Revamp to reflection.

Changes in brief:
1. Descriptor is now the entry point for all reflection.
2. IReflectedMessage has gone; there's now a Descriptor property in IMessage, which is explicitly implemented (due to the static property).
3. FieldAccessorTable has gone away
4. IFieldAccessor and OneofFieldAccessor still exist; we *could* put the functionality straight into FieldDescriptor and OneofDescriptor... I'm unsure about that.
5. There's a temporary property MessageDescriptor.FieldAccessorsByFieldNumber to make the test changes small - we probably want this to go away
6. Discovery for delegates is now via attributes applied to properties and the Clear method of a oneof

I'm happy with 1-3.
4 I'm unsure about - feedback welcome.
5 will go away
6 I'm unsure about, both in design and implementation. Should we have a ProtobufMessageAttribute too? Should we find all the relevant attributes in MessageDescriptor and pass them down, to avoid an O(N^2) scenario?

Generated code changes coming in the next commit.

15 files changed, 273 insertions, 129 deletions

diff --git a/csharp/src/Google.Protobuf/Reflection/DescriptorUtil.cs b/csharp/src/Google.Protobuf/Reflection/DescriptorUtil.cs
index af31dfb1..f5570fc4 100644
--- a/csharp/src/Google.Protobuf/Reflection/DescriptorUtil.cs
+++ b/csharp/src/Google.Protobuf/Reflection/DescriptorUtil.cs
@@ -50,9 +50,8 @@ namespace Google.Protobuf.Reflection
         /// 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)
+        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++)
diff --git a/csharp/src/Google.Protobuf/Reflection/EnumDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/EnumDescriptor.cs
index bf8f8c83..285f26f3 100644
--- a/csharp/src/Google.Protobuf/Reflection/EnumDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/EnumDescriptor.cs
@@ -30,6 +30,7 @@
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #endregion
 
+using System;
 using System.Collections.Generic;
 
 namespace Google.Protobuf.Reflection
@@ -42,11 +43,13 @@ namespace Google.Protobuf.Reflection
         private readonly EnumDescriptorProto proto;
         private readonly MessageDescriptor containingType;
         private readonly IList<EnumValueDescriptor> values;
+        private readonly Type generatedType;
 
-        internal EnumDescriptor(EnumDescriptorProto proto, FileDescriptor file, MessageDescriptor parent, int index)
+        internal EnumDescriptor(EnumDescriptorProto proto, FileDescriptor file, MessageDescriptor parent, int index, Type generatedType)
             : base(file, file.ComputeFullName(parent, proto.Name), index)
         {
             this.proto = proto;
+            this.generatedType = generatedType;
             containingType = parent;
 
             if (proto.Value.Count == 0)
@@ -69,6 +72,11 @@ namespace Google.Protobuf.Reflection
         /// </summary>
         public override string Name { get { return proto.Name; } }
 
+        /// <summary>
+        /// The generated type for this enum, or <c>null</c> if the descriptor does not represent a generated type.
+        /// </summary>
+        public Type GeneratedType { get { return generatedType; } }
+
         /// <value>
         /// If this is a nested type, get the outer descriptor, otherwise null.
         /// </value>
diff --git a/csharp/src/Google.Protobuf/Reflection/FieldAccessorBase.cs b/csharp/src/Google.Protobuf/Reflection/FieldAccessorBase.cs
index 39a63b47..0893dc3d 100644
--- a/csharp/src/Google.Protobuf/Reflection/FieldAccessorBase.cs
+++ b/csharp/src/Google.Protobuf/Reflection/FieldAccessorBase.cs
@@ -43,13 +43,8 @@ namespace Google.Protobuf.Reflection
         private readonly Func<object, object> getValueDelegate;
         private readonly FieldDescriptor descriptor;
 
-        internal FieldAccessorBase(Type type, string propertyName, FieldDescriptor descriptor)
+        internal FieldAccessorBase(PropertyInfo property, FieldDescriptor descriptor)
         {
-            PropertyInfo property = type.GetProperty(propertyName);
-            if (property == null || !property.CanRead)
-            {
-                throw new ArgumentException("Not all required properties/methods available");
-            }
             this.descriptor = descriptor;
             getValueDelegate = ReflectionUtil.CreateFuncObjectObject(property.GetGetMethod());
         }
diff --git a/csharp/src/Google.Protobuf/Reflection/FieldAccessorTable.cs b/csharp/src/Google.Protobuf/Reflection/FieldAccessorTable.cs
deleted file mode 100644
index 24fcbc64..00000000
--- a/csharp/src/Google.Protobuf/Reflection/FieldAccessorTable.cs
+++ /dev/null
@@ -1,97 +0,0 @@
-#region Copyright notice and license
-// Protocol Buffers - Google's data interchange format
-// Copyright 2008 Google Inc.  All rights reserved.
-// https://developers.google.com/protocol-buffers/
-//
-// Redistribution and use in source and binary forms, with or without
-// modification, are permitted provided that the following conditions are
-// met:
-//
-//     * Redistributions of source code must retain the above copyright
-// notice, this list of conditions and the following disclaimer.
-//     * Redistributions in binary form must reproduce the above
-// copyright notice, this list of conditions and the following disclaimer
-// in the documentation and/or other materials provided with the
-// distribution.
-//     * Neither the name of Google Inc. nor the names of its
-// contributors may be used to endorse or promote products derived from
-// this software without specific prior written permission.
-//
-// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
-// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
-// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
-// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
-// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
-// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
-// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
-// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
-// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
-// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
-// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-#endregion
-
-using System;
-using System.Collections.ObjectModel;
-
-namespace Google.Protobuf.Reflection
-{
-    /// <summary>
-    /// Provides access to fields in generated messages via reflection.
-    /// </summary>
-    public sealed class FieldAccessorTable
-    {
-        private readonly ReadOnlyCollection<IFieldAccessor> accessors;
-        private readonly ReadOnlyCollection<OneofAccessor> oneofs;
-        private readonly MessageDescriptor descriptor;
-
-        /// <summary>
-        /// Constructs a FieldAccessorTable for a particular message class.
-        /// Only one FieldAccessorTable should be constructed per class.
-        /// </summary>
-        /// <param name="type">The CLR type for the message.</param>
-        /// <param name="descriptor">The type's descriptor</param>
-        /// <param name="propertyNames">The Pascal-case names of all the field-based properties in the message.</param>
-        public FieldAccessorTable(Type type, MessageDescriptor descriptor, string[] propertyNames, string[] oneofPropertyNames)
-        {
-            this.descriptor = descriptor;
-            var accessorsArray = new IFieldAccessor[descriptor.Fields.Count];
-            for (int i = 0; i < accessorsArray.Length; i++)
-            {
-                var field = descriptor.Fields[i];
-                var name = propertyNames[i];
-                accessorsArray[i] =
-                    field.IsMap ? new MapFieldAccessor(type, name, field)
-                    : field.IsRepeated ? new RepeatedFieldAccessor(type, name, field)
-                    : (IFieldAccessor) new SingleFieldAccessor(type, name, field);
-            }
-            accessors = new ReadOnlyCollection<IFieldAccessor>(accessorsArray);
-            var oneofsArray = new OneofAccessor[descriptor.Oneofs.Count];
-            for (int i = 0; i < oneofsArray.Length; i++)
-            {
-                var oneof = descriptor.Oneofs[i];
-                oneofsArray[i] = new OneofAccessor(type, oneofPropertyNames[i], oneof);
-            }
-            oneofs = new ReadOnlyCollection<OneofAccessor>(oneofsArray);
-        }
-
-        // TODO: Validate the name here... should possibly make this type a more "general reflection access" type,
-        // bearing in mind the oneof parts to come as well.
-        /// <summary>
-        /// Returns all of the field accessors for the message type.
-        /// </summary>
-        public ReadOnlyCollection<IFieldAccessor> Accessors { get { return accessors; } }
-
-        public ReadOnlyCollection<OneofAccessor> Oneofs { get { return oneofs; } }
-
-        // TODO: Review this, as it's easy to get confused between FieldNumber and Index.
-        // Currently only used to get an accessor related to a oneof... maybe just make that simpler?
-        public IFieldAccessor this[int fieldNumber]
-        {
-            get
-            {
-                FieldDescriptor field = descriptor.FindFieldByNumber(fieldNumber);
-                return accessors[field.Index];
-            }
-        }
-    }
-}
\ No newline at end of file
diff --git a/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs
index 3d9d0d75..57378e4c 100644
--- a/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs
@@ -31,6 +31,7 @@
 #endregion
 
 using System;
+using System.Linq;
 
 namespace Google.Protobuf.Reflection
 {
@@ -45,6 +46,7 @@ namespace Google.Protobuf.Reflection
         private readonly MessageDescriptor containingType;
         private readonly OneofDescriptor containingOneof;
         private FieldType fieldType;
+        private IFieldAccessor accessor;
 
         internal FieldDescriptor(FieldDescriptorProto proto, FileDescriptor file,
                                  MessageDescriptor parent, int index)
@@ -82,6 +84,8 @@ namespace Google.Protobuf.Reflection
         public override string Name { get { return proto.Name; } }
 
         internal FieldDescriptorProto Proto { get { return proto; } }
+
+        public IFieldAccessor Accessor { get { return accessor; } }
         
         /// <summary>
         /// Maps a field type as included in the .proto file to a FieldType.
@@ -287,6 +291,30 @@ namespace Google.Protobuf.Reflection
             {
                 throw new DescriptorValidationException(this, "MessageSet format is not supported.");
             }
+
+            accessor = CreateAccessor();
+        }
+
+        private IFieldAccessor CreateAccessor()
+        {
+            // TODO: Check the performance of this with some large protos. Each message is O(N^2) in the number of fields,
+            // which isn't great...
+            if (containingType.GeneratedType == null)
+            {
+                return null;
+            }
+            var property = containingType
+                .GeneratedType
+                .GetProperties()
+                .FirstOrDefault(p => p.IsDefined(typeof(ProtobufFieldAttribute), false) &&
+                                     p.GetCustomAttributes(typeof(ProtobufFieldAttribute), false).Cast<ProtobufFieldAttribute>().Single().Number == FieldNumber);
+            if (property == null)
+            {
+                return null;
+            }
+            return IsMap ? new MapFieldAccessor(property, this)
+                : IsRepeated ? new RepeatedFieldAccessor(property, this)
+                : (IFieldAccessor) new SingleFieldAccessor(property, this);
         }
     }
 }
\ No newline at end of file
diff --git a/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
index db393480..a10e617b 100644
--- a/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
@@ -62,11 +62,12 @@ namespace Google.Protobuf.Reflection
             get { return proto.Syntax == "proto3" ? ProtoSyntax.Proto3 : ProtoSyntax.Proto2; }
         }
 
-        private FileDescriptor(FileDescriptorProto proto, FileDescriptor[] dependencies, DescriptorPool pool, bool allowUnknownDependencies)
+        private FileDescriptor(FileDescriptorProto proto, FileDescriptor[] dependencies, DescriptorPool pool, bool allowUnknownDependencies, Type[] generatedTypes)
         {
             this.pool = pool;
             this.proto = proto;
             this.dependencies = new ReadOnlyCollection<FileDescriptor>((FileDescriptor[]) dependencies.Clone());
+            IEnumerator<Type> generatedTypeIterator = generatedTypes == null ? null : ((IEnumerable<Type>)generatedTypes).GetEnumerator();
 
             publicDependencies = DeterminePublicDependencies(this, proto, dependencies, allowUnknownDependencies);
 
@@ -74,15 +75,21 @@ namespace Google.Protobuf.Reflection
 
             messageTypes = DescriptorUtil.ConvertAndMakeReadOnly(proto.MessageType,
                                                                  (message, index) =>
-                                                                 new MessageDescriptor(message, this, null, index));
+                                                                 new MessageDescriptor(message, this, null, index, generatedTypeIterator));
 
             enumTypes = DescriptorUtil.ConvertAndMakeReadOnly(proto.EnumType,
                                                               (enumType, index) =>
-                                                              new EnumDescriptor(enumType, this, null, index));
+                                                              new EnumDescriptor(enumType, this, null, index, ReflectionUtil.GetNextType(generatedTypeIterator)));
 
             services = DescriptorUtil.ConvertAndMakeReadOnly(proto.Service,
                                                              (service, index) =>
                                                              new ServiceDescriptor(service, this, index));
+
+            // We should now have consumed all the generated types.
+            if (generatedTypeIterator != null && generatedTypeIterator.MoveNext())
+            {
+                throw new ArgumentException("More generated types left over after consuming all expected ones", "generatedTypes");
+            }
         }
 
         /// <summary>
@@ -265,7 +272,7 @@ namespace Google.Protobuf.Reflection
         /// <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>
-        private static FileDescriptor BuildFrom(FileDescriptorProto proto, FileDescriptor[] dependencies, bool allowUnknownDependencies)
+        private static FileDescriptor BuildFrom(FileDescriptorProto proto, FileDescriptor[] dependencies, bool allowUnknownDependencies, Type[] generatedTypes)
         {
             // Building descriptors involves two steps: translating and linking.
             // In the translation step (implemented by FileDescriptor's
@@ -282,7 +289,7 @@ namespace Google.Protobuf.Reflection
             }
 
             DescriptorPool pool = new DescriptorPool(dependencies);
-            FileDescriptor result = new FileDescriptor(proto, dependencies, pool, allowUnknownDependencies);
+            FileDescriptor result = new FileDescriptor(proto, dependencies, pool, allowUnknownDependencies, generatedTypes);
 
             // TODO(jonskeet): Reinstate these checks, or get rid of them entirely. They aren't in the Java code,
             // and fail for the CustomOptions test right now. (We get "descriptor.proto" vs "google/protobuf/descriptor.proto".)
@@ -319,8 +326,23 @@ namespace Google.Protobuf.Reflection
             }
         }
 
+        /// <summary>
+        /// Creates an instance for generated code.
+        /// </summary>
+        /// <remarks>
+        /// The <paramref name="generatedTypes"/> parameter should be null for descriptors which don't correspond to
+        /// generated types. Otherwise, the array should be represent all the generated types in the file: messages then
+        /// enums. Within each message, there can be nested messages and enums, which must be specified "inline" in the array:
+        /// containing message, nested messages, nested enums - and of course each nested message may contain *more* nested messages,
+        /// etc. All messages within the descriptor should be represented, even if they do not have a generated type - any
+        /// type without a corresponding generated type (such as map entries) should respond to a null element.
+        /// For example, a file with a messages OuterMessage and InnerMessage, and enums OuterEnum and InnerEnum (where
+        /// InnerMessage and InnerEnum are nested within InnerMessage) would result in an array of
+        /// OuterMessage, InnerMessage, InnerEnum, OuterEnum.
+        /// </remarks>
         public static FileDescriptor InternalBuildGeneratedFileFrom(byte[] descriptorData,
-                                                                    FileDescriptor[] dependencies)
+                                                                    FileDescriptor[] dependencies,
+                                                                    Type[] generatedTypes)
         {
             FileDescriptorProto proto;
             try
@@ -336,7 +358,7 @@ namespace Google.Protobuf.Reflection
             {
                 // When building descriptors for generated code, we allow unknown
                 // dependencies by default.
-                return BuildFrom(proto, dependencies, true);
+                return BuildFrom(proto, dependencies, true, generatedTypes);
             }
             catch (DescriptorValidationException e)
             {
diff --git a/csharp/src/Google.Protobuf/Reflection/MapFieldAccessor.cs b/csharp/src/Google.Protobuf/Reflection/MapFieldAccessor.cs
index 317fbd8d..6df4c5f0 100644
--- a/csharp/src/Google.Protobuf/Reflection/MapFieldAccessor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/MapFieldAccessor.cs
@@ -32,6 +32,7 @@
 
 using System;
 using System.Collections;
+using System.Reflection;
 
 namespace Google.Protobuf.Reflection
 {
@@ -40,7 +41,7 @@ namespace Google.Protobuf.Reflection
     /// </summary>
     internal sealed class MapFieldAccessor : FieldAccessorBase
     {
-        internal MapFieldAccessor(Type type, string propertyName, FieldDescriptor descriptor) : base(type, propertyName, descriptor)
+        internal MapFieldAccessor(PropertyInfo property, FieldDescriptor descriptor) : base(property, descriptor)
         {
         }
 
diff --git a/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
index 1c22c460..9413cf61 100644
--- a/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
@@ -30,8 +30,10 @@
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #endregion
 
+using Google.Protobuf.Collections;
 using System;
 using System.Collections.Generic;
+using System.Linq;
 
 namespace Google.Protobuf.Reflection
 {
@@ -60,11 +62,15 @@ namespace Google.Protobuf.Reflection
         private readonly IList<EnumDescriptor> enumTypes;
         private readonly IList<FieldDescriptor> fields;
         private readonly IList<OneofDescriptor> oneofs;
+        // CLR representation of the type described by this descriptor, if any.
+        private readonly Type generatedType;
+        private IDictionary<int, IFieldAccessor> fieldAccessorsByFieldNumber;
         
-        internal MessageDescriptor(DescriptorProto proto, FileDescriptor file, MessageDescriptor parent, int typeIndex)
+        internal MessageDescriptor(DescriptorProto proto, FileDescriptor file, MessageDescriptor parent, int typeIndex, IEnumerator<Type> generatedTypeIterator)
             : base(file, file.ComputeFullName(parent, proto.Name), typeIndex)
         {
             this.proto = proto;
+            generatedType = ReflectionUtil.GetNextType(generatedTypeIterator);
             containingType = parent;
 
             oneofs = DescriptorUtil.ConvertAndMakeReadOnly(proto.OneofDecl,
@@ -73,11 +79,11 @@ namespace Google.Protobuf.Reflection
 
             nestedTypes = DescriptorUtil.ConvertAndMakeReadOnly(proto.NestedType,
                                                                 (type, index) =>
-                                                                new MessageDescriptor(type, file, this, index));
+                                                                new MessageDescriptor(type, file, this, index, generatedTypeIterator));
 
             enumTypes = DescriptorUtil.ConvertAndMakeReadOnly(proto.EnumType,
                                                               (type, index) =>
-                                                              new EnumDescriptor(type, file, this, index));
+                                                              new EnumDescriptor(type, file, this, index, ReflectionUtil.GetNextType(generatedTypeIterator)));
 
             // TODO(jonskeet): Sort fields first?
             fields = DescriptorUtil.ConvertAndMakeReadOnly(proto.Field,
@@ -85,6 +91,14 @@ namespace Google.Protobuf.Reflection
                                                            new FieldDescriptor(field, file, this, index));
             file.DescriptorPool.AddSymbol(this);
         }
+                
+        /// <summary>
+        /// Returns the total number of nested types and enums, recursively.
+        /// </summary>
+        private int CountTotalGeneratedTypes()
+        {
+            return nestedTypes.Sum(nested => nested.CountTotalGeneratedTypes()) + enumTypes.Count;
+        }
 
         /// <summary>
         /// The brief name of the descriptor's target.
@@ -94,6 +108,11 @@ namespace Google.Protobuf.Reflection
         internal DescriptorProto Proto { get { return proto; } }
 
         /// <summary>
+        /// The generated type for this message, or <c>null</c> if the descriptor does not represent a generated type.
+        /// </summary>
+        public Type GeneratedType { get { return generatedType; } }
+
+        /// <summary>
         /// Returns whether this message is one of the "well known types" which may have runtime/protoc support.
         /// </summary>
         internal bool IsWellKnownType
@@ -142,6 +161,13 @@ namespace Google.Protobuf.Reflection
         }
 
         /// <summary>
+        /// Returns a map from field number to accessor.
+        /// TODO: Revisit this. It's mostly in place to make the transition from FieldAccessorTable
+        /// to descriptor-based reflection simple in terms of tests. Work out what we really want.
+        /// </summary>
+        public IDictionary<int, IFieldAccessor> FieldAccessorsByFieldNumber { get { return fieldAccessorsByFieldNumber; } }
+
+        /// <summary>
         /// Finds a field by field name.
         /// </summary>
         /// <param name="name">The unqualified name of the field (e.g. "foo").</param>
@@ -192,6 +218,8 @@ namespace Google.Protobuf.Reflection
             {
                 oneof.CrossLink();
             }
+
+            fieldAccessorsByFieldNumber = new ReadOnlyDictionary<int, IFieldAccessor>(fields.ToDictionary(field => field.FieldNumber, field => field.Accessor));
         }        
     }
 }
\ No newline at end of file
diff --git a/csharp/src/Google.Protobuf/Reflection/OneofAccessor.cs b/csharp/src/Google.Protobuf/Reflection/OneofAccessor.cs
index 7a11d36b..20cbea92 100644
--- a/csharp/src/Google.Protobuf/Reflection/OneofAccessor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/OneofAccessor.cs
@@ -44,18 +44,16 @@ namespace Google.Protobuf.Reflection
         private readonly Action<object> clearDelegate;
         private OneofDescriptor descriptor;
 
-        internal OneofAccessor(Type type, string propertyName, OneofDescriptor descriptor) 
+        internal OneofAccessor(PropertyInfo caseProperty, MethodInfo clearMethod, OneofDescriptor descriptor) 
         {
-            PropertyInfo property = type.GetProperty(propertyName + "Case");
-            if (property == null || !property.CanRead)
+            if (!caseProperty.CanRead)
             {
-                throw new ArgumentException("Not all required properties/methods available");
+                throw new ArgumentException("Cannot read from property");
             }
             this.descriptor = descriptor;
-            caseDelegate = ReflectionUtil.CreateFuncObjectT<int>(property.GetGetMethod());
+            caseDelegate = ReflectionUtil.CreateFuncObjectT<int>(caseProperty.GetGetMethod());
 
             this.descriptor = descriptor;
-            MethodInfo clearMethod = type.GetMethod("Clear" + propertyName);
             clearDelegate = ReflectionUtil.CreateActionObject(clearMethod);
         }
 
diff --git a/csharp/src/Google.Protobuf/Reflection/OneofDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/OneofDescriptor.cs
index e92dc8bb..b4cc0791 100644
--- a/csharp/src/Google.Protobuf/Reflection/OneofDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/OneofDescriptor.cs
@@ -32,6 +32,7 @@
 
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
+using System.Linq;
 
 namespace Google.Protobuf.Reflection
 {
@@ -40,6 +41,7 @@ namespace Google.Protobuf.Reflection
         private readonly OneofDescriptorProto proto;
         private MessageDescriptor containingType;
         private IList<FieldDescriptor> fields;
+        private OneofAccessor accessor;
 
         internal OneofDescriptor(OneofDescriptorProto proto, FileDescriptor file, MessageDescriptor parent, int index)
             : base(file, file.ComputeFullName(parent, proto.Name), index)
@@ -62,6 +64,8 @@ namespace Google.Protobuf.Reflection
 
         public IList<FieldDescriptor> Fields { get { return fields; } }
 
+        public OneofAccessor Accessor { get { return accessor; } }
+
         internal void CrossLink()
         {
             List<FieldDescriptor> fieldCollection = new List<FieldDescriptor>();
@@ -73,6 +77,36 @@ namespace Google.Protobuf.Reflection
                 }
             }
             fields = new ReadOnlyCollection<FieldDescriptor>(fieldCollection);
+            accessor = CreateAccessor();
+        }
+
+        private OneofAccessor CreateAccessor()
+        {
+            if (containingType.GeneratedType == null)
+            {
+                return null;
+            }
+            var caseProperty = containingType
+                .GeneratedType
+                .GetProperties()
+                .FirstOrDefault(p => p.IsDefined(typeof(ProtobufOneofAttribute), false) &&
+                                     p.GetCustomAttributes(typeof(ProtobufOneofAttribute), false).Cast<ProtobufOneofAttribute>().Single().Name == Name);
+            if (caseProperty == null)
+            {
+                return null;
+            }
+
+            var clearMethod = containingType
+                 .GeneratedType
+                 .GetMethods()
+                 .FirstOrDefault(p => p.IsDefined(typeof(ProtobufOneofAttribute), false) &&
+                                      p.GetCustomAttributes(typeof(ProtobufOneofAttribute), false).Cast<ProtobufOneofAttribute>().Single().Name == Name);
+            if (clearMethod == null)
+            {
+                return null;
+            }
+
+            return new OneofAccessor(caseProperty, clearMethod, this);
         }
     }
 }
diff --git a/csharp/src/Google.Protobuf/Reflection/ProtobufFieldAttribute.cs b/csharp/src/Google.Protobuf/Reflection/ProtobufFieldAttribute.cs
new file mode 100644
index 00000000..a59caea7
--- /dev/null
+++ b/csharp/src/Google.Protobuf/Reflection/ProtobufFieldAttribute.cs
@@ -0,0 +1,58 @@
+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2015 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#endregion
+using System;
+
+namespace Google.Protobuf.Reflection
+{
+    /// <summary>
+    /// Attribute applied to a generated property corresponding to a field in a .proto file.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
+    public sealed class ProtobufFieldAttribute : Attribute
+    {
+        /// <summary>
+        /// The field number in the original .proto file.
+        /// </summary>
+        public int Number { get; set; }
+        
+        /// <summary>
+        /// The field name in the original .proto file.
+        /// </summary>
+        public string Name { get; set; }
+
+        public ProtobufFieldAttribute(int number, string name)
+        {
+            this.Number = number;
+            this.Name = name;
+        }
+    }
+}
diff --git a/csharp/src/Google.Protobuf/Reflection/ProtobufOneofAttribute.cs b/csharp/src/Google.Protobuf/Reflection/ProtobufOneofAttribute.cs
new file mode 100644
index 00000000..74ad8c53
--- /dev/null
+++ b/csharp/src/Google.Protobuf/Reflection/ProtobufOneofAttribute.cs
@@ -0,0 +1,52 @@
+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2015 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#endregion
+using System;
+
+namespace Google.Protobuf.Reflection
+{
+    /// <summary>
+    /// Attribute applied to the "case" property or "clear" method corresponding to a oneof in a .proto file.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Property | AttributeTargets.Method, AllowMultiple = false)]
+    public sealed class ProtobufOneofAttribute : Attribute
+    {
+        /// <summary>
+        /// The oneof name in the original .proto file.
+        /// </summary>
+        public string Name { get; set; }
+
+        public ProtobufOneofAttribute(string name)
+        {
+            this.Name = name;
+        }
+    }
+}
diff --git a/csharp/src/Google.Protobuf/Reflection/ReflectionUtil.cs b/csharp/src/Google.Protobuf/Reflection/ReflectionUtil.cs
index d0dc3e8b..ec222dc1 100644
--- a/csharp/src/Google.Protobuf/Reflection/ReflectionUtil.cs
+++ b/csharp/src/Google.Protobuf/Reflection/ReflectionUtil.cs
@@ -31,6 +31,7 @@
 #endregion
 
 using System;
+using System.Collections.Generic;
 using System.Linq.Expressions;
 using System.Reflection;
 
@@ -102,5 +103,24 @@ namespace Google.Protobuf.Reflection
             Expression call = Expression.Call(castTarget, method);
             return Expression.Lambda<Action<object>>(call, targetParameter).Compile();
         }
+
+        /// <summary>
+        /// Returns the next type from an iterator of types, unless the iterator is a null reference,
+        /// in which case null is returned.
+        /// </summary>
+        internal static Type GetNextType(IEnumerator<Type> generatedTypeIterator)
+        {
+            if (generatedTypeIterator == null)
+            {
+                return null;
+            }
+            if (!generatedTypeIterator.MoveNext())
+            {
+                // This parameter name corresponds to any public method supplying the generated types to start with.
+                throw new ArgumentException("More generated types left over after consuming all expected ones", "generatedTypes");
+            }
+            return generatedTypeIterator.Current;
+        }
+
     }
 }
\ No newline at end of file
diff --git a/csharp/src/Google.Protobuf/Reflection/RepeatedFieldAccessor.cs b/csharp/src/Google.Protobuf/Reflection/RepeatedFieldAccessor.cs
index 0ada7567..acb3c8d5 100644
--- a/csharp/src/Google.Protobuf/Reflection/RepeatedFieldAccessor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/RepeatedFieldAccessor.cs
@@ -32,6 +32,7 @@
 
 using System;
 using System.Collections;
+using System.Reflection;
 
 namespace Google.Protobuf.Reflection
 {
@@ -40,7 +41,7 @@ namespace Google.Protobuf.Reflection
     /// </summary>
     internal sealed class RepeatedFieldAccessor : FieldAccessorBase
     {
-        internal RepeatedFieldAccessor(Type type, string propertyName, FieldDescriptor descriptor) : base(type, propertyName, descriptor)
+        internal RepeatedFieldAccessor(PropertyInfo property, FieldDescriptor descriptor) : base(property, descriptor)
         {
         }
 
diff --git a/csharp/src/Google.Protobuf/Reflection/SingleFieldAccessor.cs b/csharp/src/Google.Protobuf/Reflection/SingleFieldAccessor.cs
index 8c24e46e..f00a51ba 100644
--- a/csharp/src/Google.Protobuf/Reflection/SingleFieldAccessor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/SingleFieldAccessor.cs
@@ -48,11 +48,8 @@ namespace Google.Protobuf.Reflection
         private readonly Action<object, object> setValueDelegate;
         private readonly Action<object> clearDelegate;
 
-        internal SingleFieldAccessor(Type type, string propertyName, FieldDescriptor descriptor) : base(type, propertyName, descriptor)
+        internal SingleFieldAccessor(PropertyInfo property, FieldDescriptor descriptor) : base(property, descriptor)
         {
-            PropertyInfo property = type.GetProperty(propertyName);
-            // We know there *is* such a property, or the base class constructor would have thrown. We should be able to write
-            // to it though.
             if (!property.CanWrite)
             {
                 throw new ArgumentException("Not all required properties/methods available");