Descriptors are pretty much complete, with a slight issue of how to find the default type for a repeated field.

17 files changed, 1194 insertions, 214 deletions

diff --git a/csharp/ProtocolBuffers/DescriptorProtos/Autogenerated.cs b/csharp/ProtocolBuffers/DescriptorProtos/Autogenerated.cs
deleted file mode 100644
index 86cfdae7..00000000
--- a/csharp/ProtocolBuffers/DescriptorProtos/Autogenerated.cs
+++ /dev/null
@@ -1,149 +0,0 @@
-// This file contains quick hacks to represent code that will be autogenerated.
-
-namespace Google.ProtocolBuffers.DescriptorProtos {
-
-  /// <summary>
-  /// This only exists until we've got the real code...
-  /// </summary>
-  /*
-  public abstract class TemporaryMessage<T> : IMessage<T> where T : IMessage<T> {
-    #region IMessage<T> Members
-
-    public IMessage<T> DefaultInstanceForType {
-      get { throw new System.NotImplementedException(); }
-    }
-
-    public IBuilder<T> CreateBuilderForType() {
-      throw new System.NotImplementedException();
-    }
-
-    #endregion
-
-    #region IMessage Members
-
-    public Google.ProtocolBuffers.Descriptors.MessageDescriptor DescriptorForType {
-      get { throw new System.NotImplementedException(); }
-    }
-
-    public System.Collections.Generic.IDictionary<Google.ProtocolBuffers.Descriptors.FieldDescriptor, object> AllFields {
-      get { throw new System.NotImplementedException(); }
-    }
-
-    public bool HasField(Google.ProtocolBuffers.Descriptors.FieldDescriptor field) {
-      throw new System.NotImplementedException();
-    }
-
-    public object this[Google.ProtocolBuffers.Descriptors.FieldDescriptor field] {
-      get { throw new System.NotImplementedException(); }
-    }
-
-    public int GetRepeatedFieldCount(Google.ProtocolBuffers.Descriptors.FieldDescriptor field) {
-      throw new System.NotImplementedException();
-    }
-
-    public object this[Google.ProtocolBuffers.Descriptors.FieldDescriptor field, int index] {
-      get { throw new System.NotImplementedException(); }
-    }
-
-    public UnknownFieldSet UnknownFields {
-      get { throw new System.NotImplementedException(); }
-    }
-
-    public bool IsInitialized {
-      get { throw new System.NotImplementedException(); }
-    }
-
-    public void WriteTo(CodedOutputStream output) {
-      throw new System.NotImplementedException();
-    }
-
-    public int SerializedSize {
-      get { throw new System.NotImplementedException(); }
-    }
-
-    public ByteString ToByteString() {
-      throw new System.NotImplementedException();
-    }
-
-    public byte[] ToByteArray() {
-      throw new System.NotImplementedException();
-    }
-
-    public void WriteTo(System.IO.Stream output) {
-      throw new System.NotImplementedException();
-    }
-
-    IMessage IMessage.DefaultInstanceForType {
-      get { throw new System.NotImplementedException(); }
-    }
-
-    IBuilder IMessage.CreateBuilderForType() {
-      throw new System.NotImplementedException();
-    }
-
-    #endregion
-  }
-
-  public partial class MessageOptions : TemporaryMessage<MessageOptions> {
-    public bool MessageSetWireFormat;
-  }
-
-  public partial class DescriptorProto : TemporaryMessage<DescriptorProto> {
-    public string Name { get; set; }
-    public string FullName { get; set; }
-    public MessageOptions Options { get; set; }
-  }
-
-  public partial class EnumDescriptorProto : TemporaryMessage<EnumDescriptorProto> {
-    public string Name { get; set; }
-    public string FullName { get; set; }
-    public EnumOptions Options { get; set; }
-  }
-
-  public partial class EnumOptions : TemporaryMessage<EnumOptions> { }
-
-  public partial class EnumValueDescriptorProto : TemporaryMessage<EnumValueDescriptorProto> {
-    public string Name { get; set; }
-    public string FullName { get; set; }
-    public EnumValueOptions Options { get; set; }
-  }
-
-  public partial class EnumValueOptions : TemporaryMessage <EnumValueOptions> { }
-
-  public partial class FieldDescriptorProto : TemporaryMessage<FieldDescriptorProto> {
-    public string Name { get; set; }
-    public string FullName { get; set; }
-    public FieldOptions Options { get; set; }
-
-  }
-
-  public partial class FieldOptions : TemporaryMessage<FieldOptions> { }
-
-  public partial class FileDescriptorProto : TemporaryMessage<FileDescriptorProto> {
-    public string Name { get; set; }
-    public string FullName { get; set; }
-    public FileOptions Options { get; set; }
-
-    public string Package { get; set; }
-  }
-
-  public partial class FileOptions : TemporaryMessage<FileOptions> { }
-
-  public partial class MethodDescriptorProto : TemporaryMessage<MethodDescriptorProto> {
-    public string Name { get; set; }
-    public string FullName { get; set; }
-    public MethodOptions Options { get; set; }
-
-  }
-
-  public partial class MethodOptions : TemporaryMessage<MethodOptions> { }
-
-  public partial class ServiceDescriptorProto : TemporaryMessage<ServiceDescriptorProto> {
-    public string Name { get; set; }
-    public string FullName { get; set; }
-    public ServiceOptions Options { get; set; }
-
-  }
-
-  public partial class ServiceOptions : TemporaryMessage<ServiceOptions> { } */
-}
diff --git a/csharp/ProtocolBuffers/Descriptors/DescriptorBase.cs b/csharp/ProtocolBuffers/Descriptors/DescriptorBase.cs
index e6be7547..0678cb6d 100644
--- a/csharp/ProtocolBuffers/Descriptors/DescriptorBase.cs
+++ b/csharp/ProtocolBuffers/Descriptors/DescriptorBase.cs
@@ -2,21 +2,39 @@
 using System.Collections.Generic;
 using System.Text;
 using Google.ProtocolBuffers.DescriptorProtos;
+using Google.ProtocolBuffers.Collections;
 
 namespace Google.ProtocolBuffers.Descriptors {
   /// <summary>
-  /// Base class for all descriptors, providing common functionality.
+  /// Base class for nearly all descriptors, providing common functionality.
   /// </summary>
   /// <typeparam name="TProto">Type of the protocol buffer form of this descriptor</typeparam>
   /// <typeparam name="TOptions">Type of the options protocol buffer for this descriptor</typeparam>
-  public abstract class DescriptorBase<TProto, TOptions> where TProto : IMessage<TProto>, IDescriptorProto<TOptions> {
+  public abstract class DescriptorBase<TProto, TOptions> : IDescriptor<TProto>
+      where TProto : IMessage, IDescriptorProto<TOptions> {
 
     private readonly TProto proto;
     private readonly FileDescriptor file;
+    private readonly string fullName;
 
-    protected DescriptorBase(TProto proto, FileDescriptor file) {
+    protected DescriptorBase(TProto proto, FileDescriptor file, string fullName) {
       this.proto = proto;
       this.file = file;
+      this.fullName = fullName;
+    }
+
+    protected static string ComputeFullName(FileDescriptor file, MessageDescriptor parent, string name) {
+      if (parent != null) {
+        return parent.FullName + "." + name;
+      }
+      if (file.Package.Length > 0) {
+        return file.Package + "." + name;
+      }
+      return name;
+    }
+
+    IMessage IDescriptor.Proto {
+      get { return proto; }
     }
 
     /// <summary>
@@ -32,10 +50,9 @@ namespace Google.ProtocolBuffers.Descriptors {
 
     /// <summary>
     /// The fully qualified name of the descriptor's target.
-    /// TODO(jonskeet): Implement!
     /// </summary>
     public string FullName {
-      get { return null; }
+      get { return fullName; }
     }
 
     /// <summary>
diff --git a/csharp/ProtocolBuffers/Descriptors/DescriptorPool.cs b/csharp/ProtocolBuffers/Descriptors/DescriptorPool.cs
index f2876976..fc30295e 100644
--- a/csharp/ProtocolBuffers/Descriptors/DescriptorPool.cs
+++ b/csharp/ProtocolBuffers/Descriptors/DescriptorPool.cs
@@ -1,4 +1,7 @@
 using System.Collections.Generic;
+using System;
+using System.Text;
+using System.Text.RegularExpressions;
 
 namespace Google.ProtocolBuffers.Descriptors {
   /// <summary>
@@ -6,17 +9,258 @@ namespace Google.ProtocolBuffers.Descriptors {
   /// </summary>
   internal class DescriptorPool {
 
-    IDictionary<string, object> byName;
+    private readonly IDictionary<string, IDescriptor> descriptorsByName =
+        new Dictionary<string, IDescriptor>();
+    private readonly IDictionary<DescriptorIntPair, FieldDescriptor> fieldsByNumber =
+        new Dictionary<DescriptorIntPair, FieldDescriptor>();
+    private readonly IDictionary<DescriptorIntPair, EnumValueDescriptor> enumValuesByNumber =
+        new Dictionary<DescriptorIntPair, EnumValueDescriptor>();
+    private readonly DescriptorPool[] dependencies;
+
+    internal DescriptorPool(FileDescriptor[] dependencyFiles) {
+      dependencies = new DescriptorPool[dependencyFiles.Length];
+      for (int i = 0; i < dependencyFiles.Length; i++) {
+        dependencies[i] = dependencyFiles[i].DescriptorPool;
+      }
+
+      foreach (FileDescriptor dependency in dependencyFiles) {
+        AddPackage(dependency.Package, dependency);
+      }
+    }
 
     /// <summary>
     /// Finds a symbol of the given name within the pool.
     /// </summary>
     /// <typeparam name="T">The type of symbol to look for</typeparam>
-    /// <param name="name">Name to look up</param>
+    /// <param name="fullName">Fully-qualified name to look up</param>
     /// <returns>The symbol with the given name and type,
     /// or null if the symbol doesn't exist or has the wrong type</returns>
-    internal T FindSymbol<T>(string name) where T : class {
-      return default(T);
+    internal T FindSymbol<T>(string fullName) where T : class, IDescriptor {
+      IDescriptor result;
+      descriptorsByName.TryGetValue(fullName, out result);
+      T descriptor = result as T;
+      if (descriptor != null) {
+        return descriptor;
+      }
+
+      foreach (DescriptorPool dependency in dependencies) {
+        dependency.descriptorsByName.TryGetValue(fullName, out result);
+        descriptor = result as T;
+        if (descriptor != null) {
+          return descriptor;
+        }
+      }
+
+      return null;
+    }
+
+    /// <summary>
+    /// Adds a package to the symbol tables. If a package by the same name
+    /// already exists, that is fine, but if some other kind of symbol
+    /// exists under the same name, an exception is thrown. If the package
+    /// has multiple components, this also adds the parent package(s).
+    /// </summary>
+    internal void AddPackage(string fullName, FileDescriptor file) {
+      int dotpos = fullName.LastIndexOf('.');
+      String name;
+      if (dotpos != -1) {
+        AddPackage(fullName.Substring(0, dotpos), file);
+        name = fullName.Substring(dotpos + 1);
+      } else {
+        name = fullName;
+      }
+
+      IDescriptor old;
+      if (descriptorsByName.TryGetValue(fullName, out old)) {
+        if (!(old is PackageDescriptor)) {
+          throw new DescriptorValidationException(file,
+           "\"" + name + "\" is already defined (as something other than a " +
+            "package) in file \"" + old.File.Name + "\".");
+        }
+      }
+      // TODO(jonskeet): Check issue 25 wrt the ordering of these parameters
+      descriptorsByName[fullName] = new PackageDescriptor(fullName, name, file);
+    }
+
+    /// <summary>
+    /// Adds a symbol to the symbol table.
+    /// </summary>
+    /// <exception cref="DescriptorValidationException">The symbol already existed
+    /// in the symbol table.</exception>
+    internal void AddSymbol(IDescriptor descriptor) {
+      ValidateSymbolName(descriptor);
+      String fullName = descriptor.FullName;
+
+      IDescriptor old;
+      if (descriptorsByName.TryGetValue(fullName, out old)) {
+        int dotPos = fullName.LastIndexOf('.');
+        string message;
+        if (descriptor.File == old.File) {
+          if (dotPos == -1) {
+            message = "\"" + fullName + "\" is already defined.";
+          } else {
+            message = "\"" + fullName.Substring(dotPos + 1) + "\" is already defined in \"" + fullName.Substring(0, dotPos) + "\".";
+          }
+        } else {
+          message = "\"" + fullName + "\" is already defined in file \"" + old.File.Name + "\".";
+        }
+        throw new DescriptorValidationException(descriptor, message);
+      }
+      descriptorsByName[fullName] = descriptor;
+    }
+
+    private static readonly Regex ValidationRegex = new Regex("^[_A-Za-z][_A-Za-z0-9]*$", RegexOptions.Compiled);
+
+    /// <summary>
+    /// Verifies that the descriptor's name is valid (i.e. it contains
+    /// only letters, digits and underscores, and does not start with a digit).
+    /// </summary>
+    /// <param name="descriptor"></param>
+    private static void ValidateSymbolName(IDescriptor descriptor) {
+      if (descriptor.Name == "") {
+        throw new DescriptorValidationException(descriptor, "Missing name.");
+      }
+      if (!ValidationRegex.IsMatch(descriptor.Name)) {
+        throw new DescriptorValidationException(descriptor,
+            "\"" + descriptor.Name + "\" is not a valid identifier.");
+      }
+    }
+
+    /// <summary>
+    /// Returns the field with the given number in the given descriptor,
+    /// or null if it can't be found.
+    /// </summary>
+    internal FieldDescriptor FindFieldByNumber(MessageDescriptor messageDescriptor, int number) {
+      FieldDescriptor ret;
+      fieldsByNumber.TryGetValue(new DescriptorIntPair(messageDescriptor, number), out ret);
+      return ret;
+    }
+
+    internal EnumValueDescriptor FindEnumValueByNumber(EnumDescriptor enumDescriptor, int number) {
+      EnumValueDescriptor ret;
+      enumValuesByNumber.TryGetValue(new DescriptorIntPair(enumDescriptor, number), out ret);
+      return ret;
+    }
+
+    /// <summary>
+    /// Adds a field to the fieldsByNumber table.
+    /// </summary>
+    /// <exception cref="DescriptorValidationException">A field with the same
+    /// containing type and number already exists.</exception>
+    internal void AddFieldByNumber(FieldDescriptor field) {
+      DescriptorIntPair key = new DescriptorIntPair(field.ContainingType, field.FieldNumber);
+      FieldDescriptor old;
+      if (fieldsByNumber.TryGetValue(key, out old)) {
+        throw new DescriptorValidationException(field, "Field number " + field.FieldNumber +
+          "has already been used in \"" + field.ContainingType.FullName +
+          "\" by field \"" + old.Name + "\".");
+      }
+      fieldsByNumber[key] = field;
+    }
+
+    /// <summary>
+    /// Adds an enum value to the enumValuesByNumber table. If an enum value
+    /// with the same type and number already exists, this method does nothing.
+    /// (This is allowed; the first value defined with the number takes precedence.)
+    /// </summary>
+    internal void AddEnumValueByNumber(EnumValueDescriptor enumValue) {
+      DescriptorIntPair key = new DescriptorIntPair(enumValue.EnumDescriptor, enumValue.Number);
+      if (!enumValuesByNumber.ContainsKey(key)) {
+        enumValuesByNumber[key] = enumValue;
+      }
+    }
+
+    /// <summary>
+    /// Looks up a descriptor by name, relative to some other descriptor.
+    /// The name may be fully-qualified (with a leading '.'), partially-qualified,
+    /// or unqualified. C++-like name lookup semantics are used to search for the
+    /// matching descriptor.
+    /// </summary>
+    public IDescriptor LookupSymbol(string name, IDescriptor relativeTo) {
+      // TODO(jonskeet):  This could be optimized in a number of ways.
+
+      IDescriptor result;
+      if (name.StartsWith(".")) {
+        // Fully-qualified name.
+        result = FindSymbol<IDescriptor>(name.Substring(1));
+      } else {
+        // If "name" is a compound identifier, we want to search for the
+        // first component of it, then search within it for the rest.
+        int firstPartLength = name.IndexOf('.');
+        string firstPart = firstPartLength == -1 ? name : name.Substring(0, firstPartLength);
+
+        // We will search each parent scope of "relativeTo" looking for the
+        // symbol.
+        StringBuilder scopeToTry = new StringBuilder(relativeTo.FullName);
+
+        while (true) {
+          // Chop off the last component of the scope.
+
+          // TODO(jonskeet): Make this more efficient. May not be worth using StringBuilder at all
+          int dotpos = scopeToTry.ToString().LastIndexOf(".");
+          if (dotpos == -1) {
+            result = FindSymbol<IDescriptor>(name);
+            break;
+          } else {
+            scopeToTry.Length = dotpos + 1;
+
+            // Append firstPart and try to find.
+            scopeToTry.Append(firstPart);
+            result = FindSymbol<IDescriptor>(scopeToTry.ToString());
+
+            if (result != null) {
+              if (firstPartLength != -1) {
+                // We only found the first part of the symbol.  Now look for
+                // the whole thing.  If this fails, we *don't* want to keep
+                // searching parent scopes.
+                scopeToTry.Length = dotpos + 1;
+                scopeToTry.Append(name);
+                result = FindSymbol<IDescriptor>(scopeToTry.ToString());
+              }
+              break;
+            }
+
+            // Not found.  Remove the name so we can try again.
+            scopeToTry.Length = dotpos;
+          }
+        }
+      }
+
+      if (result == null) {
+        throw new DescriptorValidationException(relativeTo, "\"" + name + "\" is not defined.");
+      } else {
+        return result;
+      }
+    }
+
+    /// <summary>
+    /// Struct used to hold the keys for the fieldByNumber table.
+    /// </summary>
+    struct DescriptorIntPair : IEquatable<DescriptorIntPair> {
+
+      private readonly int number;
+      private readonly IDescriptor descriptor;
+
+      internal DescriptorIntPair(IDescriptor descriptor, int number) {
+        this.number = number;
+        this.descriptor = descriptor;
+      }
+
+      public bool Equals(DescriptorIntPair other) {
+        return descriptor == other.descriptor
+            && number == other.number;
+      }
+
+      public override bool Equals(object obj) {
+        if (obj is DescriptorIntPair) {
+          return Equals((DescriptorIntPair)obj);
+        }
+        return false;
+      }
+
+      public override int GetHashCode() {
+        return descriptor.GetHashCode() * ((1 << 16) - 1) + number;
+      }
     }
   }
 }
diff --git a/csharp/ProtocolBuffers/Descriptors/DescriptorUtil.cs b/csharp/ProtocolBuffers/Descriptors/DescriptorUtil.cs
new file mode 100644
index 00000000..a75d5f92
--- /dev/null
+++ b/csharp/ProtocolBuffers/Descriptors/DescriptorUtil.cs
@@ -0,0 +1,27 @@
+using System.Collections.Generic;
+using Google.ProtocolBuffers.Collections;
+namespace Google.ProtocolBuffers.Descriptors {
+  /// <summary>
+  /// Internal class containing utility methods when working with descriptors.
+  /// </summary>
+  static class DescriptorUtil {
+    /// <summary>
+    /// Equivalent to Func[TInput, int, TOutput] but usable in .NET 2.0. Only used to convert
+    /// arrays.
+    /// </summary>
+    internal delegate TOutput IndexedConverter<TInput, TOutput>(TInput element, int index);
+
+    /// <summary>
+    /// Converts the given array into a read-only list, applying the specified conversion to
+    /// each input element.
+    /// </summary>
+    internal static IList<TOutput> ConvertAndMakeReadOnly<TInput, TOutput>(IList<TInput> input,
+        IndexedConverter<TInput, TOutput> converter) {
+      TOutput[] array = new TOutput[input.Count];
+      for (int i = 0; i < array.Length; i++) {
+        array[i] = converter(input[i], i);
+      }
+      return Lists<TOutput>.AsReadOnly(array);
+    }
+  }
+}
diff --git a/csharp/ProtocolBuffers/Descriptors/DescriptorValidationException.cs b/csharp/ProtocolBuffers/Descriptors/DescriptorValidationException.cs
new file mode 100644
index 00000000..bc0c2078
--- /dev/null
+++ b/csharp/ProtocolBuffers/Descriptors/DescriptorValidationException.cs
@@ -0,0 +1,53 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Google.ProtocolBuffers.Descriptors {
+  public class DescriptorValidationException : Exception {
+
+    private readonly String name;
+    private readonly IMessage proto;
+    private readonly string description;
+
+    /// <value>
+    /// The full name of the descriptor where the error occurred.
+    /// </value>
+    public String ProblemSymbolName { 
+      get { return name; }
+    }
+
+    /// <value>
+    /// The protocol message representation of the invalid descriptor.
+    /// </value>
+    public IMessage ProblemProto {
+      get { return proto; }
+    }
+
+    /// <value>
+    /// A human-readable description of the error. (The Message property
+    /// is made up of the descriptor's name and this description.)
+    /// </value>
+    public string Description {
+      get { return description; }
+    }
+
+    internal DescriptorValidationException(IDescriptor problemDescriptor, string description) :
+        base(problemDescriptor.FullName + ": " + description) {
+
+      // Note that problemDescriptor may be partially uninitialized, so we
+      // don't want to expose it directly to the user.  So, we only provide
+      // the name and the original proto.
+      name = problemDescriptor.FullName;
+      proto = problemDescriptor.Proto;
+      this.description = description;
+    }
+
+    internal DescriptorValidationException(IDescriptor problemDescriptor, string description, Exception cause) :
+        base(problemDescriptor.FullName + ": " + description, cause) {
+
+      name = problemDescriptor.FullName;
+      proto = problemDescriptor.Proto;
+      this.description = description;
+    }
+  }
+}
diff --git a/csharp/ProtocolBuffers/Descriptors/EnumDescriptor.cs b/csharp/ProtocolBuffers/Descriptors/EnumDescriptor.cs
index 1ebe5a7e..3dccc287 100644
--- a/csharp/ProtocolBuffers/Descriptors/EnumDescriptor.cs
+++ b/csharp/ProtocolBuffers/Descriptors/EnumDescriptor.cs
@@ -1,13 +1,61 @@
 
+using System;
+using System.Collections.Generic;
 using Google.ProtocolBuffers.DescriptorProtos;
 namespace Google.ProtocolBuffers.Descriptors {
+
   public class EnumDescriptor : IndexedDescriptorBase<EnumDescriptorProto, EnumOptions> {
-    internal EnumDescriptor(EnumDescriptorProto proto, EnumOptions options, FileDescriptor file, int index)
-        : base(proto, file, index) {
+
+    private readonly MessageDescriptor containingType;
+    private readonly IList<EnumValueDescriptor> values;
+
+    internal EnumDescriptor(EnumDescriptorProto proto, FileDescriptor file, MessageDescriptor parent, int index)
+        : base(proto, file, ComputeFullName(file, parent, proto.Name), index) {
+      containingType = parent;
+
+      if (proto.ValueCount == 0) {
+        // We cannot allow enums with no values because this would mean there
+        // would be no valid default value for fields of this type.
+        throw new DescriptorValidationException(this, "Enums must contain at least one value.");
+      }
+      
+      values = DescriptorUtil.ConvertAndMakeReadOnly(proto.ValueList,
+          (value, i) => new EnumValueDescriptor(value, file, this, i));
+
+      File.DescriptorPool.AddSymbol(this);
+    }
+
+    /// <value>
+    /// If this is a nested type, get the outer descriptor, otherwise null.
+    /// </value>
+    public MessageDescriptor ContainingType {
+      get { return containingType; }
+    }
+
+    /// <value>
+    /// An unmodifiable list of defined value descriptors for this enum.
+    /// </value>
+    public IList<EnumValueDescriptor> Values {
+      get { return values; }
+    }
+
+    /// <summary>
+    /// Finds an enum value by number. If multiple enum values have the
+    /// same number, this returns the first defined value with that number.
+    /// </summary>
+    /// <param name="number"></param>
+    /// <returns></returns>
+    internal EnumValueDescriptor FindValueByNumber(int number) {
+      return File.DescriptorPool.FindEnumValueByNumber(this, number);
     }
 
-    internal EnumValueDescriptor FindValueByNumber(int rawValue) {
-      throw new System.NotImplementedException();
+    /// <summary>
+    /// Finds an enum value by name.
+    /// </summary>
+    /// <param name="name">The unqualified name of the value (e.g. "FOO").</param>
+    /// <returns>The value's descriptor, or null if not found.</returns>
+    internal EnumValueDescriptor FindValueByName(string name) {
+      return File.DescriptorPool.FindSymbol<EnumValueDescriptor>(FullName + "." + name);
     }
   }
 }
diff --git a/csharp/ProtocolBuffers/Descriptors/EnumValueDescriptor.cs b/csharp/ProtocolBuffers/Descriptors/EnumValueDescriptor.cs
index 94bd68bf..15b901bd 100644
--- a/csharp/ProtocolBuffers/Descriptors/EnumValueDescriptor.cs
+++ b/csharp/ProtocolBuffers/Descriptors/EnumValueDescriptor.cs
@@ -6,15 +6,16 @@ namespace Google.ProtocolBuffers.Descriptors {
 
     private readonly EnumDescriptor enumDescriptor;
 
-    internal EnumValueDescriptor(EnumValueDescriptorProto proto,
-                                 FileDescriptor file,
-                                 EnumDescriptor parent,
-                                 int index) : base (proto, file, index) {
+    internal EnumValueDescriptor(EnumValueDescriptorProto proto, FileDescriptor file,
+        EnumDescriptor parent, int index) 
+        : base (proto, file, parent.FullName + "." + proto.Name, index) {
       enumDescriptor = parent;
+      file.DescriptorPool.AddSymbol(this);
+      file.DescriptorPool.AddEnumValueByNumber(this);
     }
 
     public int Number {
-      get { throw new NotImplementedException(); }
+      get { return Proto.Number; }
     }
 
     public EnumDescriptor EnumDescriptor {
diff --git a/csharp/ProtocolBuffers/Descriptors/FieldDescriptor.cs b/csharp/ProtocolBuffers/Descriptors/FieldDescriptor.cs
index 0cfc723d..1750ce1d 100644
--- a/csharp/ProtocolBuffers/Descriptors/FieldDescriptor.cs
+++ b/csharp/ProtocolBuffers/Descriptors/FieldDescriptor.cs
@@ -5,43 +5,215 @@ using Google.ProtocolBuffers.Collections;
 using Google.ProtocolBuffers.DescriptorProtos;
 
 namespace Google.ProtocolBuffers.Descriptors {
-  public class FieldDescriptor : IndexedDescriptorBase<FieldDescriptorProto, FieldOptions> {
+  public class FieldDescriptor : IndexedDescriptorBase<FieldDescriptorProto, FieldOptions>, IComparable<FieldDescriptor> {
 
-    private readonly EnumDescriptor enumType;
-    private readonly MessageDescriptor parent;
+    private readonly MessageDescriptor extensionScope;
+    private EnumDescriptor enumType;
+    private MessageDescriptor messageType;
+    private MessageDescriptor containingType;
+    private object defaultValue;
+    private FieldType fieldType;
+    private MappedType mappedType;
 
-    internal FieldDescriptor(FieldDescriptorProto proto,
-        FileDescriptor file,
-        MessageDescriptor parent,
-        int index,
-        bool isExtension) : base(proto, file, index) {
-      enumType = null;
-      this.parent = parent;
+    internal FieldDescriptor(FieldDescriptorProto proto, FileDescriptor file,
+        MessageDescriptor parent, int index, bool isExtension) 
+        : base(proto, file, ComputeFullName(file, parent, proto.Name), index) {
+      
+      if (proto.HasType) {
+        fieldType = GetFieldTypeFromProtoType(proto.Type);
+        //type = proto.Type;
+      }
+      
+      if (FieldNumber <= 0) {
+        throw new DescriptorValidationException(this,
+          "Field numbers must be positive integers.");
+      }
+
+      if (isExtension) {
+        if (!proto.HasExtendee) {
+          throw new DescriptorValidationException(this,
+              "FieldDescriptorProto.Extendee not set for extension field.");
+        }
+        containingType = null;  // Will be filled in when cross-linking
+        if (parent != null) {
+          extensionScope = parent;
+        } else {
+          extensionScope = null;
+        }
+      } else {
+        if (proto.HasExtendee) {
+          throw new DescriptorValidationException(this,
+              "FieldDescriptorProto.Extendee set for non-extension field.");
+        }
+        containingType = parent;
+        extensionScope = null;
+      }
+
+      file.DescriptorPool.AddSymbol(this);
+    }
+
+    /// <summary>
+    /// Maps a field type as included in the .proto file to a FieldType.
+    /// </summary>
+    private static FieldType GetFieldTypeFromProtoType(FieldDescriptorProto.Types.Type type) {
+      switch (type) {
+        case FieldDescriptorProto.Types.Type.TYPE_DOUBLE:   return FieldType.Double;
+        case FieldDescriptorProto.Types.Type.TYPE_FLOAT:    return FieldType.Float;
+        case FieldDescriptorProto.Types.Type.TYPE_INT64:    return FieldType.Int64;
+        case FieldDescriptorProto.Types.Type.TYPE_UINT64:   return FieldType.UInt64;
+        case FieldDescriptorProto.Types.Type.TYPE_INT32:    return FieldType.Int32;
+        case FieldDescriptorProto.Types.Type.TYPE_FIXED64:  return FieldType.Fixed64;
+        case FieldDescriptorProto.Types.Type.TYPE_FIXED32:  return FieldType.Fixed32;
+        case FieldDescriptorProto.Types.Type.TYPE_BOOL:     return FieldType.Bool;
+        case FieldDescriptorProto.Types.Type.TYPE_STRING:   return FieldType.String;
+        case FieldDescriptorProto.Types.Type.TYPE_GROUP:    return FieldType.Group;
+        case FieldDescriptorProto.Types.Type.TYPE_MESSAGE:  return FieldType.Message;
+        case FieldDescriptorProto.Types.Type.TYPE_BYTES:    return FieldType.Bytes;
+        case FieldDescriptorProto.Types.Type.TYPE_UINT32:   return FieldType.UInt32;
+        case FieldDescriptorProto.Types.Type.TYPE_ENUM:     return FieldType.Enum;
+        case FieldDescriptorProto.Types.Type.TYPE_SFIXED32: return FieldType.SFixed32;
+        case FieldDescriptorProto.Types.Type.TYPE_SFIXED64: return FieldType.SFixed64;
+        case FieldDescriptorProto.Types.Type.TYPE_SINT32:   return FieldType.SInt32;
+        case FieldDescriptorProto.Types.Type.TYPE_SINT64:   return FieldType.SInt64;
+        default:
+          throw new ArgumentException("Invalid type specified");
+      }
+    }
+
+    /// <summary>
+    /// Returns the default value for a mapped type.
+    /// </summary>
+    private static object GetDefaultValueForMappedType(MappedType type) {
+      switch (type) {
+        case MappedType.Int32: return 0;
+        case MappedType.Int64: return (long) 0;
+        case MappedType.UInt32: return (uint) 0;
+        case MappedType.UInt64: return (ulong) 0;
+        case MappedType.Single: return (float) 0;
+        case MappedType.Double: return (double) 0;
+        case MappedType.Boolean: return false;
+        case MappedType.String: return "";
+        case MappedType.ByteString: return ByteString.Empty;
+        case MappedType.Message: return null;
+        case MappedType.Enum: return null;
+        default:
+            throw new ArgumentException("Invalid type specified");
+      }
     }
 
     public bool IsRequired {
-      get;
-      set;
+      get { return Proto.Label == FieldDescriptorProto.Types.Label.LABEL_REQUIRED; }
+    }
+
+    public bool IsOptional {
+      get { return Proto.Label == FieldDescriptorProto.Types.Label.LABEL_OPTIONAL; }
     }
 
-    public MappedType MappedType { get; set; }
+    public bool IsRepeated {
+      get { return Proto.Label == FieldDescriptorProto.Types.Label.LABEL_REPEATED; }
+    }
 
-    public bool IsRepeated { get; set; }
+    /// <valule>
+    /// Indicates whether or not the field had an explicitly-defined default value.
+    /// </value>
+    public bool HasDefaultValue {
+      get { return Proto.HasDefaultValue; }
+    }
 
-    public FieldType FieldType { get; set; }
-    public int FieldNumber { get; set; }
+    /// <value>
+    /// The field's default value. Valid for all types except messages
+    /// and groups. For all other types, the object returned is of the
+    /// same class that would be returned by IMessage[this].
+    /// For repeated fields this will always be an empty list. For message fields it will
+    /// always be null. For singular values, it will depend on the descriptor.
+    /// </value>
+    public object DefaultValue {
+      get {
+        if (MappedType == MappedType.Message) {
+          throw new InvalidOperationException("FieldDescriptor.DefaultValue called on an embedded message field.");
+        }
+        return defaultValue;
+      }
+    }
 
-    public bool IsExtension { get; set; }
+    /// <value>
+    /// Indicates whether or not this field is an extension.
+    /// </value>
+    public bool IsExtension {
+      get { return Proto.HasExtendee; }
+    }
 
+    /*
+     * Get the field's containing type. For extensions, this is the type being
+     * extended, not the location where the extension was defined.  See
+     * {@link #getExtensionScope()}.
+     */
+    /// <summary>
+    /// Get the field's containing type. For extensions, this is the type being
+    /// extended, not the location where the extension was defined. See
+    /// <see cref="ExtensionScope" />.
+    /// </summary>
     public MessageDescriptor ContainingType {
-      get { return parent; }
+      get { return containingType; }
+    }
+    
+    /// <summary>
+    /// For extensions defined nested within message types, gets
+    /// the outer type. Not valid for non-extension fields.
+    /// </summary>
+    /// <example>
+    /// <code>
+    /// message Foo {
+    ///   extensions 1000 to max;
+    /// }
+    /// extend Foo {
+    ///   optional int32 baz = 1234;
+    /// }
+    /// message Bar {
+    ///   extend Foo {
+    ///     optional int32 qux = 4321;
+    ///   }
+    /// }
+    /// </code>
+    /// The containing type for both <c>baz</c> and <c>qux</c> is <c>Foo</c>.
+    /// However, the extension scope for <c>baz</c> is <c>null</c> while
+    /// the extension scope for <c>qux</c> is <c>Bar</c>.
+    /// </example>
+    public MessageDescriptor ExtensionScope {
+      get {
+        if (!IsExtension) {
+          throw new InvalidOperationException("This field is not an extension.");
+        }
+        return extensionScope;
+      }
     }
 
-    public bool IsOptional { get; set; }
+    public MappedType MappedType {
+      get { return mappedType; }
+    }
 
-    public MessageDescriptor MessageType { get; set; }
+    public FieldType FieldType {
+      get { return fieldType; }
+    }
 
-    public MessageDescriptor ExtensionScope { get; set; }
+    public int FieldNumber {
+      get { return Proto.Number; }
+    }
+
+    /// <summary>
+    /// Compares this descriptor with another one, ordering in "canonical" order
+    /// which simply means ascending order by field number. <paramref name="other"/>
+    /// must be a field of the same type, i.e. the <see cref="ContainingType"/> of
+    /// both fields must be the same.
+    /// </summary>
+    public int CompareTo(FieldDescriptor other) {
+      if (other.containingType != containingType) {
+        throw new ArgumentException("FieldDescriptors can only be compared to other FieldDescriptors " +
+          "for fields of the same message type.");
+      }
+      return FieldNumber - other.FieldNumber;
+    }
+    
 
     /// <summary>
     /// For enum fields, returns the field's type.
@@ -53,15 +225,18 @@ namespace Google.ProtocolBuffers.Descriptors {
         }
         return enumType;
       }
-    }   
+    }
 
     /// <summary>
-    /// The default value for this field. For repeated fields
-    /// this will always be an empty list. For message fields it will
-    /// always be null. For singular values, it will depend on the descriptor.
+    /// For embedded message and group fields, returns the field's type.
     /// </summary>
-    public object DefaultValue {
-      get { throw new NotImplementedException(); }
+    public MessageDescriptor MessageType {
+      get {
+        if (MappedType != MappedType.Message) {
+          throw new InvalidOperationException("MessageType is only valid for enum fields.");
+        }
+        return messageType;
+      }
     }
 
     /// <summary>
@@ -70,6 +245,7 @@ namespace Google.ProtocolBuffers.Descriptors {
     /// </summary>
     public static readonly IDictionary<FieldType, MappedType> FieldTypeToWireFormatMap = MapFieldTypes();
 
+
     private static IDictionary<FieldType, MappedType> MapFieldTypes() {
       var map = new Dictionary<FieldType, MappedType>();
       foreach (FieldInfo field in typeof(FieldType).GetFields(BindingFlags.Static | BindingFlags.Public)) {
@@ -79,5 +255,161 @@ namespace Google.ProtocolBuffers.Descriptors {
       }
       return Dictionaries.AsReadOnly(map);
     }
+
+    /// <summary>
+    /// Look up and cross-link all field types etc.
+    /// </summary>
+    internal void CrossLink() {
+      if (Proto.HasExtendee) {
+        IDescriptor extendee = File.DescriptorPool.LookupSymbol(Proto.Extendee, this);
+        if (!(extendee is MessageDescriptor)) {
+          throw new DescriptorValidationException(this, "\"" + Proto.Extendee + "\" is not a message type.");
+        }
+        containingType = (MessageDescriptor) extendee;
+
+        if (!containingType.IsExtensionNumber(FieldNumber)) {
+          throw new DescriptorValidationException(this,
+              "\"" + containingType.FullName + "\" does not declare " + FieldNumber + " as an extension number.");
+        }
+      }
+
+      if (Proto.HasTypeName) {
+        IDescriptor typeDescriptor =
+          File.DescriptorPool.LookupSymbol(Proto.TypeName, this);
+
+        if (!Proto.HasType) {
+          // Choose field type based on symbol.
+          if (typeDescriptor is MessageDescriptor) {
+            fieldType = FieldType.Message;
+            mappedType = MappedType.Message;
+          } else if (typeDescriptor is EnumDescriptor) {
+            fieldType = FieldType.Enum;
+            mappedType = MappedType.Enum;
+          } else {
+            throw new DescriptorValidationException(this, "\"" + Proto.TypeName + "\" is not a type.");
+          }
+        }
+
+        if (MappedType == MappedType.Message) {
+          if (!(typeDescriptor is MessageDescriptor)) {
+            throw new DescriptorValidationException(this, "\"" + Proto.TypeName + "\" is not a message type.");
+          }
+          messageType = (MessageDescriptor) typeDescriptor;
+
+          if (Proto.HasDefaultValue) {
+            throw new DescriptorValidationException(this, "Messages can't have default values.");
+          }
+        } else if (MappedType == Descriptors.MappedType.Enum) {
+          if (!(typeDescriptor is EnumDescriptor)) {
+            throw new DescriptorValidationException(this, "\"" + Proto.TypeName + "\" is not an enum type.");
+          }
+          enumType = (EnumDescriptor)typeDescriptor;
+        } else {
+          throw new DescriptorValidationException(this, "Field with primitive type has type_name.");
+        }
+      } else {
+        if (MappedType == MappedType.Message || MappedType == MappedType.Enum) {
+          throw new DescriptorValidationException(this, "Field with message or enum type missing type_name.");
+        }
+      }
+
+      // We don't attempt to parse the default value until here because for
+      // enums we need the enum type's descriptor.
+      if (Proto.HasDefaultValue) {
+        if (IsRepeated) {
+          throw new DescriptorValidationException(this, "Repeated fields cannot have default values.");
+        }
+
+        try {
+          // TODO(jonskeet): Check signage for Int32 and Int64.
+          switch (FieldType) {
+            case FieldType.Int32:
+            case FieldType.SInt32:
+            case FieldType.SFixed32:
+              defaultValue = TextFormat.ParseInt32(Proto.DefaultValue);
+              break;
+            case FieldType.UInt32:
+            case FieldType.Fixed32:
+              defaultValue = TextFormat.ParseUInt32(Proto.DefaultValue);
+              break;
+            case FieldType.Int64:
+            case FieldType.SInt64:
+            case FieldType.SFixed64:
+              defaultValue = TextFormat.ParseInt64(Proto.DefaultValue);
+              break;
+            case FieldType.UInt64:
+            case FieldType.Fixed64:
+              defaultValue = TextFormat.ParseUInt64(Proto.DefaultValue);
+              break;
+            case FieldType.Float:
+              defaultValue = float.Parse(Proto.DefaultValue);
+              break;
+            case FieldType.Double:
+              defaultValue = double.Parse(Proto.DefaultValue);
+              break;
+            case FieldType.Bool:
+              defaultValue = bool.Parse(Proto.DefaultValue); // TODO(jonskeet): Check this will work
+              break;
+            case FieldType.String:
+              defaultValue = Proto.DefaultValue;
+              break;
+            case FieldType.Bytes:
+              try {
+                defaultValue = TextFormat.UnescapeBytes(Proto.DefaultValue);
+              } catch (FormatException e) {
+                throw new DescriptorValidationException(this, "Couldn't parse default value: " + e.Message);
+              }
+              break;
+            case FieldType.Enum:
+              defaultValue = enumType.FindValueByName(Proto.DefaultValue);
+              if (defaultValue == null) {
+                throw new DescriptorValidationException(this, "Unknown enum default value: \"" + Proto.DefaultValue + "\"");
+              }
+              break;
+            case FieldType.Message:
+            case FieldType.Group:
+              throw new DescriptorValidationException(this, "Message type had default value.");
+          }
+        } catch (FormatException e) {
+          DescriptorValidationException validationException =
+              new DescriptorValidationException(this, "Could not parse default value: \"" + Proto.DefaultValue + "\"", e);
+          throw validationException;
+        }
+      } else {
+        // Determine the default default for this field.
+        if (IsRepeated) {
+          // FIXME(jonskeet): Find out the correct list type and use that instead.
+          defaultValue = new List<object>();
+        } else {
+          switch (MappedType) {
+            case MappedType.Enum:
+              // We guarantee elsewhere that an enum type always has at least
+              // one possible value.
+              defaultValue = enumType.Values[0];
+              break;
+            case MappedType.Message:
+              defaultValue = null;
+              break;
+            default:
+              defaultValue = GetDefaultValueForMappedType(MappedType);
+              break;
+          }
+        }
+      }
+
+      if (!IsExtension) {
+        File.DescriptorPool.AddFieldByNumber(this);
+      }
+
+      if (containingType != null && containingType.Options.MessageSetWireFormat) {
+        if (IsExtension) {
+          if (!IsOptional || FieldType != FieldType.Message) {
+            throw new DescriptorValidationException(this, "Extensions of MessageSets must be optional messages.");
+          }
+        } else {
+          throw new DescriptorValidationException(this, "MessageSets cannot have fields, only extensions.");
+        }
+      }
+    }
   }
 }
diff --git a/csharp/ProtocolBuffers/Descriptors/FileDescriptor.cs b/csharp/ProtocolBuffers/Descriptors/FileDescriptor.cs
index d552d2f0..9e065c86 100644
--- a/csharp/ProtocolBuffers/Descriptors/FileDescriptor.cs
+++ b/csharp/ProtocolBuffers/Descriptors/FileDescriptor.cs
@@ -1,17 +1,64 @@
 using System;
+using System.Collections.ObjectModel;
 using Google.ProtocolBuffers.DescriptorProtos;
 using System.Collections.Generic;
+using Google.ProtocolBuffers.Collections;
 namespace Google.ProtocolBuffers.Descriptors {
-  public class FileDescriptor : DescriptorBase<FileDescriptorProto, FileOptions> {
 
+  /// <summary>
+  /// Describes a .proto file, including everything defined within.
+  /// IDescriptor is implemented such that the File property returns this descriptor,
+  /// and the FullName is the same as the Name.
+  /// </summary>
+  public class FileDescriptor : IDescriptor<FileDescriptorProto> {
+
+    private readonly FileDescriptorProto proto;
     private readonly IList<MessageDescriptor> messageTypes;
     private readonly IList<EnumDescriptor> enumTypes;
     private readonly IList<ServiceDescriptor> services;
     private readonly IList<FieldDescriptor> extensions;
     private readonly IList<FileDescriptor> dependencies;
     private readonly DescriptorPool pool;
+    
+    private FileDescriptor(FileDescriptorProto proto, FileDescriptor[] dependencies, DescriptorPool pool) {
+      this.pool = pool;
+      this.proto = proto;
+      this.dependencies = new ReadOnlyCollection<FileDescriptor>((FileDescriptor[]) dependencies.Clone());
+
+      pool.AddPackage(Package, this);
+
+      messageTypes = DescriptorUtil.ConvertAndMakeReadOnly(proto.MessageTypeList, 
+          (message, index) => new MessageDescriptor(message, this, null, index));
+
+      enumTypes = DescriptorUtil.ConvertAndMakeReadOnly(proto.EnumTypeList,
+        (enumType, index) => new EnumDescriptor(enumType, this, null, index));
+
+      services = DescriptorUtil.ConvertAndMakeReadOnly(proto.ServiceList,
+        (service, index) => new ServiceDescriptor(service, this, index));
+
+      extensions = DescriptorUtil.ConvertAndMakeReadOnly(proto.ExtensionList,
+        (field, index) => new FieldDescriptor(field, this, null, index, true));
+    }
+
+    /// <value>
+    /// The descriptor in its protocol message representation.
+    /// </value>
+    public FileDescriptorProto Proto {
+      get { return proto; }
+    }
 
-    public FileDescriptor(FileDescriptorProto proto, FileDescriptor file) : base(proto, file) {
+    /// <value>
+    /// The <see cref="FileOptions" /> defined in <c>descriptor.proto</c>.
+    /// </value>
+    public FileOptions Options {
+      get { return proto.Options; }
+    }
+
+    /// <value>
+    /// The file name.
+    /// </value>
+    public string Name {
+      get { return proto.Name; }
     }
 
     /// <summary>
@@ -19,7 +66,7 @@ namespace Google.ProtocolBuffers.Descriptors {
     /// be equivalent to the .NET namespace of the generated classes.
     /// </summary>
     public string Package {
-      get { return Proto.Package; }
+      get { return proto.Package; }
     }
 
     /// <value>
@@ -57,10 +104,110 @@ namespace Google.ProtocolBuffers.Descriptors {
       get { return dependencies; }
     }
 
-    public static FileDescriptor BuildFrom(FileDescriptorProto proto,
-        FileDescriptor[] dependencies) {
-      throw new NotImplementedException();
+    /// <value>
+    /// Implementation of IDescriptor.FullName - just returns the same as Name.
+    /// </value>
+    string IDescriptor.FullName {
+      get { return Name; }
+    }
+
+    /// <value>
+    /// Implementation of IDescriptor.File - just returns this descriptor.
+    /// </value>
+    FileDescriptor IDescriptor.File {
+      get { return this; }
+    }
+
+    /// <value>
+    /// Protocol buffer describing this descriptor.
+    /// </value>
+    IMessage IDescriptor.Proto {
+      get { return Proto; }
+    }
+
+    /// <value>
+    /// Pool containing symbol descriptors.
+    /// </value>
+    internal DescriptorPool DescriptorPool {
+      get { return pool; }
+    }
+    
+    /// <summary>
+    /// Finds a type (message, enum, service or extension) in the file by name. Does not find nested types.
+    /// </summary>
+    /// <param name="name">The unqualified type name to look for.</param>
+    /// <typeparam name="T">The type of descriptor to look for (or ITypeDescriptor for any)</typeparam>
+    /// <returns>The type's descriptor, or null if not found.</returns>
+    public T FindTypeByName<T>(String name) 
+        where T : class, IDescriptor {
+      // Don't allow looking up nested types.  This will make optimization
+      // easier later.
+      if (name.IndexOf('.') != -1) {
+        return null;
+      }
+      if (Package.Length > 0) {
+        name = Package + "." + name;
+      }
+      T result = pool.FindSymbol<T>(name);
+      if (result != null && result.File == this) {
+        return result;
+      }
+      return null;
+    }
+
+    /// <summary>
+    /// Builds a FileDescriptor from its protocol buffer representation.
+    /// </summary>
+    /// <param name="proto">The protocol message form of the FileDescriptor.</param>
+    /// <param name="dependencies">FileDescriptors corresponding to all of the
+    /// file's dependencies, in the exact order listed in the .proto file</param>
+    /// <exception cref="DescriptorValidationException">If <paramref name="proto"/> is not
+    /// a valid descriptor. This can occur for a number of reasons, such as a field
+    /// having an undefined type or because two messages were defined with the same name.</exception>
+    public static FileDescriptor BuildFrom(FileDescriptorProto proto, FileDescriptor[] dependencies) {
+      // Building decsriptors involves two steps:  translating and linking.
+      // In the translation step (implemented by FileDescriptor's
+      // constructor), we build an object tree mirroring the
+      // FileDescriptorProto's tree and put all of the descriptors into the
+      // DescriptorPool's lookup tables.  In the linking step, we look up all
+      // type references in the DescriptorPool, so that, for example, a
+      // FieldDescriptor for an embedded message contains a pointer directly
+      // to the Descriptor for that message's type.  We also detect undefined
+      // types in the linking step.
+      DescriptorPool pool = new DescriptorPool(dependencies);
+      FileDescriptor result = new FileDescriptor(proto, dependencies, pool);
+
+      if (dependencies.Length != proto.DependencyCount) {
+        throw new DescriptorValidationException(result,
+          "Dependencies passed to FileDescriptor.BuildFrom() don't match " +
+          "those listed in the FileDescriptorProto.");
+      }
+      for (int i = 0; i < proto.DependencyCount; i++) {
+        if (dependencies[i].Name != proto.DependencyList[i]) {
+          throw new DescriptorValidationException(result,
+            "Dependencies passed to FileDescriptor.BuildFrom() don't match " +
+            "those listed in the FileDescriptorProto.");
+        }
+      }
+
+      result.CrossLink();
+      return result;
+    }                                 
+
+    private void CrossLink() {
+      foreach (MessageDescriptor message in messageTypes) {
+        message.CrossLink();
+      }
+
+      foreach (ServiceDescriptor service in services) {
+        service.CrossLink();
+      }
+
+      foreach (FieldDescriptor extension in extensions) {
+        extension.CrossLink();
+      }
     }
+    
     /// <summary>
     /// This method is to be called by generated code only.  It is equivalent
     /// to BuilderFrom except that the FileDescriptorProto is encoded in
diff --git a/csharp/ProtocolBuffers/Descriptors/IDescriptor.cs b/csharp/ProtocolBuffers/Descriptors/IDescriptor.cs
new file mode 100644
index 00000000..ff562840
--- /dev/null
+++ b/csharp/ProtocolBuffers/Descriptors/IDescriptor.cs
@@ -0,0 +1,25 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Google.ProtocolBuffers.Descriptors {
+
+  /// <summary>
+  /// The non-generic form of the IDescriptor interface. Useful for describing a general
+  /// descriptor.
+  /// </summary>
+  public interface IDescriptor {
+    string Name { get; }
+    string FullName { get; }
+    FileDescriptor File { get; }
+    IMessage Proto { get; }
+  }
+
+  /// <summary>
+  /// Strongly-typed form of the IDescriptor interface.
+  /// </summary>
+  /// <typeparam name="TProto">Protocol buffer type underlying this descriptor type</typeparam>
+  public interface IDescriptor<TProto> : IDescriptor where TProto : IMessage {
+    new TProto Proto { get; }
+  }
+}
diff --git a/csharp/ProtocolBuffers/Descriptors/IndexedDescriptorBase.cs b/csharp/ProtocolBuffers/Descriptors/IndexedDescriptorBase.cs
index b9452163..5415357d 100644
--- a/csharp/ProtocolBuffers/Descriptors/IndexedDescriptorBase.cs
+++ b/csharp/ProtocolBuffers/Descriptors/IndexedDescriptorBase.cs
@@ -13,8 +13,8 @@ namespace Google.ProtocolBuffers.Descriptors {
 
     private readonly int index;
 
-    protected IndexedDescriptorBase(TProto proto, FileDescriptor file, int index)
-        : base(proto, file) {
+    protected IndexedDescriptorBase(TProto proto, FileDescriptor file, string fullName, int index)
+        : base(proto, file, fullName) {
       this.index = index;
     }
 
diff --git a/csharp/ProtocolBuffers/Descriptors/MessageDescriptor.cs b/csharp/ProtocolBuffers/Descriptors/MessageDescriptor.cs
index fd8c6e5b..548f226b 100644
--- a/csharp/ProtocolBuffers/Descriptors/MessageDescriptor.cs
+++ b/csharp/ProtocolBuffers/Descriptors/MessageDescriptor.cs
@@ -1,27 +1,113 @@
-
-using System.Collections.Generic;
+using System.Collections.Generic;
 using Google.ProtocolBuffers.DescriptorProtos;
 
 namespace Google.ProtocolBuffers.Descriptors {
-  public class MessageDescriptor : DescriptorBase<DescriptorProto, MessageOptions> {
-    public IList<FieldDescriptor> Fields;
 
-    internal MessageDescriptor(DescriptorProto proto, FileDescriptor file) : base(proto, file) {
+  /// <summary>
+  /// Describes a message type.
+  /// </summary>
+  public class MessageDescriptor : IndexedDescriptorBase<DescriptorProto, MessageOptions> {
+
+    private readonly MessageDescriptor containingType;
+    private readonly IList<MessageDescriptor> nestedTypes;
+    private readonly IList<EnumDescriptor> enumTypes;
+    private readonly IList<FieldDescriptor> fields;
+    private readonly IList<FieldDescriptor> extensions;
+
+    internal MessageDescriptor(DescriptorProto proto, FileDescriptor file, MessageDescriptor parent, int typeIndex)
+        : base(proto, file, ComputeFullName(file, parent, proto.Name), typeIndex) {
+      containingType = parent;
+
+      nestedTypes = DescriptorUtil.ConvertAndMakeReadOnly(proto.NestedTypeList,
+          (type, index) => new MessageDescriptor(type, file, null, index));
+
+      enumTypes = DescriptorUtil.ConvertAndMakeReadOnly(proto.EnumTypeList,
+          (type, index) => new EnumDescriptor(type, file, null, index));
+
+      fields = DescriptorUtil.ConvertAndMakeReadOnly(proto.FieldList,
+          (field, index) => new FieldDescriptor(field, file, null, index, false));
+
+      extensions = DescriptorUtil.ConvertAndMakeReadOnly(proto.ExtensionList,
+          (field, index) => new FieldDescriptor(field, file, null, index, true));
+
+      file.DescriptorPool.AddSymbol(this);
     }
 
-    internal bool IsExtensionNumber(int fieldNumber) {
-      throw new System.NotImplementedException();
+    /// <value>
+    /// An unmodifiable list of this message type's fields.
+    /// </value>
+    public IList<FieldDescriptor> Fields {
+      get { return fields; }
     }
 
-    internal FieldDescriptor FindFieldByNumber(int fieldNumber) {
-      throw new System.NotImplementedException();
+    /// <value>
+    /// An unmodifiable list of this message type's extensions.
+    /// </value>
+    public IList<FieldDescriptor> Extensions {
+      get { return extensions; }
     }
 
     /// <value>
-    /// List of message types nested within this one.
+    /// An unmodifiable list of this message type's nested types.
     /// </value>
     public IList<MessageDescriptor> NestedTypes {
-      get { throw new System.NotImplementedException(); }
+      get { return nestedTypes; }
+    }
+
+    /// <value>
+    /// An unmodifiable list of this message type's enum types.
+    /// </value>
+    public IList<EnumDescriptor> EnumTypes {
+      get { return enumTypes; }
+    }
+
+    /// <summary>
+    /// Determines if the given field number is an extension.
+    /// </summary>
+    public bool IsExtensionNumber(int number) {
+      foreach (DescriptorProto.Types.ExtensionRange range in Proto.ExtensionRangeList) {
+        if (range.Start <= number && number < range.End) {
+          return true;
+        }
+      }
+      return false;
+    }
+
+    /// <summary>
+    /// Finds a field by field number.
+    /// </summary>
+    /// <param name="number">The field number within this message type.</param>
+    /// <returns>The field's descriptor, or null if not found.</returns>
+    public FieldDescriptor FindFieldByNumber(int number) {
+      return File.DescriptorPool.FindFieldByNumber(this, number);
+    }
+
+    /// <summary>
+    /// Finds a nested descriptor by name. The is valid for fields, nested
+    /// message types and enums.
+    /// </summary>
+    /// <param name="name">The unqualified name of the descriptor, e.g. "Foo"</param>
+    /// <returns>The descriptor, or null if not found.</returns>
+    public T FindDescriptor<T>(string name) 
+        where T : class, IDescriptor {
+      return File.DescriptorPool.FindSymbol<T>(FullName + "." + name);
+    }
+
+    /// <summary>
+    /// Looks up and cross-links all fields, nested types, and extensions.
+    /// </summary>
+    internal void CrossLink() {
+      foreach (MessageDescriptor message in nestedTypes) {
+        message.CrossLink();
+      }
+
+      foreach (FieldDescriptor field in fields) {
+        field.CrossLink();
+      }
+
+      foreach (FieldDescriptor extension in extensions) {
+        extension.CrossLink();
+      }
     }
   }
 }
diff --git a/csharp/ProtocolBuffers/Descriptors/MethodDescriptor.cs b/csharp/ProtocolBuffers/Descriptors/MethodDescriptor.cs
new file mode 100644
index 00000000..efc3fa4c
--- /dev/null
+++ b/csharp/ProtocolBuffers/Descriptors/MethodDescriptor.cs
@@ -0,0 +1,55 @@
+using Google.ProtocolBuffers.DescriptorProtos;
+
+namespace Google.ProtocolBuffers.Descriptors {
+  /// <summary>
+  /// Describes a single method in a service.
+  /// </summary>
+  public class MethodDescriptor : IndexedDescriptorBase<MethodDescriptorProto, MethodOptions> {
+
+    private readonly ServiceDescriptor service;
+    private MessageDescriptor inputType;
+    private MessageDescriptor outputType;
+
+    /// <value>
+    /// The service this method belongs to.
+    /// </value>
+    public ServiceDescriptor Service {
+      get { return service; }
+    }
+
+    /// <value>
+    /// The method's input type.
+    /// </value>
+    public MessageDescriptor InputType {
+      get { return inputType; }
+    }
+
+    /// <value>
+    /// The method's input type.
+    /// </value>
+    public MessageDescriptor OutputType {
+      get { return inputType; }
+    }
+
+    internal MethodDescriptor(MethodDescriptorProto proto, FileDescriptor file,
+        ServiceDescriptor parent, int index) 
+        : base(proto, file, parent.FullName + "." + proto.Name, index) {
+      service = parent;
+      file.DescriptorPool.AddSymbol(this);
+    }
+
+    internal void CrossLink() {
+      IDescriptor lookup = File.DescriptorPool.LookupSymbol(Proto.InputType, this);
+      if (!(lookup is MessageDescriptor)) {
+        throw new DescriptorValidationException(this, "\"" + Proto.InputType + "\" is not a message type.");
+      }
+      inputType = (MessageDescriptor) lookup;
+
+      lookup = File.DescriptorPool.LookupSymbol(Proto.OutputType, this);
+      if (!(lookup is MessageDescriptor)) {
+        throw new DescriptorValidationException(this, "\"" + Proto.OutputType + "\" is not a message type.");
+      }
+      outputType = (MessageDescriptor) lookup;
+    }
+  }
+}
diff --git a/csharp/ProtocolBuffers/Descriptors/PackageDescriptor.cs b/csharp/ProtocolBuffers/Descriptors/PackageDescriptor.cs
new file mode 100644
index 00000000..acef6200
--- /dev/null
+++ b/csharp/ProtocolBuffers/Descriptors/PackageDescriptor.cs
@@ -0,0 +1,44 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Google.ProtocolBuffers.Descriptors {
+    /*
+     * Represents a package in the symbol table.  We use PackageDescriptors
+     * just as placeholders so that someone cannot define, say, a message type
+     * that has the same name as an existing package.
+     */
+  /// <summary>
+  /// Represents a package in the symbol table.  We use PackageDescriptors
+  /// just as placeholders so that someone cannot define, say, a message type
+  /// that has the same name as an existing package.
+  /// </summary>
+    internal class PackageDescriptor : IDescriptor<IMessage> {
+
+      private readonly string name;
+      private readonly string fullName;
+      private readonly FileDescriptor file;
+
+      internal PackageDescriptor(string name, string fullName, FileDescriptor file) {
+        this.file = file;
+        this.fullName = fullName;
+        this.name = name;
+      }
+
+      public IMessage Proto {
+        get { return file.Proto; }
+      }
+
+      public string Name {
+        get { return name; }
+      }
+
+      public string FullName {
+        get { return fullName; }
+      }
+
+      public FileDescriptor File {
+        get { return file; }
+      }
+    }
+}
diff --git a/csharp/ProtocolBuffers/Descriptors/ServiceDescriptor.cs b/csharp/ProtocolBuffers/Descriptors/ServiceDescriptor.cs
index dbe93f99..d7c6f3a3 100644
--- a/csharp/ProtocolBuffers/Descriptors/ServiceDescriptor.cs
+++ b/csharp/ProtocolBuffers/Descriptors/ServiceDescriptor.cs
@@ -4,9 +4,34 @@ using System.Text;
 using Google.ProtocolBuffers.DescriptorProtos;
 
 namespace Google.ProtocolBuffers.Descriptors {
+
+  /// <summary>
+  /// Describes a service type.
+  /// </summary>
   public class ServiceDescriptor : IndexedDescriptorBase<ServiceDescriptorProto, ServiceOptions> {
+
+    private readonly IList<MethodDescriptor> methods;
+
     public ServiceDescriptor(ServiceDescriptorProto proto, FileDescriptor file, int index)
-        : base(proto, file, index) {
+        : base(proto, file, ComputeFullName(file, null, proto.Name), index) {
+
+      methods = DescriptorUtil.ConvertAndMakeReadOnly(proto.MethodList,
+        (method, i) => new MethodDescriptor(method, file, this, i));
+
+      file.DescriptorPool.AddSymbol(this);
+    }
+
+    /// <value>
+    /// An unmodifiable list of methods in this service.
+    /// </value>
+    public IList<MethodDescriptor> Methods {
+      get { return methods; }
+    }
+
+    internal void CrossLink() {
+      foreach (MethodDescriptor method in methods) {
+        method.CrossLink();
+      }
     }
   }
 }
diff --git a/csharp/ProtocolBuffers/ProtocolBuffers.csproj b/csharp/ProtocolBuffers/ProtocolBuffers.csproj
index b5106738..d1f7ad26 100644
--- a/csharp/ProtocolBuffers/ProtocolBuffers.csproj
+++ b/csharp/ProtocolBuffers/ProtocolBuffers.csproj
@@ -52,6 +52,8 @@
     <Compile Include="DescriptorProtos\PartialClasses.cs" />
     <Compile Include="Descriptors\DescriptorBase.cs" />
     <Compile Include="Descriptors\DescriptorPool.cs" />
+    <Compile Include="Descriptors\DescriptorUtil.cs" />
+    <Compile Include="Descriptors\DescriptorValidationException.cs" />
     <Compile Include="Descriptors\EnumDescriptor.cs" />
     <Compile Include="Descriptors\EnumDescriptorIndexAttribute.cs" />
     <Compile Include="Descriptors\EnumValueDescriptor.cs" />
@@ -59,9 +61,12 @@
     <Compile Include="Descriptors\FieldMappingAttribute.cs" />
     <Compile Include="Descriptors\FieldType.cs" />
     <Compile Include="Descriptors\FileDescriptor.cs" />
+    <Compile Include="Descriptors\IDescriptor.cs" />
     <Compile Include="Descriptors\IndexedDescriptorBase.cs" />
     <Compile Include="Descriptors\MappedType.cs" />
     <Compile Include="Descriptors\MessageDescriptor.cs" />
+    <Compile Include="Descriptors\MethodDescriptor.cs" />
+    <Compile Include="Descriptors\PackageDescriptor.cs" />
     <Compile Include="Descriptors\ServiceDescriptor.cs" />
     <Compile Include="ExtensionInfo.cs" />
     <Compile Include="ExtensionRegistry.cs" />
diff --git a/csharp/ProtocolBuffers/TextFormat.cs b/csharp/ProtocolBuffers/TextFormat.cs
index e789d0d7..87c5e211 100644
--- a/csharp/ProtocolBuffers/TextFormat.cs
+++ b/csharp/ProtocolBuffers/TextFormat.cs
@@ -11,5 +11,25 @@ namespace Google.ProtocolBuffers {
     internal static string PrintToString(UnknownFieldSet unknownFieldSet) {
       throw new NotImplementedException();
     }
+
+    internal static object ParseUInt64(string p) {
+      throw new NotImplementedException();
+    }
+
+    internal static object ParseInt64(string p) {
+      throw new NotImplementedException();
+    }
+
+    internal static object ParseUInt32(string p) {
+      throw new NotImplementedException();
+    }
+
+    internal static object ParseInt32(string p) {
+      throw new NotImplementedException();
+    }
+
+    internal static object UnescapeBytes(string p) {
+      throw new NotImplementedException();
+    }
   }
 }