +33,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestLiteImportsNonlite : pb::GeneratedMessageLite<TestLiteImportsNonlite, TestLiteImportsNonlite.Builder> {

     private TestLiteImportsNonlite() { }

-    private static readonly TestLiteImportsNonlite defaultInstance = new Builder().BuildPartial();

+    private static readonly TestLiteImportsNonlite defaultInstance = new TestLiteImportsNonlite().MakeReadOnly();

     private static readonly string[] _testLiteImportsNonliteFieldNames = new string[] { "message" };

     private static readonly uint[] _testLiteImportsNonliteFieldTags = new uint[] { 10 };

     public static TestLiteImportsNonlite DefaultInstance {

@@ -41,7 +41,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestLiteImportsNonlite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestLiteImportsNonlite ThisMessage {

@@ -50,12 +50,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int MessageFieldNumber = 1;

     private bool hasMessage;

-    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes message_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes message_;

     public bool HasMessage {

       get { return hasMessage; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestAllTypes Message {

-      get { return message_; }

+      get { return message_ ?? global::Google.ProtocolBuffers.TestProtos.TestAllTypes.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -136,6 +136,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestLiteImportsNonlite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestLiteImportsNonlite MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -151,22 +155,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestLiteImportsNonlite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestLiteImportsNonlite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestLiteImportsNonlite result;

+      private bool resultIsReadOnly;

+      private TestLiteImportsNonlite result;

       

       private TestLiteImportsNonlite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestLiteImportsNonlite original = result;

           result = new TestLiteImportsNonlite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -181,13 +185,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestLiteImportsNonlite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -199,11 +203,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestLiteImportsNonlite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -305,7 +309,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearMessage() {

         PrepareBuilder();

         result.hasMessage = false;

-        result.message_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.DefaultInstance;

+        result.message_ = null;

         return this;

       }

     }

diff --git a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteProtoFile.cs b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteProtoFile.cs
index 90c792b3..c682ad1c 100644
--- a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteProtoFile.cs
+++ b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteProtoFile.cs
@@ -1229,7 +1229,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestAllTypesLite : pb::GeneratedMessageLite<TestAllTypesLite, TestAllTypesLite.Builder> {

     private TestAllTypesLite() { }

-    private static readonly TestAllTypesLite defaultInstance = new Builder().BuildPartial();

+    private static readonly TestAllTypesLite defaultInstance = new TestAllTypesLite().MakeReadOnly();

     private static readonly string[] _testAllTypesLiteFieldNames = new string[] { "default_bool", "default_bytes", "default_cord", "default_double", "default_fixed32", "default_fixed64", "default_float", "default_foreign_enum", "default_import_enum", "default_int32", "default_int64", "default_nested_enum", "default_sfixed32", "default_sfixed64", "default_sint32", "default_sint64", "default_string", "default_string_piece", "default_uint32", "default_uint64", "optional_bool", "optional_bytes", "optional_cord", "optional_double", "optional_fixed32", "optional_fixed64", "optional_float", "optional_foreign_enum", "optional_foreign_message", "optional_import_enum", "optional_import_message", "optional_int32", "optional_int64", "optional_nested_enum", "optional_nested_message", "optional_sfixed32", "optional_sfixed64", "optional_sint32", "optional_sint64", "optional_string", "optional_string_piece", "optional_uint32", "optional_uint64", "optionalgroup", "repeated_bool", "repeated_bytes", "repeated_cord", "repeated_double", "repeated_fixed32", "repeated_fixed64", "repeated_float", "repeated_foreign_enum", "repeated_foreign_message", "repeated_import_enum", "repeated_import_message", "repeated_int32", "repeated_int64", "repeated_nested_enum", "repeated_nested_message", "repeated_sfixed32", "repeated_sfixed64", "repeated_sint32", "repeated_sint64", "repeated_string", "repeated_string_piece", "repeated_uint32", "repeated_uint64", "repeatedgroup" };

     private static readonly uint[] _testAllTypesLiteFieldTags = new uint[] { 584, 602, 682, 577, 541, 545, 573, 656, 664, 488, 496, 648, 557, 561, 520, 528, 594, 674, 504, 512, 104, 122, 202, 97, 61, 65, 93, 176, 154, 184, 162, 8, 16, 168, 146, 77, 81, 40, 48, 114, 194, 24, 32, 131, 344, 362, 442, 337, 301, 305, 333, 416, 394, 424, 402, 248, 256, 408, 386, 317, 321, 280, 288, 354, 434, 264, 272, 371 };

     public static TestAllTypesLite DefaultInstance {

@@ -1237,7 +1237,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestAllTypesLite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestAllTypesLite ThisMessage {

@@ -1262,7 +1262,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class NestedMessage : pb::GeneratedMessageLite<NestedMessage, NestedMessage.Builder> {

         private NestedMessage() { }

-        private static readonly NestedMessage defaultInstance = new Builder().BuildPartial();

+        private static readonly NestedMessage defaultInstance = new NestedMessage().MakeReadOnly();

         private static readonly string[] _nestedMessageFieldNames = new string[] { "bb" };

         private static readonly uint[] _nestedMessageFieldTags = new uint[] { 8 };

         public static NestedMessage DefaultInstance {

@@ -1270,7 +1270,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override NestedMessage DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override NestedMessage ThisMessage {

@@ -1365,6 +1365,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static NestedMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private NestedMessage MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1380,22 +1384,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new NestedMessage();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(NestedMessage cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          NestedMessage result;

+          private bool resultIsReadOnly;

+          private NestedMessage result;

           

           private NestedMessage PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               NestedMessage original = result;

               result = new NestedMessage();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -1410,13 +1414,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new NestedMessage();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -1428,11 +1432,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override NestedMessage BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -1523,7 +1527,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class OptionalGroup : pb::GeneratedMessageLite<OptionalGroup, OptionalGroup.Builder> {

         private OptionalGroup() { }

-        private static readonly OptionalGroup defaultInstance = new Builder().BuildPartial();

+        private static readonly OptionalGroup defaultInstance = new OptionalGroup().MakeReadOnly();

         private static readonly string[] _optionalGroupFieldNames = new string[] { "a" };

         private static readonly uint[] _optionalGroupFieldTags = new uint[] { 136 };

         public static OptionalGroup DefaultInstance {

@@ -1531,7 +1535,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override OptionalGroup DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override OptionalGroup ThisMessage {

@@ -1626,6 +1630,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static OptionalGroup ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private OptionalGroup MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1641,22 +1649,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new OptionalGroup();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(OptionalGroup cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          OptionalGroup result;

+          private bool resultIsReadOnly;

+          private OptionalGroup result;

           

           private OptionalGroup PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               OptionalGroup original = result;

               result = new OptionalGroup();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -1671,13 +1679,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new OptionalGroup();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -1689,11 +1697,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override OptionalGroup BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -1784,7 +1792,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class RepeatedGroup : pb::GeneratedMessageLite<RepeatedGroup, RepeatedGroup.Builder> {

         private RepeatedGroup() { }

-        private static readonly RepeatedGroup defaultInstance = new Builder().BuildPartial();

+        private static readonly RepeatedGroup defaultInstance = new RepeatedGroup().MakeReadOnly();

         private static readonly string[] _repeatedGroupFieldNames = new string[] { "a" };

         private static readonly uint[] _repeatedGroupFieldTags = new uint[] { 376 };

         public static RepeatedGroup DefaultInstance {

@@ -1792,7 +1800,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override RepeatedGroup DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override RepeatedGroup ThisMessage {

@@ -1887,6 +1895,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static RepeatedGroup ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private RepeatedGroup MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1902,22 +1914,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new RepeatedGroup();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(RepeatedGroup cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          RepeatedGroup result;

+          private bool resultIsReadOnly;

+          private RepeatedGroup result;

           

           private RepeatedGroup PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               RepeatedGroup original = result;

               result = new RepeatedGroup();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -1932,13 +1944,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new RepeatedGroup();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -1950,11 +1962,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override RepeatedGroup BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -2199,42 +2211,42 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int OptionalGroupFieldNumber = 16;

     private bool hasOptionalGroup;

-    private global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.OptionalGroup optionalGroup_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.OptionalGroup.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.OptionalGroup optionalGroup_;

     public bool HasOptionalGroup {

       get { return hasOptionalGroup; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.OptionalGroup OptionalGroup {

-      get { return optionalGroup_; }

+      get { return optionalGroup_ ?? global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.OptionalGroup.DefaultInstance; }

     }

     

     public const int OptionalNestedMessageFieldNumber = 18;

     private bool hasOptionalNestedMessage;

-    private global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage optionalNestedMessage_;

     public bool HasOptionalNestedMessage {

       get { return hasOptionalNestedMessage; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage OptionalNestedMessage {

-      get { return optionalNestedMessage_; }

+      get { return optionalNestedMessage_ ?? global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage.DefaultInstance; }

     }

     

     public const int OptionalForeignMessageFieldNumber = 19;

     private bool hasOptionalForeignMessage;

-    private global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite optionalForeignMessage_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite optionalForeignMessage_;

     public bool HasOptionalForeignMessage {

       get { return hasOptionalForeignMessage; }

     }

     public global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite OptionalForeignMessage {

-      get { return optionalForeignMessage_; }

+      get { return optionalForeignMessage_ ?? global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite.DefaultInstance; }

     }

     

     public const int OptionalImportMessageFieldNumber = 20;

     private bool hasOptionalImportMessage;

-    private global::Google.ProtocolBuffers.TestProtos.ImportMessageLite optionalImportMessage_ = global::Google.ProtocolBuffers.TestProtos.ImportMessageLite.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.ImportMessageLite optionalImportMessage_;

     public bool HasOptionalImportMessage {

       get { return hasOptionalImportMessage; }

     }

     public global::Google.ProtocolBuffers.TestProtos.ImportMessageLite OptionalImportMessage {

-      get { return optionalImportMessage_; }

+      get { return optionalImportMessage_ ?? global::Google.ProtocolBuffers.TestProtos.ImportMessageLite.DefaultInstance; }

     }

     

     public const int OptionalNestedEnumFieldNumber = 21;

@@ -3632,6 +3644,34 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestAllTypesLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestAllTypesLite MakeReadOnly() {

+      repeatedInt32_.MakeReadOnly();

+      repeatedInt64_.MakeReadOnly();

+      repeatedUint32_.MakeReadOnly();

+      repeatedUint64_.MakeReadOnly();

+      repeatedSint32_.MakeReadOnly();

+      repeatedSint64_.MakeReadOnly();

+      repeatedFixed32_.MakeReadOnly();

+      repeatedFixed64_.MakeReadOnly();

+      repeatedSfixed32_.MakeReadOnly();

+      repeatedSfixed64_.MakeReadOnly();

+      repeatedFloat_.MakeReadOnly();

+      repeatedDouble_.MakeReadOnly();

+      repeatedBool_.MakeReadOnly();

+      repeatedString_.MakeReadOnly();

+      repeatedBytes_.MakeReadOnly();

+      repeatedGroup_.MakeReadOnly();

+      repeatedNestedMessage_.MakeReadOnly();

+      repeatedForeignMessage_.MakeReadOnly();

+      repeatedImportMessage_.MakeReadOnly();

+      repeatedNestedEnum_.MakeReadOnly();

+      repeatedForeignEnum_.MakeReadOnly();

+      repeatedImportEnum_.MakeReadOnly();

+      repeatedStringPiece_.MakeReadOnly();

+      repeatedCord_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -3647,22 +3687,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestAllTypesLite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestAllTypesLite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestAllTypesLite result;

+      private bool resultIsReadOnly;

+      private TestAllTypesLite result;

       

       private TestAllTypesLite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestAllTypesLite original = result;

           result = new TestAllTypesLite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -3677,13 +3717,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestAllTypesLite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -3695,35 +3735,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestAllTypesLite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.repeatedInt32_.MakeReadOnly();

-        result.repeatedInt64_.MakeReadOnly();

-        result.repeatedUint32_.MakeReadOnly();

-        result.repeatedUint64_.MakeReadOnly();

-        result.repeatedSint32_.MakeReadOnly();

-        result.repeatedSint64_.MakeReadOnly();

-        result.repeatedFixed32_.MakeReadOnly();

-        result.repeatedFixed64_.MakeReadOnly();

-        result.repeatedSfixed32_.MakeReadOnly();

-        result.repeatedSfixed64_.MakeReadOnly();

-        result.repeatedFloat_.MakeReadOnly();

-        result.repeatedDouble_.MakeReadOnly();

-        result.repeatedBool_.MakeReadOnly();

-        result.repeatedString_.MakeReadOnly();

-        result.repeatedBytes_.MakeReadOnly();

-        result.repeatedGroup_.MakeReadOnly();

-        result.repeatedNestedMessage_.MakeReadOnly();

-        result.repeatedForeignMessage_.MakeReadOnly();

-        result.repeatedImportMessage_.MakeReadOnly();

-        result.repeatedNestedEnum_.MakeReadOnly();

-        result.repeatedForeignEnum_.MakeReadOnly();

-        result.repeatedImportEnum_.MakeReadOnly();

-        result.repeatedStringPiece_.MakeReadOnly();

-        result.repeatedCord_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -4662,7 +4678,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalGroup() {

         PrepareBuilder();

         result.hasOptionalGroup = false;

-        result.optionalGroup_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.OptionalGroup.DefaultInstance;

+        result.optionalGroup_ = null;

         return this;

       }

       

@@ -4702,7 +4718,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalNestedMessage() {

         PrepareBuilder();

         result.hasOptionalNestedMessage = false;

-        result.optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypesLite.Types.NestedMessage.DefaultInstance;

+        result.optionalNestedMessage_ = null;

         return this;

       }

       

@@ -4742,7 +4758,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalForeignMessage() {

         PrepareBuilder();

         result.hasOptionalForeignMessage = false;

-        result.optionalForeignMessage_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessageLite.DefaultInstance;

+        result.optionalForeignMessage_ = null;

         return this;

       }

       

@@ -4782,7 +4798,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalImportMessage() {

         PrepareBuilder();

         result.hasOptionalImportMessage = false;

-        result.optionalImportMessage_ = global::Google.ProtocolBuffers.TestProtos.ImportMessageLite.DefaultInstance;

+        result.optionalImportMessage_ = null;

         return this;

       }

       

@@ -6114,7 +6130,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class ForeignMessageLite : pb::GeneratedMessageLite<ForeignMessageLite, ForeignMessageLite.Builder> {

     private ForeignMessageLite() { }

-    private static readonly ForeignMessageLite defaultInstance = new Builder().BuildPartial();

+    private static readonly ForeignMessageLite defaultInstance = new ForeignMessageLite().MakeReadOnly();

     private static readonly string[] _foreignMessageLiteFieldNames = new string[] { "c" };

     private static readonly uint[] _foreignMessageLiteFieldTags = new uint[] { 8 };

     public static ForeignMessageLite DefaultInstance {

@@ -6122,7 +6138,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override ForeignMessageLite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override ForeignMessageLite ThisMessage {

@@ -6217,6 +6233,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static ForeignMessageLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private ForeignMessageLite MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -6232,22 +6252,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new ForeignMessageLite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(ForeignMessageLite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      ForeignMessageLite result;

+      private bool resultIsReadOnly;

+      private ForeignMessageLite result;

       

       private ForeignMessageLite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           ForeignMessageLite original = result;

           result = new ForeignMessageLite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -6262,13 +6282,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new ForeignMessageLite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -6280,11 +6300,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ForeignMessageLite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -6375,7 +6395,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestPackedTypesLite : pb::GeneratedMessageLite<TestPackedTypesLite, TestPackedTypesLite.Builder> {

     private TestPackedTypesLite() { }

-    private static readonly TestPackedTypesLite defaultInstance = new Builder().BuildPartial();

+    private static readonly TestPackedTypesLite defaultInstance = new TestPackedTypesLite().MakeReadOnly();

     private static readonly string[] _testPackedTypesLiteFieldNames = new string[] { "packed_bool", "packed_double", "packed_enum", "packed_fixed32", "packed_fixed64", "packed_float", "packed_int32", "packed_int64", "packed_sfixed32", "packed_sfixed64", "packed_sint32", "packed_sint64", "packed_uint32", "packed_uint64" };

     private static readonly uint[] _testPackedTypesLiteFieldTags = new uint[] { 818, 810, 826, 770, 778, 802, 722, 730, 786, 794, 754, 762, 738, 746 };

     public static TestPackedTypesLite DefaultInstance {

@@ -6383,7 +6403,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestPackedTypesLite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestPackedTypesLite ThisMessage {

@@ -6916,6 +6936,24 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestPackedTypesLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestPackedTypesLite MakeReadOnly() {

+      packedInt32_.MakeReadOnly();

+      packedInt64_.MakeReadOnly();

+      packedUint32_.MakeReadOnly();

+      packedUint64_.MakeReadOnly();

+      packedSint32_.MakeReadOnly();

+      packedSint64_.MakeReadOnly();

+      packedFixed32_.MakeReadOnly();

+      packedFixed64_.MakeReadOnly();

+      packedSfixed32_.MakeReadOnly();

+      packedSfixed64_.MakeReadOnly();

+      packedFloat_.MakeReadOnly();

+      packedDouble_.MakeReadOnly();

+      packedBool_.MakeReadOnly();

+      packedEnum_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -6931,22 +6969,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestPackedTypesLite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestPackedTypesLite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestPackedTypesLite result;

+      private bool resultIsReadOnly;

+      private TestPackedTypesLite result;

       

       private TestPackedTypesLite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestPackedTypesLite original = result;

           result = new TestPackedTypesLite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -6961,13 +6999,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestPackedTypesLite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -6979,25 +7017,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestPackedTypesLite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.packedInt32_.MakeReadOnly();

-        result.packedInt64_.MakeReadOnly();

-        result.packedUint32_.MakeReadOnly();

-        result.packedUint64_.MakeReadOnly();

-        result.packedSint32_.MakeReadOnly();

-        result.packedSint64_.MakeReadOnly();

-        result.packedFixed32_.MakeReadOnly();

-        result.packedFixed64_.MakeReadOnly();

-        result.packedSfixed32_.MakeReadOnly();

-        result.packedSfixed64_.MakeReadOnly();

-        result.packedFloat_.MakeReadOnly();

-        result.packedDouble_.MakeReadOnly();

-        result.packedBool_.MakeReadOnly();

-        result.packedEnum_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -7614,7 +7638,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestUnpackedTypesLite : pb::GeneratedMessageLite<TestUnpackedTypesLite, TestUnpackedTypesLite.Builder> {

     private TestUnpackedTypesLite() { }

-    private static readonly TestUnpackedTypesLite defaultInstance = new Builder().BuildPartial();

+    private static readonly TestUnpackedTypesLite defaultInstance = new TestUnpackedTypesLite().MakeReadOnly();

     private static readonly string[] _testUnpackedTypesLiteFieldNames = new string[] { "unpacked_bool", "unpacked_double", "unpacked_enum", "unpacked_fixed32", "unpacked_fixed64", "unpacked_float", "unpacked_int32", "unpacked_int64", "unpacked_sfixed32", "unpacked_sfixed64", "unpacked_sint32", "unpacked_sint64", "unpacked_uint32", "unpacked_uint64" };

     private static readonly uint[] _testUnpackedTypesLiteFieldTags = new uint[] { 816, 809, 824, 773, 777, 805, 720, 728, 789, 793, 752, 760, 736, 744 };

     public static TestUnpackedTypesLite DefaultInstance {

@@ -7622,7 +7646,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestUnpackedTypesLite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestUnpackedTypesLite ThisMessage {

@@ -8100,6 +8124,24 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestUnpackedTypesLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestUnpackedTypesLite MakeReadOnly() {

+      unpackedInt32_.MakeReadOnly();

+      unpackedInt64_.MakeReadOnly();

+      unpackedUint32_.MakeReadOnly();

+      unpackedUint64_.MakeReadOnly();

+      unpackedSint32_.MakeReadOnly();

+      unpackedSint64_.MakeReadOnly();

+      unpackedFixed32_.MakeReadOnly();

+      unpackedFixed64_.MakeReadOnly();

+      unpackedSfixed32_.MakeReadOnly();

+      unpackedSfixed64_.MakeReadOnly();

+      unpackedFloat_.MakeReadOnly();

+      unpackedDouble_.MakeReadOnly();

+      unpackedBool_.MakeReadOnly();

+      unpackedEnum_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -8115,22 +8157,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestUnpackedTypesLite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestUnpackedTypesLite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestUnpackedTypesLite result;

+      private bool resultIsReadOnly;

+      private TestUnpackedTypesLite result;

       

       private TestUnpackedTypesLite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestUnpackedTypesLite original = result;

           result = new TestUnpackedTypesLite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -8145,13 +8187,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestUnpackedTypesLite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -8163,25 +8205,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestUnpackedTypesLite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.unpackedInt32_.MakeReadOnly();

-        result.unpackedInt64_.MakeReadOnly();

-        result.unpackedUint32_.MakeReadOnly();

-        result.unpackedUint64_.MakeReadOnly();

-        result.unpackedSint32_.MakeReadOnly();

-        result.unpackedSint64_.MakeReadOnly();

-        result.unpackedFixed32_.MakeReadOnly();

-        result.unpackedFixed64_.MakeReadOnly();

-        result.unpackedSfixed32_.MakeReadOnly();

-        result.unpackedSfixed64_.MakeReadOnly();

-        result.unpackedFloat_.MakeReadOnly();

-        result.unpackedDouble_.MakeReadOnly();

-        result.unpackedBool_.MakeReadOnly();

-        result.unpackedEnum_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -8798,7 +8826,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestAllExtensionsLite : pb::ExtendableMessageLite<TestAllExtensionsLite, TestAllExtensionsLite.Builder> {

     private TestAllExtensionsLite() { }

-    private static readonly TestAllExtensionsLite defaultInstance = new Builder().BuildPartial();

+    private static readonly TestAllExtensionsLite defaultInstance = new TestAllExtensionsLite().MakeReadOnly();

     private static readonly string[] _testAllExtensionsLiteFieldNames = new string[] {  };

     private static readonly uint[] _testAllExtensionsLiteFieldTags = new uint[] {  };

     public static TestAllExtensionsLite DefaultInstance {

@@ -8806,7 +8834,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestAllExtensionsLite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestAllExtensionsLite ThisMessage {

@@ -8889,6 +8917,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestAllExtensionsLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestAllExtensionsLite MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -8904,22 +8936,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestAllExtensionsLite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestAllExtensionsLite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestAllExtensionsLite result;

+      private bool resultIsReadOnly;

+      private TestAllExtensionsLite result;

       

       private TestAllExtensionsLite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestAllExtensionsLite original = result;

           result = new TestAllExtensionsLite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -8934,13 +8966,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestAllExtensionsLite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -8952,11 +8984,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestAllExtensionsLite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -9021,7 +9053,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class OptionalGroup_extension_lite : pb::GeneratedMessageLite<OptionalGroup_extension_lite, OptionalGroup_extension_lite.Builder> {

     private OptionalGroup_extension_lite() { }

-    private static readonly OptionalGroup_extension_lite defaultInstance = new Builder().BuildPartial();

+    private static readonly OptionalGroup_extension_lite defaultInstance = new OptionalGroup_extension_lite().MakeReadOnly();

     private static readonly string[] _optionalGroupExtensionLiteFieldNames = new string[] { "a" };

     private static readonly uint[] _optionalGroupExtensionLiteFieldTags = new uint[] { 136 };

     public static OptionalGroup_extension_lite DefaultInstance {

@@ -9029,7 +9061,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override OptionalGroup_extension_lite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override OptionalGroup_extension_lite ThisMessage {

@@ -9124,6 +9156,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static OptionalGroup_extension_lite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private OptionalGroup_extension_lite MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -9139,22 +9175,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new OptionalGroup_extension_lite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(OptionalGroup_extension_lite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      OptionalGroup_extension_lite result;

+      private bool resultIsReadOnly;

+      private OptionalGroup_extension_lite result;

       

       private OptionalGroup_extension_lite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           OptionalGroup_extension_lite original = result;

           result = new OptionalGroup_extension_lite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -9169,13 +9205,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new OptionalGroup_extension_lite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -9187,11 +9223,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OptionalGroup_extension_lite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -9282,7 +9318,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class RepeatedGroup_extension_lite : pb::GeneratedMessageLite<RepeatedGroup_extension_lite, RepeatedGroup_extension_lite.Builder> {

     private RepeatedGroup_extension_lite() { }

-    private static readonly RepeatedGroup_extension_lite defaultInstance = new Builder().BuildPartial();

+    private static readonly RepeatedGroup_extension_lite defaultInstance = new RepeatedGroup_extension_lite().MakeReadOnly();

     private static readonly string[] _repeatedGroupExtensionLiteFieldNames = new string[] { "a" };

     private static readonly uint[] _repeatedGroupExtensionLiteFieldTags = new uint[] { 376 };

     public static RepeatedGroup_extension_lite DefaultInstance {

@@ -9290,7 +9326,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override RepeatedGroup_extension_lite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override RepeatedGroup_extension_lite ThisMessage {

@@ -9385,6 +9421,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static RepeatedGroup_extension_lite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private RepeatedGroup_extension_lite MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -9400,22 +9440,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new RepeatedGroup_extension_lite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(RepeatedGroup_extension_lite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      RepeatedGroup_extension_lite result;

+      private bool resultIsReadOnly;

+      private RepeatedGroup_extension_lite result;

       

       private RepeatedGroup_extension_lite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           RepeatedGroup_extension_lite original = result;

           result = new RepeatedGroup_extension_lite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -9430,13 +9470,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new RepeatedGroup_extension_lite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -9448,11 +9488,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override RepeatedGroup_extension_lite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -9543,7 +9583,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestPackedExtensionsLite : pb::ExtendableMessageLite<TestPackedExtensionsLite, TestPackedExtensionsLite.Builder> {

     private TestPackedExtensionsLite() { }

-    private static readonly TestPackedExtensionsLite defaultInstance = new Builder().BuildPartial();

+    private static readonly TestPackedExtensionsLite defaultInstance = new TestPackedExtensionsLite().MakeReadOnly();

     private static readonly string[] _testPackedExtensionsLiteFieldNames = new string[] {  };

     private static readonly uint[] _testPackedExtensionsLiteFieldTags = new uint[] {  };

     public static TestPackedExtensionsLite DefaultInstance {

@@ -9551,7 +9591,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestPackedExtensionsLite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestPackedExtensionsLite ThisMessage {

@@ -9634,6 +9674,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestPackedExtensionsLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestPackedExtensionsLite MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -9649,22 +9693,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestPackedExtensionsLite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestPackedExtensionsLite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestPackedExtensionsLite result;

+      private bool resultIsReadOnly;

+      private TestPackedExtensionsLite result;

       

       private TestPackedExtensionsLite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestPackedExtensionsLite original = result;

           result = new TestPackedExtensionsLite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -9679,13 +9723,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestPackedExtensionsLite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -9697,11 +9741,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestPackedExtensionsLite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -9766,7 +9810,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestUnpackedExtensionsLite : pb::ExtendableMessageLite<TestUnpackedExtensionsLite, TestUnpackedExtensionsLite.Builder> {

     private TestUnpackedExtensionsLite() { }

-    private static readonly TestUnpackedExtensionsLite defaultInstance = new Builder().BuildPartial();

+    private static readonly TestUnpackedExtensionsLite defaultInstance = new TestUnpackedExtensionsLite().MakeReadOnly();

     private static readonly string[] _testUnpackedExtensionsLiteFieldNames = new string[] {  };

     private static readonly uint[] _testUnpackedExtensionsLiteFieldTags = new uint[] {  };

     public static TestUnpackedExtensionsLite DefaultInstance {

@@ -9774,7 +9818,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestUnpackedExtensionsLite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestUnpackedExtensionsLite ThisMessage {

@@ -9857,6 +9901,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestUnpackedExtensionsLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestUnpackedExtensionsLite MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -9872,22 +9920,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestUnpackedExtensionsLite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestUnpackedExtensionsLite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestUnpackedExtensionsLite result;

+      private bool resultIsReadOnly;

+      private TestUnpackedExtensionsLite result;

       

       private TestUnpackedExtensionsLite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestUnpackedExtensionsLite original = result;

           result = new TestUnpackedExtensionsLite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -9902,13 +9950,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestUnpackedExtensionsLite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -9920,11 +9968,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestUnpackedExtensionsLite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -9989,7 +10037,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestNestedExtensionLite : pb::GeneratedMessageLite<TestNestedExtensionLite, TestNestedExtensionLite.Builder> {

     private TestNestedExtensionLite() { }

-    private static readonly TestNestedExtensionLite defaultInstance = new Builder().BuildPartial();

+    private static readonly TestNestedExtensionLite defaultInstance = new TestNestedExtensionLite().MakeReadOnly();

     private static readonly string[] _testNestedExtensionLiteFieldNames = new string[] {  };

     private static readonly uint[] _testNestedExtensionLiteFieldTags = new uint[] {  };

     public static TestNestedExtensionLite DefaultInstance {

@@ -9997,7 +10045,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestNestedExtensionLite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestNestedExtensionLite ThisMessage {

@@ -10075,6 +10123,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestNestedExtensionLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestNestedExtensionLite MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -10090,22 +10142,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestNestedExtensionLite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestNestedExtensionLite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestNestedExtensionLite result;

+      private bool resultIsReadOnly;

+      private TestNestedExtensionLite result;

       

       private TestNestedExtensionLite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestNestedExtensionLite original = result;

           result = new TestNestedExtensionLite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -10120,13 +10172,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestNestedExtensionLite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -10138,11 +10190,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestNestedExtensionLite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -10206,7 +10258,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestDeprecatedLite : pb::GeneratedMessageLite<TestDeprecatedLite, TestDeprecatedLite.Builder> {

     private TestDeprecatedLite() { }

-    private static readonly TestDeprecatedLite defaultInstance = new Builder().BuildPartial();

+    private static readonly TestDeprecatedLite defaultInstance = new TestDeprecatedLite().MakeReadOnly();

     private static readonly string[] _testDeprecatedLiteFieldNames = new string[] { "deprecated_field" };

     private static readonly uint[] _testDeprecatedLiteFieldTags = new uint[] { 8 };

     public static TestDeprecatedLite DefaultInstance {

@@ -10214,7 +10266,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestDeprecatedLite DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestDeprecatedLite ThisMessage {

@@ -10311,6 +10363,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestDeprecatedLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestDeprecatedLite MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -10326,22 +10382,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestDeprecatedLite();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestDeprecatedLite cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestDeprecatedLite result;

+      private bool resultIsReadOnly;

+      private TestDeprecatedLite result;

       

       private TestDeprecatedLite PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestDeprecatedLite original = result;

           result = new TestDeprecatedLite();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -10356,13 +10412,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestDeprecatedLite();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -10374,11 +10430,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestDeprecatedLite BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

diff --git a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestProtoFile.cs b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestProtoFile.cs
index 547d712a..35064953 100644
--- a/src/ProtocolBuffersLite.Test/TestProtos/UnitTestProtoFile.cs
+++ b/src/ProtocolBuffersLite.Test/TestProtos/UnitTestProtoFile.cs
@@ -1123,7 +1123,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestAllTypes : pb::GeneratedMessage<TestAllTypes, TestAllTypes.Builder> {

     private TestAllTypes() { }

-    private static readonly TestAllTypes defaultInstance = new Builder().BuildPartial();

+    private static readonly TestAllTypes defaultInstance = new TestAllTypes().MakeReadOnly();

     private static readonly string[] _testAllTypesFieldNames = new string[] { "default_bool", "default_bytes", "default_cord", "default_double", "default_fixed32", "default_fixed64", "default_float", "default_foreign_enum", "default_import_enum", "default_int32", "default_int64", "default_nested_enum", "default_sfixed32", "default_sfixed64", "default_sint32", "default_sint64", "default_string", "default_string_piece", "default_uint32", "default_uint64", "optional_bool", "optional_bytes", "optional_cord", "optional_double", "optional_fixed32", "optional_fixed64", "optional_float", "optional_foreign_enum", "optional_foreign_message", "optional_import_enum", "optional_import_message", "optional_int32", "optional_int64", "optional_nested_enum", "optional_nested_message", "optional_sfixed32", "optional_sfixed64", "optional_sint32", "optional_sint64", "optional_string", "optional_string_piece", "optional_uint32", "optional_uint64", "optionalgroup", "repeated_bool", "repeated_bytes", "repeated_cord", "repeated_double", "repeated_fixed32", "repeated_fixed64", "repeated_float", "repeated_foreign_enum", "repeated_foreign_message", "repeated_import_enum", "repeated_import_message", "repeated_int32", "repeated_int64", "repeated_nested_enum", "repeated_nested_message", "repeated_sfixed32", "repeated_sfixed64", "repeated_sint32", "repeated_sint64", "repeated_string", "repeated_string_piece", "repeated_uint32", "repeated_uint64", "repeatedgroup" };

     private static readonly uint[] _testAllTypesFieldTags = new uint[] { 584, 602, 682, 577, 541, 545, 573, 656, 664, 488, 496, 648, 557, 561, 520, 528, 594, 674, 504, 512, 104, 122, 202, 97, 61, 65, 93, 176, 154, 184, 162, 8, 16, 168, 146, 77, 81, 40, 48, 114, 194, 24, 32, 131, 344, 362, 442, 337, 301, 305, 333, 416, 394, 424, 402, 248, 256, 408, 386, 317, 321, 280, 288, 354, 434, 264, 272, 371 };

     public static TestAllTypes DefaultInstance {

@@ -1131,7 +1131,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestAllTypes DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestAllTypes ThisMessage {

@@ -1164,7 +1164,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class NestedMessage : pb::GeneratedMessage<NestedMessage, NestedMessage.Builder> {

         private NestedMessage() { }

-        private static readonly NestedMessage defaultInstance = new Builder().BuildPartial();

+        private static readonly NestedMessage defaultInstance = new NestedMessage().MakeReadOnly();

         private static readonly string[] _nestedMessageFieldNames = new string[] { "bb" };

         private static readonly uint[] _nestedMessageFieldTags = new uint[] { 8 };

         public static NestedMessage DefaultInstance {

@@ -1172,7 +1172,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override NestedMessage DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override NestedMessage ThisMessage {

@@ -1258,6 +1258,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static NestedMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private NestedMessage MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1273,22 +1277,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new NestedMessage();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(NestedMessage cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          NestedMessage result;

+          private bool resultIsReadOnly;

+          private NestedMessage result;

           

           private NestedMessage PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               NestedMessage original = result;

               result = new NestedMessage();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -1303,13 +1307,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new NestedMessage();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -1325,11 +1329,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override NestedMessage BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -1434,7 +1438,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class OptionalGroup : pb::GeneratedMessage<OptionalGroup, OptionalGroup.Builder> {

         private OptionalGroup() { }

-        private static readonly OptionalGroup defaultInstance = new Builder().BuildPartial();

+        private static readonly OptionalGroup defaultInstance = new OptionalGroup().MakeReadOnly();

         private static readonly string[] _optionalGroupFieldNames = new string[] { "a" };

         private static readonly uint[] _optionalGroupFieldTags = new uint[] { 136 };

         public static OptionalGroup DefaultInstance {

@@ -1442,7 +1446,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override OptionalGroup DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override OptionalGroup ThisMessage {

@@ -1528,6 +1532,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static OptionalGroup ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private OptionalGroup MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1543,22 +1551,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new OptionalGroup();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(OptionalGroup cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          OptionalGroup result;

+          private bool resultIsReadOnly;

+          private OptionalGroup result;

           

           private OptionalGroup PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               OptionalGroup original = result;

               result = new OptionalGroup();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -1573,13 +1581,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new OptionalGroup();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -1595,11 +1603,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override OptionalGroup BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -1704,7 +1712,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class RepeatedGroup : pb::GeneratedMessage<RepeatedGroup, RepeatedGroup.Builder> {

         private RepeatedGroup() { }

-        private static readonly RepeatedGroup defaultInstance = new Builder().BuildPartial();

+        private static readonly RepeatedGroup defaultInstance = new RepeatedGroup().MakeReadOnly();

         private static readonly string[] _repeatedGroupFieldNames = new string[] { "a" };

         private static readonly uint[] _repeatedGroupFieldTags = new uint[] { 376 };

         public static RepeatedGroup DefaultInstance {

@@ -1712,7 +1720,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override RepeatedGroup DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override RepeatedGroup ThisMessage {

@@ -1798,6 +1806,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static RepeatedGroup ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private RepeatedGroup MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -1813,22 +1825,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new RepeatedGroup();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(RepeatedGroup cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          RepeatedGroup result;

+          private bool resultIsReadOnly;

+          private RepeatedGroup result;

           

           private RepeatedGroup PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               RepeatedGroup original = result;

               result = new RepeatedGroup();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -1843,13 +1855,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new RepeatedGroup();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -1865,11 +1877,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override RepeatedGroup BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -2128,42 +2140,42 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int OptionalGroupFieldNumber = 16;

     private bool hasOptionalGroup;

-    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup optionalGroup_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup optionalGroup_;

     public bool HasOptionalGroup {

       get { return hasOptionalGroup; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup OptionalGroup {

-      get { return optionalGroup_; }

+      get { return optionalGroup_ ?? global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.DefaultInstance; }

     }

     

     public const int OptionalNestedMessageFieldNumber = 18;

     private bool hasOptionalNestedMessage;

-    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage optionalNestedMessage_;

     public bool HasOptionalNestedMessage {

       get { return hasOptionalNestedMessage; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage OptionalNestedMessage {

-      get { return optionalNestedMessage_; }

+      get { return optionalNestedMessage_ ?? global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance; }

     }

     

     public const int OptionalForeignMessageFieldNumber = 19;

     private bool hasOptionalForeignMessage;

-    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage optionalForeignMessage_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage optionalForeignMessage_;

     public bool HasOptionalForeignMessage {

       get { return hasOptionalForeignMessage; }

     }

     public global::Google.ProtocolBuffers.TestProtos.ForeignMessage OptionalForeignMessage {

-      get { return optionalForeignMessage_; }

+      get { return optionalForeignMessage_ ?? global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance; }

     }

     

     public const int OptionalImportMessageFieldNumber = 20;

     private bool hasOptionalImportMessage;

-    private global::Google.ProtocolBuffers.TestProtos.ImportMessage optionalImportMessage_ = global::Google.ProtocolBuffers.TestProtos.ImportMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.ImportMessage optionalImportMessage_;

     public bool HasOptionalImportMessage {

       get { return hasOptionalImportMessage; }

     }

     public global::Google.ProtocolBuffers.TestProtos.ImportMessage OptionalImportMessage {

-      get { return optionalImportMessage_; }

+      get { return optionalImportMessage_ ?? global::Google.ProtocolBuffers.TestProtos.ImportMessage.DefaultInstance; }

     }

     

     public const int OptionalNestedEnumFieldNumber = 21;

@@ -3271,6 +3283,34 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestAllTypes ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestAllTypes MakeReadOnly() {

+      repeatedInt32_.MakeReadOnly();

+      repeatedInt64_.MakeReadOnly();

+      repeatedUint32_.MakeReadOnly();

+      repeatedUint64_.MakeReadOnly();

+      repeatedSint32_.MakeReadOnly();

+      repeatedSint64_.MakeReadOnly();

+      repeatedFixed32_.MakeReadOnly();

+      repeatedFixed64_.MakeReadOnly();

+      repeatedSfixed32_.MakeReadOnly();

+      repeatedSfixed64_.MakeReadOnly();

+      repeatedFloat_.MakeReadOnly();

+      repeatedDouble_.MakeReadOnly();

+      repeatedBool_.MakeReadOnly();

+      repeatedString_.MakeReadOnly();

+      repeatedBytes_.MakeReadOnly();

+      repeatedGroup_.MakeReadOnly();

+      repeatedNestedMessage_.MakeReadOnly();

+      repeatedForeignMessage_.MakeReadOnly();

+      repeatedImportMessage_.MakeReadOnly();

+      repeatedNestedEnum_.MakeReadOnly();

+      repeatedForeignEnum_.MakeReadOnly();

+      repeatedImportEnum_.MakeReadOnly();

+      repeatedStringPiece_.MakeReadOnly();

+      repeatedCord_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -3286,22 +3326,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestAllTypes();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestAllTypes cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestAllTypes result;

+      private bool resultIsReadOnly;

+      private TestAllTypes result;

       

       private TestAllTypes PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestAllTypes original = result;

           result = new TestAllTypes();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -3316,13 +3356,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestAllTypes();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -3338,35 +3378,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestAllTypes BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.repeatedInt32_.MakeReadOnly();

-        result.repeatedInt64_.MakeReadOnly();

-        result.repeatedUint32_.MakeReadOnly();

-        result.repeatedUint64_.MakeReadOnly();

-        result.repeatedSint32_.MakeReadOnly();

-        result.repeatedSint64_.MakeReadOnly();

-        result.repeatedFixed32_.MakeReadOnly();

-        result.repeatedFixed64_.MakeReadOnly();

-        result.repeatedSfixed32_.MakeReadOnly();

-        result.repeatedSfixed64_.MakeReadOnly();

-        result.repeatedFloat_.MakeReadOnly();

-        result.repeatedDouble_.MakeReadOnly();

-        result.repeatedBool_.MakeReadOnly();

-        result.repeatedString_.MakeReadOnly();

-        result.repeatedBytes_.MakeReadOnly();

-        result.repeatedGroup_.MakeReadOnly();

-        result.repeatedNestedMessage_.MakeReadOnly();

-        result.repeatedForeignMessage_.MakeReadOnly();

-        result.repeatedImportMessage_.MakeReadOnly();

-        result.repeatedNestedEnum_.MakeReadOnly();

-        result.repeatedForeignEnum_.MakeReadOnly();

-        result.repeatedImportEnum_.MakeReadOnly();

-        result.repeatedStringPiece_.MakeReadOnly();

-        result.repeatedCord_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -4367,7 +4383,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalGroup() {

         PrepareBuilder();

         result.hasOptionalGroup = false;

-        result.optionalGroup_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.OptionalGroup.DefaultInstance;

+        result.optionalGroup_ = null;

         return this;

       }

       

@@ -4407,7 +4423,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalNestedMessage() {

         PrepareBuilder();

         result.hasOptionalNestedMessage = false;

-        result.optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance;

+        result.optionalNestedMessage_ = null;

         return this;

       }

       

@@ -4447,7 +4463,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalForeignMessage() {

         PrepareBuilder();

         result.hasOptionalForeignMessage = false;

-        result.optionalForeignMessage_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

+        result.optionalForeignMessage_ = null;

         return this;

       }

       

@@ -4487,7 +4503,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalImportMessage() {

         PrepareBuilder();

         result.hasOptionalImportMessage = false;

-        result.optionalImportMessage_ = global::Google.ProtocolBuffers.TestProtos.ImportMessage.DefaultInstance;

+        result.optionalImportMessage_ = null;

         return this;

       }

       

@@ -5819,7 +5835,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestDeprecatedFields : pb::GeneratedMessage<TestDeprecatedFields, TestDeprecatedFields.Builder> {

     private TestDeprecatedFields() { }

-    private static readonly TestDeprecatedFields defaultInstance = new Builder().BuildPartial();

+    private static readonly TestDeprecatedFields defaultInstance = new TestDeprecatedFields().MakeReadOnly();

     private static readonly string[] _testDeprecatedFieldsFieldNames = new string[] { "deprecated_int32" };

     private static readonly uint[] _testDeprecatedFieldsFieldTags = new uint[] { 8 };

     public static TestDeprecatedFields DefaultInstance {

@@ -5827,7 +5843,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestDeprecatedFields DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestDeprecatedFields ThisMessage {

@@ -5915,6 +5931,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestDeprecatedFields ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestDeprecatedFields MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -5930,22 +5950,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestDeprecatedFields();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestDeprecatedFields cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestDeprecatedFields result;

+      private bool resultIsReadOnly;

+      private TestDeprecatedFields result;

       

       private TestDeprecatedFields PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestDeprecatedFields original = result;

           result = new TestDeprecatedFields();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -5960,13 +5980,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestDeprecatedFields();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -5982,11 +6002,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestDeprecatedFields BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -6095,7 +6115,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class ForeignMessage : pb::GeneratedMessage<ForeignMessage, ForeignMessage.Builder> {

     private ForeignMessage() { }

-    private static readonly ForeignMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly ForeignMessage defaultInstance = new ForeignMessage().MakeReadOnly();

     private static readonly string[] _foreignMessageFieldNames = new string[] { "c" };

     private static readonly uint[] _foreignMessageFieldTags = new uint[] { 8 };

     public static ForeignMessage DefaultInstance {

@@ -6103,7 +6123,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override ForeignMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override ForeignMessage ThisMessage {

@@ -6189,6 +6209,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static ForeignMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private ForeignMessage MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -6204,22 +6228,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new ForeignMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(ForeignMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      ForeignMessage result;

+      private bool resultIsReadOnly;

+      private ForeignMessage result;

       

       private ForeignMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           ForeignMessage original = result;

           result = new ForeignMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -6234,13 +6258,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new ForeignMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -6256,11 +6280,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override ForeignMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -6365,7 +6389,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestAllExtensions : pb::ExtendableMessage<TestAllExtensions, TestAllExtensions.Builder> {

     private TestAllExtensions() { }

-    private static readonly TestAllExtensions defaultInstance = new Builder().BuildPartial();

+    private static readonly TestAllExtensions defaultInstance = new TestAllExtensions().MakeReadOnly();

     private static readonly string[] _testAllExtensionsFieldNames = new string[] {  };

     private static readonly uint[] _testAllExtensionsFieldTags = new uint[] {  };

     public static TestAllExtensions DefaultInstance {

@@ -6373,7 +6397,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestAllExtensions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestAllExtensions ThisMessage {

@@ -6447,6 +6471,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestAllExtensions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestAllExtensions MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -6462,22 +6490,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestAllExtensions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestAllExtensions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestAllExtensions result;

+      private bool resultIsReadOnly;

+      private TestAllExtensions result;

       

       private TestAllExtensions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestAllExtensions original = result;

           result = new TestAllExtensions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -6492,13 +6520,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestAllExtensions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -6514,11 +6542,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestAllExtensions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -6597,7 +6625,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class OptionalGroup_extension : pb::GeneratedMessage<OptionalGroup_extension, OptionalGroup_extension.Builder> {

     private OptionalGroup_extension() { }

-    private static readonly OptionalGroup_extension defaultInstance = new Builder().BuildPartial();

+    private static readonly OptionalGroup_extension defaultInstance = new OptionalGroup_extension().MakeReadOnly();

     private static readonly string[] _optionalGroupExtensionFieldNames = new string[] { "a" };

     private static readonly uint[] _optionalGroupExtensionFieldTags = new uint[] { 136 };

     public static OptionalGroup_extension DefaultInstance {

@@ -6605,7 +6633,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override OptionalGroup_extension DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override OptionalGroup_extension ThisMessage {

@@ -6691,6 +6719,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static OptionalGroup_extension ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private OptionalGroup_extension MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -6706,22 +6738,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new OptionalGroup_extension();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(OptionalGroup_extension cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      OptionalGroup_extension result;

+      private bool resultIsReadOnly;

+      private OptionalGroup_extension result;

       

       private OptionalGroup_extension PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           OptionalGroup_extension original = result;

           result = new OptionalGroup_extension();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -6736,13 +6768,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new OptionalGroup_extension();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -6758,11 +6790,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OptionalGroup_extension BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -6867,7 +6899,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class RepeatedGroup_extension : pb::GeneratedMessage<RepeatedGroup_extension, RepeatedGroup_extension.Builder> {

     private RepeatedGroup_extension() { }

-    private static readonly RepeatedGroup_extension defaultInstance = new Builder().BuildPartial();

+    private static readonly RepeatedGroup_extension defaultInstance = new RepeatedGroup_extension().MakeReadOnly();

     private static readonly string[] _repeatedGroupExtensionFieldNames = new string[] { "a" };

     private static readonly uint[] _repeatedGroupExtensionFieldTags = new uint[] { 376 };

     public static RepeatedGroup_extension DefaultInstance {

@@ -6875,7 +6907,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override RepeatedGroup_extension DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override RepeatedGroup_extension ThisMessage {

@@ -6961,6 +6993,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static RepeatedGroup_extension ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private RepeatedGroup_extension MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -6976,22 +7012,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new RepeatedGroup_extension();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(RepeatedGroup_extension cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      RepeatedGroup_extension result;

+      private bool resultIsReadOnly;

+      private RepeatedGroup_extension result;

       

       private RepeatedGroup_extension PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           RepeatedGroup_extension original = result;

           result = new RepeatedGroup_extension();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -7006,13 +7042,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new RepeatedGroup_extension();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -7028,11 +7064,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override RepeatedGroup_extension BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -7137,7 +7173,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestNestedExtension : pb::GeneratedMessage<TestNestedExtension, TestNestedExtension.Builder> {

     private TestNestedExtension() { }

-    private static readonly TestNestedExtension defaultInstance = new Builder().BuildPartial();

+    private static readonly TestNestedExtension defaultInstance = new TestNestedExtension().MakeReadOnly();

     private static readonly string[] _testNestedExtensionFieldNames = new string[] {  };

     private static readonly uint[] _testNestedExtensionFieldTags = new uint[] {  };

     public static TestNestedExtension DefaultInstance {

@@ -7145,7 +7181,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestNestedExtension DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestNestedExtension ThisMessage {

@@ -7217,6 +7253,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestNestedExtension ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestNestedExtension MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -7232,22 +7272,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestNestedExtension();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestNestedExtension cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestNestedExtension result;

+      private bool resultIsReadOnly;

+      private TestNestedExtension result;

       

       private TestNestedExtension PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestNestedExtension original = result;

           result = new TestNestedExtension();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -7262,13 +7302,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestNestedExtension();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -7284,11 +7324,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestNestedExtension BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -7366,7 +7406,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestRequired : pb::GeneratedMessage<TestRequired, TestRequired.Builder> {

     private TestRequired() { }

-    private static readonly TestRequired defaultInstance = new Builder().BuildPartial();

+    private static readonly TestRequired defaultInstance = new TestRequired().MakeReadOnly();

     private static readonly string[] _testRequiredFieldNames = new string[] { "a", "b", "c", "dummy10", "dummy11", "dummy12", "dummy13", "dummy14", "dummy15", "dummy16", "dummy17", "dummy18", "dummy19", "dummy2", "dummy20", "dummy21", "dummy22", "dummy23", "dummy24", "dummy25", "dummy26", "dummy27", "dummy28", "dummy29", "dummy30", "dummy31", "dummy32", "dummy4", "dummy5", "dummy6", "dummy7", "dummy8", "dummy9" };

     private static readonly uint[] _testRequiredFieldTags = new uint[] { 8, 24, 264, 80, 88, 96, 104, 112, 120, 128, 136, 144, 152, 16, 160, 168, 176, 184, 192, 200, 208, 216, 224, 232, 240, 248, 256, 32, 40, 48, 56, 64, 72 };

     public static TestRequired DefaultInstance {

@@ -7374,7 +7414,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestRequired DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestRequired ThisMessage {

@@ -7979,6 +8019,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestRequired ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestRequired MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -7994,22 +8038,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestRequired();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestRequired cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestRequired result;

+      private bool resultIsReadOnly;

+      private TestRequired result;

       

       private TestRequired PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestRequired original = result;

           result = new TestRequired();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -8024,13 +8068,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestRequired();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -8046,11 +8090,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRequired BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -9019,7 +9063,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestRequiredForeign : pb::GeneratedMessage<TestRequiredForeign, TestRequiredForeign.Builder> {

     private TestRequiredForeign() { }

-    private static readonly TestRequiredForeign defaultInstance = new Builder().BuildPartial();

+    private static readonly TestRequiredForeign defaultInstance = new TestRequiredForeign().MakeReadOnly();

     private static readonly string[] _testRequiredForeignFieldNames = new string[] { "dummy", "optional_message", "repeated_message" };

     private static readonly uint[] _testRequiredForeignFieldTags = new uint[] { 24, 10, 18 };

     public static TestRequiredForeign DefaultInstance {

@@ -9027,7 +9071,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestRequiredForeign DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestRequiredForeign ThisMessage {

@@ -9044,12 +9088,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int OptionalMessageFieldNumber = 1;

     private bool hasOptionalMessage;

-    private global::Google.ProtocolBuffers.TestProtos.TestRequired optionalMessage_ = global::Google.ProtocolBuffers.TestProtos.TestRequired.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestRequired optionalMessage_;

     public bool HasOptionalMessage {

       get { return hasOptionalMessage; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestRequired OptionalMessage {

-      get { return optionalMessage_; }

+      get { return optionalMessage_ ?? global::Google.ProtocolBuffers.TestProtos.TestRequired.DefaultInstance; }

     }

     

     public const int RepeatedMessageFieldNumber = 2;

@@ -9153,6 +9197,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestRequiredForeign ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestRequiredForeign MakeReadOnly() {

+      repeatedMessage_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -9168,22 +9217,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestRequiredForeign();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestRequiredForeign cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestRequiredForeign result;

+      private bool resultIsReadOnly;

+      private TestRequiredForeign result;

       

       private TestRequiredForeign PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestRequiredForeign original = result;

           result = new TestRequiredForeign();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -9198,13 +9247,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestRequiredForeign();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -9220,12 +9269,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRequiredForeign BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.repeatedMessage_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -9355,7 +9403,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalMessage() {

         PrepareBuilder();

         result.hasOptionalMessage = false;

-        result.optionalMessage_ = global::Google.ProtocolBuffers.TestProtos.TestRequired.DefaultInstance;

+        result.optionalMessage_ = null;

         return this;

       }

       

@@ -9433,7 +9481,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestForeignNested : pb::GeneratedMessage<TestForeignNested, TestForeignNested.Builder> {

     private TestForeignNested() { }

-    private static readonly TestForeignNested defaultInstance = new Builder().BuildPartial();

+    private static readonly TestForeignNested defaultInstance = new TestForeignNested().MakeReadOnly();

     private static readonly string[] _testForeignNestedFieldNames = new string[] { "foreign_nested" };

     private static readonly uint[] _testForeignNestedFieldTags = new uint[] { 10 };

     public static TestForeignNested DefaultInstance {

@@ -9441,7 +9489,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestForeignNested DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestForeignNested ThisMessage {

@@ -9458,12 +9506,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int ForeignNestedFieldNumber = 1;

     private bool hasForeignNested;

-    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage foreignNested_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage foreignNested_;

     public bool HasForeignNested {

       get { return hasForeignNested; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage ForeignNested {

-      get { return foreignNested_; }

+      get { return foreignNested_ ?? global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -9527,6 +9575,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestForeignNested ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestForeignNested MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -9542,22 +9594,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestForeignNested();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestForeignNested cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestForeignNested result;

+      private bool resultIsReadOnly;

+      private TestForeignNested result;

       

       private TestForeignNested PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestForeignNested original = result;

           result = new TestForeignNested();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -9572,13 +9624,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestForeignNested();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -9594,11 +9646,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestForeignNested BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -9714,7 +9766,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearForeignNested() {

         PrepareBuilder();

         result.hasForeignNested = false;

-        result.foreignNested_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.Types.NestedMessage.DefaultInstance;

+        result.foreignNested_ = null;

         return this;

       }

     }

@@ -9728,7 +9780,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestEmptyMessage : pb::GeneratedMessage<TestEmptyMessage, TestEmptyMessage.Builder> {

     private TestEmptyMessage() { }

-    private static readonly TestEmptyMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly TestEmptyMessage defaultInstance = new TestEmptyMessage().MakeReadOnly();

     private static readonly string[] _testEmptyMessageFieldNames = new string[] {  };

     private static readonly uint[] _testEmptyMessageFieldTags = new uint[] {  };

     public static TestEmptyMessage DefaultInstance {

@@ -9736,7 +9788,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestEmptyMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestEmptyMessage ThisMessage {

@@ -9806,6 +9858,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestEmptyMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestEmptyMessage MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -9821,22 +9877,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestEmptyMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestEmptyMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestEmptyMessage result;

+      private bool resultIsReadOnly;

+      private TestEmptyMessage result;

       

       private TestEmptyMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestEmptyMessage original = result;

           result = new TestEmptyMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -9851,13 +9907,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestEmptyMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -9873,11 +9929,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestEmptyMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -9955,7 +10011,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestEmptyMessageWithExtensions : pb::ExtendableMessage<TestEmptyMessageWithExtensions, TestEmptyMessageWithExtensions.Builder> {

     private TestEmptyMessageWithExtensions() { }

-    private static readonly TestEmptyMessageWithExtensions defaultInstance = new Builder().BuildPartial();

+    private static readonly TestEmptyMessageWithExtensions defaultInstance = new TestEmptyMessageWithExtensions().MakeReadOnly();

     private static readonly string[] _testEmptyMessageWithExtensionsFieldNames = new string[] {  };

     private static readonly uint[] _testEmptyMessageWithExtensionsFieldTags = new uint[] {  };

     public static TestEmptyMessageWithExtensions DefaultInstance {

@@ -9963,7 +10019,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestEmptyMessageWithExtensions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestEmptyMessageWithExtensions ThisMessage {

@@ -10037,6 +10093,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestEmptyMessageWithExtensions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestEmptyMessageWithExtensions MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -10052,22 +10112,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestEmptyMessageWithExtensions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestEmptyMessageWithExtensions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestEmptyMessageWithExtensions result;

+      private bool resultIsReadOnly;

+      private TestEmptyMessageWithExtensions result;

       

       private TestEmptyMessageWithExtensions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestEmptyMessageWithExtensions original = result;

           result = new TestEmptyMessageWithExtensions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -10082,13 +10142,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestEmptyMessageWithExtensions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -10104,11 +10164,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestEmptyMessageWithExtensions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -10187,7 +10247,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestMultipleExtensionRanges : pb::ExtendableMessage<TestMultipleExtensionRanges, TestMultipleExtensionRanges.Builder> {

     private TestMultipleExtensionRanges() { }

-    private static readonly TestMultipleExtensionRanges defaultInstance = new Builder().BuildPartial();

+    private static readonly TestMultipleExtensionRanges defaultInstance = new TestMultipleExtensionRanges().MakeReadOnly();

     private static readonly string[] _testMultipleExtensionRangesFieldNames = new string[] {  };

     private static readonly uint[] _testMultipleExtensionRangesFieldTags = new uint[] {  };

     public static TestMultipleExtensionRanges DefaultInstance {

@@ -10195,7 +10255,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestMultipleExtensionRanges DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestMultipleExtensionRanges ThisMessage {

@@ -10271,6 +10331,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMultipleExtensionRanges ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestMultipleExtensionRanges MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -10286,22 +10350,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestMultipleExtensionRanges();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestMultipleExtensionRanges cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestMultipleExtensionRanges result;

+      private bool resultIsReadOnly;

+      private TestMultipleExtensionRanges result;

       

       private TestMultipleExtensionRanges PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestMultipleExtensionRanges original = result;

           result = new TestMultipleExtensionRanges();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -10316,13 +10380,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestMultipleExtensionRanges();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -10338,11 +10402,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMultipleExtensionRanges BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -10421,7 +10485,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestReallyLargeTagNumber : pb::GeneratedMessage<TestReallyLargeTagNumber, TestReallyLargeTagNumber.Builder> {

     private TestReallyLargeTagNumber() { }

-    private static readonly TestReallyLargeTagNumber defaultInstance = new Builder().BuildPartial();

+    private static readonly TestReallyLargeTagNumber defaultInstance = new TestReallyLargeTagNumber().MakeReadOnly();

     private static readonly string[] _testReallyLargeTagNumberFieldNames = new string[] { "a", "bb" };

     private static readonly uint[] _testReallyLargeTagNumberFieldTags = new uint[] { 8, 2147483640 };

     public static TestReallyLargeTagNumber DefaultInstance {

@@ -10429,7 +10493,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestReallyLargeTagNumber DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestReallyLargeTagNumber ThisMessage {

@@ -10531,6 +10595,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestReallyLargeTagNumber ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestReallyLargeTagNumber MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -10546,22 +10614,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestReallyLargeTagNumber();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestReallyLargeTagNumber cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestReallyLargeTagNumber result;

+      private bool resultIsReadOnly;

+      private TestReallyLargeTagNumber result;

       

       private TestReallyLargeTagNumber PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestReallyLargeTagNumber original = result;

           result = new TestReallyLargeTagNumber();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -10576,13 +10644,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestReallyLargeTagNumber();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -10598,11 +10666,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestReallyLargeTagNumber BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -10734,7 +10802,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestRecursiveMessage : pb::GeneratedMessage<TestRecursiveMessage, TestRecursiveMessage.Builder> {

     private TestRecursiveMessage() { }

-    private static readonly TestRecursiveMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly TestRecursiveMessage defaultInstance = new TestRecursiveMessage().MakeReadOnly();

     private static readonly string[] _testRecursiveMessageFieldNames = new string[] { "a", "i" };

     private static readonly uint[] _testRecursiveMessageFieldTags = new uint[] { 10, 16 };

     public static TestRecursiveMessage DefaultInstance {

@@ -10742,7 +10810,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestRecursiveMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestRecursiveMessage ThisMessage {

@@ -10759,12 +10827,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int AFieldNumber = 1;

     private bool hasA;

-    private global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage a_ = global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage a_;

     public bool HasA {

       get { return hasA; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage A {

-      get { return a_; }

+      get { return a_ ?? global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.DefaultInstance; }

     }

     

     public const int IFieldNumber = 2;

@@ -10844,6 +10912,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestRecursiveMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestRecursiveMessage MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -10859,22 +10931,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestRecursiveMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestRecursiveMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestRecursiveMessage result;

+      private bool resultIsReadOnly;

+      private TestRecursiveMessage result;

       

       private TestRecursiveMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestRecursiveMessage original = result;

           result = new TestRecursiveMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -10889,13 +10961,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestRecursiveMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -10911,11 +10983,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRecursiveMessage BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -11038,7 +11110,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearA() {

         PrepareBuilder();

         result.hasA = false;

-        result.a_ = global::Google.ProtocolBuffers.TestProtos.TestRecursiveMessage.DefaultInstance;

+        result.a_ = null;

         return this;

       }

       

@@ -11072,7 +11144,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestMutualRecursionA : pb::GeneratedMessage<TestMutualRecursionA, TestMutualRecursionA.Builder> {

     private TestMutualRecursionA() { }

-    private static readonly TestMutualRecursionA defaultInstance = new Builder().BuildPartial();

+    private static readonly TestMutualRecursionA defaultInstance = new TestMutualRecursionA().MakeReadOnly();

     private static readonly string[] _testMutualRecursionAFieldNames = new string[] { "bb" };

     private static readonly uint[] _testMutualRecursionAFieldTags = new uint[] { 10 };

     public static TestMutualRecursionA DefaultInstance {

@@ -11080,7 +11152,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestMutualRecursionA DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestMutualRecursionA ThisMessage {

@@ -11097,12 +11169,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int BbFieldNumber = 1;

     private bool hasBb;

-    private global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB bb_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB bb_;

     public bool HasBb {

       get { return hasBb; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB Bb {

-      get { return bb_; }

+      get { return bb_ ?? global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -11166,6 +11238,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMutualRecursionA ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestMutualRecursionA MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -11181,22 +11257,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestMutualRecursionA();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestMutualRecursionA cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestMutualRecursionA result;

+      private bool resultIsReadOnly;

+      private TestMutualRecursionA result;

       

       private TestMutualRecursionA PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestMutualRecursionA original = result;

           result = new TestMutualRecursionA();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -11211,13 +11287,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestMutualRecursionA();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -11233,11 +11309,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMutualRecursionA BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -11353,7 +11429,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearBb() {

         PrepareBuilder();

         result.hasBb = false;

-        result.bb_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionB.DefaultInstance;

+        result.bb_ = null;

         return this;

       }

     }

@@ -11367,7 +11443,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestMutualRecursionB : pb::GeneratedMessage<TestMutualRecursionB, TestMutualRecursionB.Builder> {

     private TestMutualRecursionB() { }

-    private static readonly TestMutualRecursionB defaultInstance = new Builder().BuildPartial();

+    private static readonly TestMutualRecursionB defaultInstance = new TestMutualRecursionB().MakeReadOnly();

     private static readonly string[] _testMutualRecursionBFieldNames = new string[] { "a", "optional_int32" };

     private static readonly uint[] _testMutualRecursionBFieldTags = new uint[] { 10, 16 };

     public static TestMutualRecursionB DefaultInstance {

@@ -11375,7 +11451,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestMutualRecursionB DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestMutualRecursionB ThisMessage {

@@ -11392,12 +11468,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int AFieldNumber = 1;

     private bool hasA;

-    private global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA a_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA a_;

     public bool HasA {

       get { return hasA; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA A {

-      get { return a_; }

+      get { return a_ ?? global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.DefaultInstance; }

     }

     

     public const int OptionalInt32FieldNumber = 2;

@@ -11477,6 +11553,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMutualRecursionB ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestMutualRecursionB MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -11492,22 +11572,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestMutualRecursionB();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestMutualRecursionB cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestMutualRecursionB result;

+      private bool resultIsReadOnly;

+      private TestMutualRecursionB result;

       

       private TestMutualRecursionB PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestMutualRecursionB original = result;

           result = new TestMutualRecursionB();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -11522,13 +11602,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestMutualRecursionB();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -11544,11 +11624,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestMutualRecursionB BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -11671,7 +11751,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearA() {

         PrepareBuilder();

         result.hasA = false;

-        result.a_ = global::Google.ProtocolBuffers.TestProtos.TestMutualRecursionA.DefaultInstance;

+        result.a_ = null;

         return this;

       }

       

@@ -11705,7 +11785,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestDupFieldNumber : pb::GeneratedMessage<TestDupFieldNumber, TestDupFieldNumber.Builder> {

     private TestDupFieldNumber() { }

-    private static readonly TestDupFieldNumber defaultInstance = new Builder().BuildPartial();

+    private static readonly TestDupFieldNumber defaultInstance = new TestDupFieldNumber().MakeReadOnly();

     private static readonly string[] _testDupFieldNumberFieldNames = new string[] { "a", "bar", "foo" };

     private static readonly uint[] _testDupFieldNumberFieldTags = new uint[] { 8, 27, 19 };

     public static TestDupFieldNumber DefaultInstance {

@@ -11713,7 +11793,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestDupFieldNumber DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestDupFieldNumber ThisMessage {

@@ -11738,7 +11818,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class Foo : pb::GeneratedMessage<Foo, Foo.Builder> {

         private Foo() { }

-        private static readonly Foo defaultInstance = new Builder().BuildPartial();

+        private static readonly Foo defaultInstance = new Foo().MakeReadOnly();

         private static readonly string[] _fooFieldNames = new string[] { "a" };

         private static readonly uint[] _fooFieldTags = new uint[] { 8 };

         public static Foo DefaultInstance {

@@ -11746,7 +11826,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override Foo DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override Foo ThisMessage {

@@ -11832,6 +11912,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Foo ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private Foo MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -11847,22 +11931,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new Foo();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(Foo cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          Foo result;

+          private bool resultIsReadOnly;

+          private Foo result;

           

           private Foo PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               Foo original = result;

               result = new Foo();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -11877,13 +11961,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new Foo();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -11899,11 +11983,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Foo BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -12008,7 +12092,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class Bar : pb::GeneratedMessage<Bar, Bar.Builder> {

         private Bar() { }

-        private static readonly Bar defaultInstance = new Builder().BuildPartial();

+        private static readonly Bar defaultInstance = new Bar().MakeReadOnly();

         private static readonly string[] _barFieldNames = new string[] { "a" };

         private static readonly uint[] _barFieldTags = new uint[] { 8 };

         public static Bar DefaultInstance {

@@ -12016,7 +12100,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override Bar DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override Bar ThisMessage {

@@ -12102,6 +12186,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Bar ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private Bar MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -12117,22 +12205,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new Bar();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(Bar cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          Bar result;

+          private bool resultIsReadOnly;

+          private Bar result;

           

           private Bar PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               Bar original = result;

               result = new Bar();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -12147,13 +12235,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new Bar();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -12169,11 +12257,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Bar BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -12288,22 +12376,22 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int FooFieldNumber = 2;

     private bool hasFoo;

-    private global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo foo_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo foo_;

     public bool HasFoo {

       get { return hasFoo; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo Foo {

-      get { return foo_; }

+      get { return foo_ ?? global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.DefaultInstance; }

     }

     

     public const int BarFieldNumber = 3;

     private bool hasBar;

-    private global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar bar_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar bar_;

     public bool HasBar {

       get { return hasBar; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar Bar {

-      get { return bar_; }

+      get { return bar_ ?? global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -12379,6 +12467,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestDupFieldNumber ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestDupFieldNumber MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -12394,22 +12486,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestDupFieldNumber();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestDupFieldNumber cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestDupFieldNumber result;

+      private bool resultIsReadOnly;

+      private TestDupFieldNumber result;

       

       private TestDupFieldNumber PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestDupFieldNumber original = result;

           result = new TestDupFieldNumber();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -12424,13 +12516,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestDupFieldNumber();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -12446,11 +12538,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestDupFieldNumber BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -12605,7 +12697,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearFoo() {

         PrepareBuilder();

         result.hasFoo = false;

-        result.foo_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Foo.DefaultInstance;

+        result.foo_ = null;

         return this;

       }

       

@@ -12645,7 +12737,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearBar() {

         PrepareBuilder();

         result.hasBar = false;

-        result.bar_ = global::Google.ProtocolBuffers.TestProtos.TestDupFieldNumber.Types.Bar.DefaultInstance;

+        result.bar_ = null;

         return this;

       }

     }

@@ -12659,7 +12751,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestNestedMessageHasBits : pb::GeneratedMessage<TestNestedMessageHasBits, TestNestedMessageHasBits.Builder> {

     private TestNestedMessageHasBits() { }

-    private static readonly TestNestedMessageHasBits defaultInstance = new Builder().BuildPartial();

+    private static readonly TestNestedMessageHasBits defaultInstance = new TestNestedMessageHasBits().MakeReadOnly();

     private static readonly string[] _testNestedMessageHasBitsFieldNames = new string[] { "optional_nested_message" };

     private static readonly uint[] _testNestedMessageHasBitsFieldTags = new uint[] { 10 };

     public static TestNestedMessageHasBits DefaultInstance {

@@ -12667,7 +12759,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestNestedMessageHasBits DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestNestedMessageHasBits ThisMessage {

@@ -12692,7 +12784,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class NestedMessage : pb::GeneratedMessage<NestedMessage, NestedMessage.Builder> {

         private NestedMessage() { }

-        private static readonly NestedMessage defaultInstance = new Builder().BuildPartial();

+        private static readonly NestedMessage defaultInstance = new NestedMessage().MakeReadOnly();

         private static readonly string[] _nestedMessageFieldNames = new string[] { "nestedmessage_repeated_foreignmessage", "nestedmessage_repeated_int32" };

         private static readonly uint[] _nestedMessageFieldTags = new uint[] { 18, 8 };

         public static NestedMessage DefaultInstance {

@@ -12700,7 +12792,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override NestedMessage DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override NestedMessage ThisMessage {

@@ -12811,6 +12903,12 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static NestedMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private NestedMessage MakeReadOnly() {

+          nestedmessageRepeatedInt32_.MakeReadOnly();

+          nestedmessageRepeatedForeignmessage_.MakeReadOnly();

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -12826,22 +12924,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new NestedMessage();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(NestedMessage cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          NestedMessage result;

+          private bool resultIsReadOnly;

+          private NestedMessage result;

           

           private NestedMessage PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               NestedMessage original = result;

               result = new NestedMessage();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -12856,13 +12954,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new NestedMessage();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -12878,13 +12976,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override NestedMessage BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            result.nestedmessageRepeatedInt32_.MakeReadOnly();

-            result.nestedmessageRepeatedForeignmessage_.MakeReadOnly();

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -13051,12 +13147,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int OptionalNestedMessageFieldNumber = 1;

     private bool hasOptionalNestedMessage;

-    private global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage optionalNestedMessage_;

     public bool HasOptionalNestedMessage {

       get { return hasOptionalNestedMessage; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage OptionalNestedMessage {

-      get { return optionalNestedMessage_; }

+      get { return optionalNestedMessage_ ?? global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.DefaultInstance; }

     }

     

     public override bool IsInitialized {

@@ -13120,6 +13216,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestNestedMessageHasBits ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestNestedMessageHasBits MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -13135,22 +13235,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestNestedMessageHasBits();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestNestedMessageHasBits cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestNestedMessageHasBits result;

+      private bool resultIsReadOnly;

+      private TestNestedMessageHasBits result;

       

       private TestNestedMessageHasBits PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestNestedMessageHasBits original = result;

           result = new TestNestedMessageHasBits();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -13165,13 +13265,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestNestedMessageHasBits();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -13187,11 +13287,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestNestedMessageHasBits BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -13307,7 +13407,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalNestedMessage() {

         PrepareBuilder();

         result.hasOptionalNestedMessage = false;

-        result.optionalNestedMessage_ = global::Google.ProtocolBuffers.TestProtos.TestNestedMessageHasBits.Types.NestedMessage.DefaultInstance;

+        result.optionalNestedMessage_ = null;

         return this;

       }

     }

@@ -13321,7 +13421,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestCamelCaseFieldNames : pb::GeneratedMessage<TestCamelCaseFieldNames, TestCamelCaseFieldNames.Builder> {

     private TestCamelCaseFieldNames() { }

-    private static readonly TestCamelCaseFieldNames defaultInstance = new Builder().BuildPartial();

+    private static readonly TestCamelCaseFieldNames defaultInstance = new TestCamelCaseFieldNames().MakeReadOnly();

     private static readonly string[] _testCamelCaseFieldNamesFieldNames = new string[] { "CordField", "EnumField", "MessageField", "PrimitiveField", "RepeatedCordField", "RepeatedEnumField", "RepeatedMessageField", "RepeatedPrimitiveField", "RepeatedStringField", "RepeatedStringPieceField", "StringField", "StringPieceField" };

     private static readonly uint[] _testCamelCaseFieldNamesFieldTags = new uint[] { 50, 24, 34, 8, 98, 72, 82, 56, 66, 90, 18, 42 };

     public static TestCamelCaseFieldNames DefaultInstance {

@@ -13329,7 +13429,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestCamelCaseFieldNames DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestCamelCaseFieldNames ThisMessage {

@@ -13376,12 +13476,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int MessageFieldFieldNumber = 4;

     private bool hasMessageField;

-    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage messageField_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage messageField_;

     public bool HasMessageField {

       get { return hasMessageField; }

     }

     public global::Google.ProtocolBuffers.TestProtos.ForeignMessage MessageField {

-      get { return messageField_; }

+      get { return messageField_ ?? global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance; }

     }

     

     public const int StringPieceFieldFieldNumber = 5;

@@ -13630,6 +13730,16 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestCamelCaseFieldNames ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestCamelCaseFieldNames MakeReadOnly() {

+      repeatedPrimitiveField_.MakeReadOnly();

+      repeatedStringField_.MakeReadOnly();

+      repeatedEnumField_.MakeReadOnly();

+      repeatedMessageField_.MakeReadOnly();

+      repeatedStringPieceField_.MakeReadOnly();

+      repeatedCordField_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -13645,22 +13755,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestCamelCaseFieldNames();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestCamelCaseFieldNames cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestCamelCaseFieldNames result;

+      private bool resultIsReadOnly;

+      private TestCamelCaseFieldNames result;

       

       private TestCamelCaseFieldNames PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestCamelCaseFieldNames original = result;

           result = new TestCamelCaseFieldNames();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -13675,13 +13785,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestCamelCaseFieldNames();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -13697,17 +13807,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestCamelCaseFieldNames BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.repeatedPrimitiveField_.MakeReadOnly();

-        result.repeatedStringField_.MakeReadOnly();

-        result.repeatedEnumField_.MakeReadOnly();

-        result.repeatedMessageField_.MakeReadOnly();

-        result.repeatedStringPieceField_.MakeReadOnly();

-        result.repeatedCordField_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -13980,7 +14084,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearMessageField() {

         PrepareBuilder();

         result.hasMessageField = false;

-        result.messageField_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

+        result.messageField_ = null;

         return this;

       }

       

@@ -14236,7 +14340,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestFieldOrderings : pb::ExtendableMessage<TestFieldOrderings, TestFieldOrderings.Builder> {

     private TestFieldOrderings() { }

-    private static readonly TestFieldOrderings defaultInstance = new Builder().BuildPartial();

+    private static readonly TestFieldOrderings defaultInstance = new TestFieldOrderings().MakeReadOnly();

     private static readonly string[] _testFieldOrderingsFieldNames = new string[] { "my_float", "my_int", "my_string" };

     private static readonly uint[] _testFieldOrderingsFieldTags = new uint[] { 813, 8, 90 };

     public static TestFieldOrderings DefaultInstance {

@@ -14244,7 +14348,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestFieldOrderings DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestFieldOrderings ThisMessage {

@@ -14367,6 +14471,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestFieldOrderings ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestFieldOrderings MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -14382,22 +14490,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestFieldOrderings();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestFieldOrderings cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestFieldOrderings result;

+      private bool resultIsReadOnly;

+      private TestFieldOrderings result;

       

       private TestFieldOrderings PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestFieldOrderings original = result;

           result = new TestFieldOrderings();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -14412,13 +14520,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestFieldOrderings();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -14434,11 +14542,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestFieldOrderings BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -14599,7 +14707,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestExtremeDefaultValues : pb::GeneratedMessage<TestExtremeDefaultValues, TestExtremeDefaultValues.Builder> {

     private TestExtremeDefaultValues() { }

-    private static readonly TestExtremeDefaultValues defaultInstance = new Builder().BuildPartial();

+    private static readonly TestExtremeDefaultValues defaultInstance = new TestExtremeDefaultValues().MakeReadOnly();

     private static readonly string[] _testExtremeDefaultValuesFieldNames = new string[] { "escaped_bytes", "inf_double", "inf_float", "large_float", "large_uint32", "large_uint64", "nan_double", "nan_float", "neg_inf_double", "neg_inf_float", "negative_float", "negative_one_float", "one_float", "small_float", "small_int32", "small_int64", "small_negative_float", "utf8_string", "zero_float" };

     private static readonly uint[] _testExtremeDefaultValuesFieldTags = new uint[] { 10, 113, 141, 101, 16, 24, 129, 157, 121, 149, 93, 85, 69, 77, 32, 40, 109, 50, 61 };

     public static TestExtremeDefaultValues DefaultInstance {

@@ -14607,7 +14715,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestExtremeDefaultValues DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestExtremeDefaultValues ThisMessage {

@@ -14983,6 +15091,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestExtremeDefaultValues ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestExtremeDefaultValues MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -14998,22 +15110,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestExtremeDefaultValues();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestExtremeDefaultValues cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestExtremeDefaultValues result;

+      private bool resultIsReadOnly;

+      private TestExtremeDefaultValues result;

       

       private TestExtremeDefaultValues PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestExtremeDefaultValues original = result;

           result = new TestExtremeDefaultValues();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -15028,13 +15140,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestExtremeDefaultValues();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -15050,11 +15162,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestExtremeDefaultValues BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -15651,7 +15763,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class OneString : pb::GeneratedMessage<OneString, OneString.Builder> {

     private OneString() { }

-    private static readonly OneString defaultInstance = new Builder().BuildPartial();

+    private static readonly OneString defaultInstance = new OneString().MakeReadOnly();

     private static readonly string[] _oneStringFieldNames = new string[] { "data" };

     private static readonly uint[] _oneStringFieldTags = new uint[] { 10 };

     public static OneString DefaultInstance {

@@ -15659,7 +15771,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override OneString DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override OneString ThisMessage {

@@ -15745,6 +15857,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static OneString ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private OneString MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -15760,22 +15876,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new OneString();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(OneString cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      OneString result;

+      private bool resultIsReadOnly;

+      private OneString result;

       

       private OneString PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           OneString original = result;

           result = new OneString();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -15790,13 +15906,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new OneString();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -15812,11 +15928,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OneString BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -15922,7 +16038,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class OneBytes : pb::GeneratedMessage<OneBytes, OneBytes.Builder> {

     private OneBytes() { }

-    private static readonly OneBytes defaultInstance = new Builder().BuildPartial();

+    private static readonly OneBytes defaultInstance = new OneBytes().MakeReadOnly();

     private static readonly string[] _oneBytesFieldNames = new string[] { "data" };

     private static readonly uint[] _oneBytesFieldTags = new uint[] { 10 };

     public static OneBytes DefaultInstance {

@@ -15930,7 +16046,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override OneBytes DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override OneBytes ThisMessage {

@@ -16016,6 +16132,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static OneBytes ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private OneBytes MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -16031,22 +16151,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new OneBytes();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(OneBytes cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      OneBytes result;

+      private bool resultIsReadOnly;

+      private OneBytes result;

       

       private OneBytes PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           OneBytes original = result;

           result = new OneBytes();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -16061,13 +16181,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new OneBytes();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -16083,11 +16203,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override OneBytes BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -16193,7 +16313,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestPackedTypes : pb::GeneratedMessage<TestPackedTypes, TestPackedTypes.Builder> {

     private TestPackedTypes() { }

-    private static readonly TestPackedTypes defaultInstance = new Builder().BuildPartial();

+    private static readonly TestPackedTypes defaultInstance = new TestPackedTypes().MakeReadOnly();

     private static readonly string[] _testPackedTypesFieldNames = new string[] { "packed_bool", "packed_double", "packed_enum", "packed_fixed32", "packed_fixed64", "packed_float", "packed_int32", "packed_int64", "packed_sfixed32", "packed_sfixed64", "packed_sint32", "packed_sint64", "packed_uint32", "packed_uint64" };

     private static readonly uint[] _testPackedTypesFieldTags = new uint[] { 818, 810, 826, 770, 778, 802, 722, 730, 786, 794, 754, 762, 738, 746 };

     public static TestPackedTypes DefaultInstance {

@@ -16201,7 +16321,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestPackedTypes DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestPackedTypes ThisMessage {

@@ -16644,6 +16764,24 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestPackedTypes ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestPackedTypes MakeReadOnly() {

+      packedInt32_.MakeReadOnly();

+      packedInt64_.MakeReadOnly();

+      packedUint32_.MakeReadOnly();

+      packedUint64_.MakeReadOnly();

+      packedSint32_.MakeReadOnly();

+      packedSint64_.MakeReadOnly();

+      packedFixed32_.MakeReadOnly();

+      packedFixed64_.MakeReadOnly();

+      packedSfixed32_.MakeReadOnly();

+      packedSfixed64_.MakeReadOnly();

+      packedFloat_.MakeReadOnly();

+      packedDouble_.MakeReadOnly();

+      packedBool_.MakeReadOnly();

+      packedEnum_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -16659,22 +16797,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestPackedTypes();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestPackedTypes cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestPackedTypes result;

+      private bool resultIsReadOnly;

+      private TestPackedTypes result;

       

       private TestPackedTypes PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestPackedTypes original = result;

           result = new TestPackedTypes();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -16689,13 +16827,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestPackedTypes();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -16711,25 +16849,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestPackedTypes BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.packedInt32_.MakeReadOnly();

-        result.packedInt64_.MakeReadOnly();

-        result.packedUint32_.MakeReadOnly();

-        result.packedUint64_.MakeReadOnly();

-        result.packedSint32_.MakeReadOnly();

-        result.packedSint64_.MakeReadOnly();

-        result.packedFixed32_.MakeReadOnly();

-        result.packedFixed64_.MakeReadOnly();

-        result.packedSfixed32_.MakeReadOnly();

-        result.packedSfixed64_.MakeReadOnly();

-        result.packedFloat_.MakeReadOnly();

-        result.packedDouble_.MakeReadOnly();

-        result.packedBool_.MakeReadOnly();

-        result.packedEnum_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -17368,7 +17492,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestUnpackedTypes : pb::GeneratedMessage<TestUnpackedTypes, TestUnpackedTypes.Builder> {

     private TestUnpackedTypes() { }

-    private static readonly TestUnpackedTypes defaultInstance = new Builder().BuildPartial();

+    private static readonly TestUnpackedTypes defaultInstance = new TestUnpackedTypes().MakeReadOnly();

     private static readonly string[] _testUnpackedTypesFieldNames = new string[] { "unpacked_bool", "unpacked_double", "unpacked_enum", "unpacked_fixed32", "unpacked_fixed64", "unpacked_float", "unpacked_int32", "unpacked_int64", "unpacked_sfixed32", "unpacked_sfixed64", "unpacked_sint32", "unpacked_sint64", "unpacked_uint32", "unpacked_uint64" };

     private static readonly uint[] _testUnpackedTypesFieldTags = new uint[] { 816, 809, 824, 773, 777, 805, 720, 728, 789, 793, 752, 760, 736, 744 };

     public static TestUnpackedTypes DefaultInstance {

@@ -17376,7 +17500,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestUnpackedTypes DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestUnpackedTypes ThisMessage {

@@ -17764,6 +17888,24 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestUnpackedTypes ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestUnpackedTypes MakeReadOnly() {

+      unpackedInt32_.MakeReadOnly();

+      unpackedInt64_.MakeReadOnly();

+      unpackedUint32_.MakeReadOnly();

+      unpackedUint64_.MakeReadOnly();

+      unpackedSint32_.MakeReadOnly();

+      unpackedSint64_.MakeReadOnly();

+      unpackedFixed32_.MakeReadOnly();

+      unpackedFixed64_.MakeReadOnly();

+      unpackedSfixed32_.MakeReadOnly();

+      unpackedSfixed64_.MakeReadOnly();

+      unpackedFloat_.MakeReadOnly();

+      unpackedDouble_.MakeReadOnly();

+      unpackedBool_.MakeReadOnly();

+      unpackedEnum_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -17779,22 +17921,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestUnpackedTypes();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestUnpackedTypes cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestUnpackedTypes result;

+      private bool resultIsReadOnly;

+      private TestUnpackedTypes result;

       

       private TestUnpackedTypes PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestUnpackedTypes original = result;

           result = new TestUnpackedTypes();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -17809,13 +17951,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestUnpackedTypes();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -17831,25 +17973,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestUnpackedTypes BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.unpackedInt32_.MakeReadOnly();

-        result.unpackedInt64_.MakeReadOnly();

-        result.unpackedUint32_.MakeReadOnly();

-        result.unpackedUint64_.MakeReadOnly();

-        result.unpackedSint32_.MakeReadOnly();

-        result.unpackedSint64_.MakeReadOnly();

-        result.unpackedFixed32_.MakeReadOnly();

-        result.unpackedFixed64_.MakeReadOnly();

-        result.unpackedSfixed32_.MakeReadOnly();

-        result.unpackedSfixed64_.MakeReadOnly();

-        result.unpackedFloat_.MakeReadOnly();

-        result.unpackedDouble_.MakeReadOnly();

-        result.unpackedBool_.MakeReadOnly();

-        result.unpackedEnum_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -18488,7 +18616,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestPackedExtensions : pb::ExtendableMessage<TestPackedExtensions, TestPackedExtensions.Builder> {

     private TestPackedExtensions() { }

-    private static readonly TestPackedExtensions defaultInstance = new Builder().BuildPartial();

+    private static readonly TestPackedExtensions defaultInstance = new TestPackedExtensions().MakeReadOnly();

     private static readonly string[] _testPackedExtensionsFieldNames = new string[] {  };

     private static readonly uint[] _testPackedExtensionsFieldTags = new uint[] {  };

     public static TestPackedExtensions DefaultInstance {

@@ -18496,7 +18624,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestPackedExtensions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestPackedExtensions ThisMessage {

@@ -18570,6 +18698,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestPackedExtensions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestPackedExtensions MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -18585,22 +18717,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestPackedExtensions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestPackedExtensions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestPackedExtensions result;

+      private bool resultIsReadOnly;

+      private TestPackedExtensions result;

       

       private TestPackedExtensions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestPackedExtensions original = result;

           result = new TestPackedExtensions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -18615,13 +18747,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestPackedExtensions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -18637,11 +18769,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestPackedExtensions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -18720,7 +18852,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestUnpackedExtensions : pb::ExtendableMessage<TestUnpackedExtensions, TestUnpackedExtensions.Builder> {

     private TestUnpackedExtensions() { }

-    private static readonly TestUnpackedExtensions defaultInstance = new Builder().BuildPartial();

+    private static readonly TestUnpackedExtensions defaultInstance = new TestUnpackedExtensions().MakeReadOnly();

     private static readonly string[] _testUnpackedExtensionsFieldNames = new string[] {  };

     private static readonly uint[] _testUnpackedExtensionsFieldTags = new uint[] {  };

     public static TestUnpackedExtensions DefaultInstance {

@@ -18728,7 +18860,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestUnpackedExtensions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestUnpackedExtensions ThisMessage {

@@ -18802,6 +18934,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestUnpackedExtensions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestUnpackedExtensions MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -18817,22 +18953,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestUnpackedExtensions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestUnpackedExtensions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestUnpackedExtensions result;

+      private bool resultIsReadOnly;

+      private TestUnpackedExtensions result;

       

       private TestUnpackedExtensions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestUnpackedExtensions original = result;

           result = new TestUnpackedExtensions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -18847,13 +18983,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestUnpackedExtensions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -18869,11 +19005,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestUnpackedExtensions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -18952,7 +19088,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestDynamicExtensions : pb::GeneratedMessage<TestDynamicExtensions, TestDynamicExtensions.Builder> {

     private TestDynamicExtensions() { }

-    private static readonly TestDynamicExtensions defaultInstance = new Builder().BuildPartial();

+    private static readonly TestDynamicExtensions defaultInstance = new TestDynamicExtensions().MakeReadOnly();

     private static readonly string[] _testDynamicExtensionsFieldNames = new string[] { "dynamic_enum_extension", "dynamic_message_extension", "enum_extension", "message_extension", "packed_extension", "repeated_extension", "scalar_extension" };

     private static readonly uint[] _testDynamicExtensionsFieldTags = new uint[] { 16016, 16034, 16008, 16026, 16050, 16042, 16005 };

     public static TestDynamicExtensions DefaultInstance {

@@ -18960,7 +19096,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestDynamicExtensions DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestDynamicExtensions ThisMessage {

@@ -18993,7 +19129,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

       public sealed partial class DynamicMessageType : pb::GeneratedMessage<DynamicMessageType, DynamicMessageType.Builder> {

         private DynamicMessageType() { }

-        private static readonly DynamicMessageType defaultInstance = new Builder().BuildPartial();

+        private static readonly DynamicMessageType defaultInstance = new DynamicMessageType().MakeReadOnly();

         private static readonly string[] _dynamicMessageTypeFieldNames = new string[] { "dynamic_field" };

         private static readonly uint[] _dynamicMessageTypeFieldTags = new uint[] { 16800 };

         public static DynamicMessageType DefaultInstance {

@@ -19001,7 +19137,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }

         

         public override DynamicMessageType DefaultInstanceForType {

-          get { return defaultInstance; }

+          get { return DefaultInstance; }

         }

         

         protected override DynamicMessageType ThisMessage {

@@ -19087,6 +19223,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static DynamicMessageType ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

         }

+        private DynamicMessageType MakeReadOnly() {

+          return this;

+        }

+        

         public static Builder CreateBuilder() { return new Builder(); }

         public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -19102,22 +19242,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }

           }

           public Builder() {

-            result = DefaultInstance ?? new DynamicMessageType();

-            builderIsReadOnly = result == DefaultInstance;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

           }

           internal Builder(DynamicMessageType cloneFrom) {

             result = cloneFrom;

-            builderIsReadOnly = true;

+            resultIsReadOnly = true;

           }

           

-          bool builderIsReadOnly;

-          DynamicMessageType result;

+          private bool resultIsReadOnly;

+          private DynamicMessageType result;

           

           private DynamicMessageType PrepareBuilder() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               DynamicMessageType original = result;

               result = new DynamicMessageType();

-              builderIsReadOnly = false;

+              resultIsReadOnly = false;

               MergeFrom(original);

             }

             return result;

@@ -19132,13 +19272,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override Builder Clear() {

-            result = DefaultInstance ?? new DynamicMessageType();

-            builderIsReadOnly = true;

+            result = DefaultInstance;

+            resultIsReadOnly = true;

             return this;

           }

           

           public override Builder Clone() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return new Builder(result);

             } else {

               return new Builder().MergeFrom(result);

@@ -19154,11 +19294,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }

           

           public override DynamicMessageType BuildPartial() {

-            if (builderIsReadOnly) {

+            if (resultIsReadOnly) {

               return result;

             }

-            builderIsReadOnly = true;

-            return result;

+            resultIsReadOnly = true;

+            return result.MakeReadOnly();

           }

           

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

@@ -19294,22 +19434,22 @@ namespace Google.ProtocolBuffers.TestProtos {
     

     public const int MessageExtensionFieldNumber = 2003;

     private bool hasMessageExtension;

-    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage messageExtension_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage messageExtension_;

     public bool HasMessageExtension {

       get { return hasMessageExtension; }

     }

     public global::Google.ProtocolBuffers.TestProtos.ForeignMessage MessageExtension {

-      get { return messageExtension_; }

+      get { return messageExtension_ ?? global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance; }

     }

     

     public const int DynamicMessageExtensionFieldNumber = 2004;

     private bool hasDynamicMessageExtension;

-    private global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType dynamicMessageExtension_ = global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.DefaultInstance;

+    private global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType dynamicMessageExtension_;

     public bool HasDynamicMessageExtension {

       get { return hasDynamicMessageExtension; }

     }

     public global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType DynamicMessageExtension {

-      get { return dynamicMessageExtension_; }

+      get { return dynamicMessageExtension_ ?? global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.DefaultInstance; }

     }

     

     public const int RepeatedExtensionFieldNumber = 2005;

@@ -19447,6 +19587,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestDynamicExtensions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestDynamicExtensions MakeReadOnly() {

+      repeatedExtension_.MakeReadOnly();

+      packedExtension_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -19462,22 +19608,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestDynamicExtensions();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestDynamicExtensions cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestDynamicExtensions result;

+      private bool resultIsReadOnly;

+      private TestDynamicExtensions result;

       

       private TestDynamicExtensions PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestDynamicExtensions original = result;

           result = new TestDynamicExtensions();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -19492,13 +19638,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestDynamicExtensions();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -19514,13 +19660,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestDynamicExtensions BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.repeatedExtension_.MakeReadOnly();

-        result.packedExtension_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -19762,7 +19906,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearMessageExtension() {

         PrepareBuilder();

         result.hasMessageExtension = false;

-        result.messageExtension_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;

+        result.messageExtension_ = null;

         return this;

       }

       

@@ -19802,7 +19946,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearDynamicMessageExtension() {

         PrepareBuilder();

         result.hasDynamicMessageExtension = false;

-        result.dynamicMessageExtension_ = global::Google.ProtocolBuffers.TestProtos.TestDynamicExtensions.Types.DynamicMessageType.DefaultInstance;

+        result.dynamicMessageExtension_ = null;

         return this;

       }

       

@@ -19878,7 +20022,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class TestRepeatedScalarDifferentTagSizes : pb::GeneratedMessage<TestRepeatedScalarDifferentTagSizes, TestRepeatedScalarDifferentTagSizes.Builder> {

     private TestRepeatedScalarDifferentTagSizes() { }

-    private static readonly TestRepeatedScalarDifferentTagSizes defaultInstance = new Builder().BuildPartial();

+    private static readonly TestRepeatedScalarDifferentTagSizes defaultInstance = new TestRepeatedScalarDifferentTagSizes().MakeReadOnly();

     private static readonly string[] _testRepeatedScalarDifferentTagSizesFieldNames = new string[] { "repeated_fixed32", "repeated_fixed64", "repeated_float", "repeated_int32", "repeated_int64", "repeated_uint64" };

     private static readonly uint[] _testRepeatedScalarDifferentTagSizesFieldTags = new uint[] { 101, 16369, 2097141, 104, 16376, 2097144 };

     public static TestRepeatedScalarDifferentTagSizes DefaultInstance {

@@ -19886,7 +20030,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override TestRepeatedScalarDifferentTagSizes DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override TestRepeatedScalarDifferentTagSizes ThisMessage {

@@ -20094,6 +20238,16 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestRepeatedScalarDifferentTagSizes ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private TestRepeatedScalarDifferentTagSizes MakeReadOnly() {

+      repeatedFixed32_.MakeReadOnly();

+      repeatedInt32_.MakeReadOnly();

+      repeatedFixed64_.MakeReadOnly();

+      repeatedInt64_.MakeReadOnly();

+      repeatedFloat_.MakeReadOnly();

+      repeatedUint64_.MakeReadOnly();

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -20109,22 +20263,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new TestRepeatedScalarDifferentTagSizes();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(TestRepeatedScalarDifferentTagSizes cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      TestRepeatedScalarDifferentTagSizes result;

+      private bool resultIsReadOnly;

+      private TestRepeatedScalarDifferentTagSizes result;

       

       private TestRepeatedScalarDifferentTagSizes PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           TestRepeatedScalarDifferentTagSizes original = result;

           result = new TestRepeatedScalarDifferentTagSizes();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -20139,13 +20293,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new TestRepeatedScalarDifferentTagSizes();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -20161,17 +20315,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override TestRepeatedScalarDifferentTagSizes BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        result.repeatedFixed32_.MakeReadOnly();

-        result.repeatedInt32_.MakeReadOnly();

-        result.repeatedFixed64_.MakeReadOnly();

-        result.repeatedInt64_.MakeReadOnly();

-        result.repeatedFloat_.MakeReadOnly();

-        result.repeatedUint64_.MakeReadOnly();

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -20492,7 +20640,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class FooRequest : pb::GeneratedMessage<FooRequest, FooRequest.Builder> {

     private FooRequest() { }

-    private static readonly FooRequest defaultInstance = new Builder().BuildPartial();

+    private static readonly FooRequest defaultInstance = new FooRequest().MakeReadOnly();

     private static readonly string[] _fooRequestFieldNames = new string[] {  };

     private static readonly uint[] _fooRequestFieldTags = new uint[] {  };

     public static FooRequest DefaultInstance {

@@ -20500,7 +20648,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override FooRequest DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override FooRequest ThisMessage {

@@ -20570,6 +20718,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static FooRequest ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private FooRequest MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -20585,22 +20737,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new FooRequest();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(FooRequest cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      FooRequest result;

+      private bool resultIsReadOnly;

+      private FooRequest result;

       

       private FooRequest PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           FooRequest original = result;

           result = new FooRequest();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -20615,13 +20767,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new FooRequest();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -20637,11 +20789,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override FooRequest BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -20719,7 +20871,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class FooResponse : pb::GeneratedMessage<FooResponse, FooResponse.Builder> {

     private FooResponse() { }

-    private static readonly FooResponse defaultInstance = new Builder().BuildPartial();

+    private static readonly FooResponse defaultInstance = new FooResponse().MakeReadOnly();

     private static readonly string[] _fooResponseFieldNames = new string[] {  };

     private static readonly uint[] _fooResponseFieldTags = new uint[] {  };

     public static FooResponse DefaultInstance {

@@ -20727,7 +20879,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override FooResponse DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override FooResponse ThisMessage {

@@ -20797,6 +20949,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static FooResponse ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private FooResponse MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -20812,22 +20968,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new FooResponse();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(FooResponse cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      FooResponse result;

+      private bool resultIsReadOnly;

+      private FooResponse result;

       

       private FooResponse PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           FooResponse original = result;

           result = new FooResponse();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -20842,13 +20998,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new FooResponse();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -20864,11 +21020,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override FooResponse BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -20946,7 +21102,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class BarRequest : pb::GeneratedMessage<BarRequest, BarRequest.Builder> {

     private BarRequest() { }

-    private static readonly BarRequest defaultInstance = new Builder().BuildPartial();

+    private static readonly BarRequest defaultInstance = new BarRequest().MakeReadOnly();

     private static readonly string[] _barRequestFieldNames = new string[] {  };

     private static readonly uint[] _barRequestFieldTags = new uint[] {  };

     public static BarRequest DefaultInstance {

@@ -20954,7 +21110,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override BarRequest DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override BarRequest ThisMessage {

@@ -21024,6 +21180,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static BarRequest ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private BarRequest MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -21039,22 +21199,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new BarRequest();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(BarRequest cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      BarRequest result;

+      private bool resultIsReadOnly;

+      private BarRequest result;

       

       private BarRequest PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           BarRequest original = result;

           result = new BarRequest();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -21069,13 +21229,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new BarRequest();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -21091,11 +21251,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override BarRequest BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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

@@ -21173,7 +21333,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class BarResponse : pb::GeneratedMessage<BarResponse, BarResponse.Builder> {

     private BarResponse() { }

-    private static readonly BarResponse defaultInstance = new Builder().BuildPartial();

+    private static readonly BarResponse defaultInstance = new BarResponse().MakeReadOnly();

     private static readonly string[] _barResponseFieldNames = new string[] {  };

     private static readonly uint[] _barResponseFieldTags = new uint[] {  };

     public static BarResponse DefaultInstance {

@@ -21181,7 +21341,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }

     

     public override BarResponse DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override BarResponse ThisMessage {

@@ -21251,6 +21411,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static BarResponse ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private BarResponse MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

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

@@ -21266,22 +21430,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new BarResponse();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(BarResponse cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      BarResponse result;

+      private bool resultIsReadOnly;

+      private BarResponse result;

       

       private BarResponse PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           BarResponse original = result;

           result = new BarResponse();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -21296,13 +21460,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new BarResponse();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -21318,11 +21482,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }

       

       public override BarResponse BuildPartial() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return result;

         }

-        builderIsReadOnly = true;

-        return result;

+        resultIsReadOnly = true;

+        return result.MakeReadOnly();

       }

       

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