First pass at making field access simpler.

This is definitely not ready to ship - I'm "troubled" by the disconnect between a list of fields in declaration order, and a mapping of field accessors by field number/name. Discussion required, but I find that easier when we've got code to look at :)

7 files changed, 309 insertions, 175 deletions

diff --git a/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.cs b/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.cs
index 180976d1..528a4023 100644
--- a/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.cs
+++ b/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.cs
@@ -597,165 +597,5 @@ namespace Google.Protobuf
             Assert.AreEqual(message, message2);

             Assert.AreEqual(TestAllTypes.OneofFieldOneofCase.OneofUint32, message2.OneofFieldCase);

         }

-

-        // TODO: Consider moving these tests to a separate reflection test - although they do require generated messages.

-

-        [Test]

-        public void Reflection_GetValue()

-        {

-            var message = SampleMessages.CreateFullTestAllTypes();

-            var fields = TestAllTypes.Descriptor.FieldAccessorsByFieldNumber;

-            Assert.AreEqual(message.SingleBool, fields[TestAllTypes.SingleBoolFieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleBytes, fields[TestAllTypes.SingleBytesFieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleDouble, fields[TestAllTypes.SingleDoubleFieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleFixed32, fields[TestAllTypes.SingleFixed32FieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleFixed64, fields[TestAllTypes.SingleFixed64FieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleFloat, fields[TestAllTypes.SingleFloatFieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleForeignEnum, fields[TestAllTypes.SingleForeignEnumFieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleForeignMessage, fields[TestAllTypes.SingleForeignMessageFieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleImportEnum, fields[TestAllTypes.SingleImportEnumFieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleImportMessage, fields[TestAllTypes.SingleImportMessageFieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleInt32, fields[TestAllTypes.SingleInt32FieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleInt64, fields[TestAllTypes.SingleInt64FieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleNestedEnum, fields[TestAllTypes.SingleNestedEnumFieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleNestedMessage, fields[TestAllTypes.SingleNestedMessageFieldNumber].GetValue(message));

-            Assert.AreEqual(message.SinglePublicImportMessage, fields[TestAllTypes.SinglePublicImportMessageFieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleSint32, fields[TestAllTypes.SingleSint32FieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleSint64, fields[TestAllTypes.SingleSint64FieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleString, fields[TestAllTypes.SingleStringFieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleSfixed32, fields[TestAllTypes.SingleSfixed32FieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleSfixed64, fields[TestAllTypes.SingleSfixed64FieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleUint32, fields[TestAllTypes.SingleUint32FieldNumber].GetValue(message));

-            Assert.AreEqual(message.SingleUint64, fields[TestAllTypes.SingleUint64FieldNumber].GetValue(message));

-            Assert.AreEqual(message.OneofBytes, fields[TestAllTypes.OneofBytesFieldNumber].GetValue(message));

-            Assert.AreEqual(message.OneofString, fields[TestAllTypes.OneofStringFieldNumber].GetValue(message));

-            Assert.AreEqual(message.OneofNestedMessage, fields[TestAllTypes.OneofNestedMessageFieldNumber].GetValue(message));

-            Assert.AreEqual(message.OneofUint32, fields[TestAllTypes.OneofUint32FieldNumber].GetValue(message));

-

-            // Just one example for repeated fields - they're all just returning the list

-            var list = (IList)fields[TestAllTypes.RepeatedInt32FieldNumber].GetValue(message);

-            Assert.AreEqual(message.RepeatedInt32, list);

-            Assert.AreEqual(message.RepeatedInt32[0], list[0]); // Just in case there was any doubt...

-

-            // Just a single map field, for the same reason

-            var mapMessage = new TestMap { MapStringString = { { "key1", "value1" }, { "key2", "value2" } } };

-            fields = TestMap.Descriptor.FieldAccessorsByFieldNumber;

-            var dictionary = (IDictionary) fields[TestMap.MapStringStringFieldNumber].GetValue(mapMessage);

-            Assert.AreEqual(mapMessage.MapStringString, dictionary);

-            Assert.AreEqual("value1", dictionary["key1"]);

-        }

-

-        [Test]

-        public void Reflection_Clear()

-        {

-            var message = SampleMessages.CreateFullTestAllTypes();

-            var fields = TestAllTypes.Descriptor.FieldAccessorsByFieldNumber;

-            fields[TestAllTypes.SingleBoolFieldNumber].Clear(message);

-            fields[TestAllTypes.SingleInt32FieldNumber].Clear(message);

-            fields[TestAllTypes.SingleStringFieldNumber].Clear(message);

-            fields[TestAllTypes.SingleBytesFieldNumber].Clear(message);

-            fields[TestAllTypes.SingleForeignEnumFieldNumber].Clear(message);

-            fields[TestAllTypes.SingleForeignMessageFieldNumber].Clear(message);

-            fields[TestAllTypes.RepeatedDoubleFieldNumber].Clear(message);

-

-            var expected = new TestAllTypes(SampleMessages.CreateFullTestAllTypes())

-            {

-                SingleBool = false,

-                SingleInt32 = 0,

-                SingleString = "",

-                SingleBytes = ByteString.Empty,

-                SingleForeignEnum = 0,

-                SingleForeignMessage = null,

-            };

-            expected.RepeatedDouble.Clear();

-

-            Assert.AreEqual(expected, message);

-

-            // Separately, maps.

-            var mapMessage = new TestMap { MapStringString = { { "key1", "value1" }, { "key2", "value2" } } };

-            fields = TestMap.Descriptor.FieldAccessorsByFieldNumber;

-            fields[TestMap.MapStringStringFieldNumber].Clear(mapMessage);

-            Assert.AreEqual(0, mapMessage.MapStringString.Count);

-        }

-

-        [Test]

-        public void Reflection_SetValue_SingleFields()

-        {

-            // Just a sample (primitives, messages, enums, strings, byte strings)

-            var message = SampleMessages.CreateFullTestAllTypes();

-            var fields = TestAllTypes.Descriptor.FieldAccessorsByFieldNumber;

-            fields[TestAllTypes.SingleBoolFieldNumber].SetValue(message, false);

-            fields[TestAllTypes.SingleInt32FieldNumber].SetValue(message, 500);

-            fields[TestAllTypes.SingleStringFieldNumber].SetValue(message, "It's a string");

-            fields[TestAllTypes.SingleBytesFieldNumber].SetValue(message, ByteString.CopyFrom(99, 98, 97));

-            fields[TestAllTypes.SingleForeignEnumFieldNumber].SetValue(message, ForeignEnum.FOREIGN_FOO);

-            fields[TestAllTypes.SingleForeignMessageFieldNumber].SetValue(message, new ForeignMessage { C = 12345 });

-            fields[TestAllTypes.SingleDoubleFieldNumber].SetValue(message, 20150701.5);

-

-            var expected = new TestAllTypes(SampleMessages.CreateFullTestAllTypes())

-            {

-                SingleBool = false,

-                SingleInt32 = 500,

-                SingleString = "It's a string",

-                SingleBytes = ByteString.CopyFrom(99, 98, 97),

-                SingleForeignEnum = ForeignEnum.FOREIGN_FOO,

-                SingleForeignMessage = new ForeignMessage { C = 12345 },

-                SingleDouble = 20150701.5

-            };

-

-            Assert.AreEqual(expected, message);

-        }

-

-        [Test]

-        public void Reflection_SetValue_SingleFields_WrongType()

-        {

-            IMessage message = SampleMessages.CreateFullTestAllTypes();

-            var fields = message.Descriptor.FieldAccessorsByFieldNumber;

-            Assert.Throws<InvalidCastException>(() => fields[TestAllTypes.SingleBoolFieldNumber].SetValue(message, "This isn't a bool"));

-        }

-

-        [Test]

-        public void Reflection_SetValue_MapFields()

-        {

-            IMessage message = new TestMap();

-            var fields = message.Descriptor.FieldAccessorsByFieldNumber;

-            Assert.Throws<InvalidOperationException>(() => fields[TestMap.MapStringStringFieldNumber].SetValue(message, new Dictionary<string, string>()));

-        }

-

-        [Test]

-        public void Reflection_SetValue_RepeatedFields()

-        {

-            IMessage message = SampleMessages.CreateFullTestAllTypes();

-            var fields = message.Descriptor.FieldAccessorsByFieldNumber;

-            Assert.Throws<InvalidOperationException>(() => fields[TestAllTypes.RepeatedDoubleFieldNumber].SetValue(message, new double[10]));

-        }

-

-        [Test]

-        public void Reflection_GetValue_IncorrectType()

-        {

-            IMessage message = SampleMessages.CreateFullTestAllTypes();

-            var fields = message.Descriptor.FieldAccessorsByFieldNumber;

-            Assert.Throws<InvalidCastException>(() => fields[TestAllTypes.SingleBoolFieldNumber].GetValue(new TestMap()));

-        }

-

-        [Test]

-        public void Reflection_Oneof()

-        {

-            var message = new TestAllTypes();

-            var descriptor = TestAllTypes.Descriptor;

-            Assert.AreEqual(1, descriptor.Oneofs.Count);

-            var oneof = descriptor.Oneofs[0];

-            Assert.AreEqual("oneof_field", oneof.Name);

-            Assert.IsNull(oneof.Accessor.GetCaseFieldDescriptor(message));

-

-            message.OneofString = "foo";

-            Assert.AreSame(descriptor.FieldAccessorsByFieldNumber[TestAllTypes.OneofStringFieldNumber].Descriptor, oneof.Accessor.GetCaseFieldDescriptor(message));

-

-            message.OneofUint32 = 10;

-            Assert.AreSame(descriptor.FieldAccessorsByFieldNumber[TestAllTypes.OneofUint32FieldNumber].Descriptor, oneof.Accessor.GetCaseFieldDescriptor(message));

-

-            oneof.Accessor.Clear(message);

-            Assert.AreEqual(TestAllTypes.OneofFieldOneofCase.None, message.OneofFieldCase);

-        }

     }

 }

diff --git a/csharp/src/Google.Protobuf.Test/Google.Protobuf.Test.csproj b/csharp/src/Google.Protobuf.Test/Google.Protobuf.Test.csproj
index 6d8b4de2..2522901e 100644
--- a/csharp/src/Google.Protobuf.Test/Google.Protobuf.Test.csproj
+++ b/csharp/src/Google.Protobuf.Test/Google.Protobuf.Test.csproj
@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="utf-8"?>

+<?xml version="1.0" encoding="utf-8"?>

 <Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">

   <PropertyGroup>

     <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>

@@ -82,6 +82,7 @@
     <Compile Include="Collections\RepeatedFieldTest.cs" />

     <Compile Include="JsonFormatterTest.cs" />

     <Compile Include="Reflection\DescriptorsTest.cs" />

+    <Compile Include="Reflection\FieldAccessTest.cs" />

     <Compile Include="SampleEnum.cs" />

     <Compile Include="SampleMessages.cs" />

     <Compile Include="TestProtos\MapUnittestProto3.cs" />

diff --git a/csharp/src/Google.Protobuf.Test/Reflection/DescriptorsTest.cs b/csharp/src/Google.Protobuf.Test/Reflection/DescriptorsTest.cs
index 0aff0a6c..ed4291a4 100644
--- a/csharp/src/Google.Protobuf.Test/Reflection/DescriptorsTest.cs
+++ b/csharp/src/Google.Protobuf.Test/Reflection/DescriptorsTest.cs
@@ -33,6 +33,7 @@
 using System.Linq;
 using Google.Protobuf.TestProtos;
 using NUnit.Framework;
+using UnitTest.Issues.TestProtos;
 
 namespace Google.Protobuf.Reflection
 {
@@ -220,5 +221,19 @@ namespace Google.Protobuf.Reflection
 
             CollectionAssert.AreEquivalent(expectedFields, descriptor.Fields);
         }
+
+        [Test]
+        public void ConstructionWithoutGeneratedCodeInfo()
+        {
+            var data = UnittestIssues.Descriptor.Proto.ToByteArray();
+            var newDescriptor = Google.Protobuf.Reflection.FileDescriptor.InternalBuildGeneratedFileFrom(data, new Reflection.FileDescriptor[] { }, null);
+
+            // We should still be able to get at a field...
+            var messageDescriptor = newDescriptor.FindTypeByName<MessageDescriptor>("ItemField");
+            var fieldDescriptor = messageDescriptor.FindFieldByName("item");
+            // But there shouldn't be an accessor (or a generated type for the message)
+            Assert.IsNull(fieldDescriptor.Accessor);
+            Assert.IsNull(messageDescriptor.GeneratedType);
+        }
     }
 }
\ No newline at end of file
diff --git a/csharp/src/Google.Protobuf.Test/Reflection/FieldAccessTest.cs b/csharp/src/Google.Protobuf.Test/Reflection/FieldAccessTest.cs
new file mode 100644
index 00000000..5d6e777f
--- /dev/null
+++ b/csharp/src/Google.Protobuf.Test/Reflection/FieldAccessTest.cs
@@ -0,0 +1,218 @@
+#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 Google.Protobuf.TestProtos;
+using NUnit.Framework;
+using System;
+using System.Collections;
+using System.Collections.Generic;
+
+namespace Google.Protobuf.Reflection
+{
+    public class FieldAccessTest
+    {
+        [Test]
+        public void GetValue()
+        {
+            var message = SampleMessages.CreateFullTestAllTypes();
+            var fields = TestAllTypes.Descriptor.FieldAccessors;
+            Assert.AreEqual(message.SingleBool, fields[TestAllTypes.SingleBoolFieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleBytes, fields[TestAllTypes.SingleBytesFieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleDouble, fields[TestAllTypes.SingleDoubleFieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleFixed32, fields[TestAllTypes.SingleFixed32FieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleFixed64, fields[TestAllTypes.SingleFixed64FieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleFloat, fields[TestAllTypes.SingleFloatFieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleForeignEnum, fields[TestAllTypes.SingleForeignEnumFieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleForeignMessage, fields[TestAllTypes.SingleForeignMessageFieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleImportEnum, fields[TestAllTypes.SingleImportEnumFieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleImportMessage, fields[TestAllTypes.SingleImportMessageFieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleInt32, fields[TestAllTypes.SingleInt32FieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleInt64, fields[TestAllTypes.SingleInt64FieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleNestedEnum, fields[TestAllTypes.SingleNestedEnumFieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleNestedMessage, fields[TestAllTypes.SingleNestedMessageFieldNumber].GetValue(message));
+            Assert.AreEqual(message.SinglePublicImportMessage, fields[TestAllTypes.SinglePublicImportMessageFieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleSint32, fields[TestAllTypes.SingleSint32FieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleSint64, fields[TestAllTypes.SingleSint64FieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleString, fields[TestAllTypes.SingleStringFieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleSfixed32, fields[TestAllTypes.SingleSfixed32FieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleSfixed64, fields[TestAllTypes.SingleSfixed64FieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleUint32, fields[TestAllTypes.SingleUint32FieldNumber].GetValue(message));
+            Assert.AreEqual(message.SingleUint64, fields[TestAllTypes.SingleUint64FieldNumber].GetValue(message));
+            Assert.AreEqual(message.OneofBytes, fields[TestAllTypes.OneofBytesFieldNumber].GetValue(message));
+            Assert.AreEqual(message.OneofString, fields[TestAllTypes.OneofStringFieldNumber].GetValue(message));
+            Assert.AreEqual(message.OneofNestedMessage, fields[TestAllTypes.OneofNestedMessageFieldNumber].GetValue(message));
+            Assert.AreEqual(message.OneofUint32, fields[TestAllTypes.OneofUint32FieldNumber].GetValue(message));
+
+            // Just one example for repeated fields - they're all just returning the list
+            var list = (IList) fields[TestAllTypes.RepeatedInt32FieldNumber].GetValue(message);
+            Assert.AreEqual(message.RepeatedInt32, list);
+            Assert.AreEqual(message.RepeatedInt32[0], list[0]); // Just in case there was any doubt...
+
+            // Just a single map field, for the same reason
+            var mapMessage = new TestMap { MapStringString = { { "key1", "value1" }, { "key2", "value2" } } };
+            fields = TestMap.Descriptor.FieldAccessors;
+            var dictionary = (IDictionary) fields[TestMap.MapStringStringFieldNumber].GetValue(mapMessage);
+            Assert.AreEqual(mapMessage.MapStringString, dictionary);
+            Assert.AreEqual("value1", dictionary["key1"]);
+        }
+
+        [Test]
+        public void Clear()
+        {
+            var message = SampleMessages.CreateFullTestAllTypes();
+            var fields = TestAllTypes.Descriptor.FieldAccessors;
+            fields[TestAllTypes.SingleBoolFieldNumber].Clear(message);
+            fields[TestAllTypes.SingleInt32FieldNumber].Clear(message);
+            fields[TestAllTypes.SingleStringFieldNumber].Clear(message);
+            fields[TestAllTypes.SingleBytesFieldNumber].Clear(message);
+            fields[TestAllTypes.SingleForeignEnumFieldNumber].Clear(message);
+            fields[TestAllTypes.SingleForeignMessageFieldNumber].Clear(message);
+            fields[TestAllTypes.RepeatedDoubleFieldNumber].Clear(message);
+
+            var expected = new TestAllTypes(SampleMessages.CreateFullTestAllTypes())
+            {
+                SingleBool = false,
+                SingleInt32 = 0,
+                SingleString = "",
+                SingleBytes = ByteString.Empty,
+                SingleForeignEnum = 0,
+                SingleForeignMessage = null,
+            };
+            expected.RepeatedDouble.Clear();
+
+            Assert.AreEqual(expected, message);
+
+            // Separately, maps.
+            var mapMessage = new TestMap { MapStringString = { { "key1", "value1" }, { "key2", "value2" } } };
+            fields = TestMap.Descriptor.FieldAccessors;
+            fields[TestMap.MapStringStringFieldNumber].Clear(mapMessage);
+            Assert.AreEqual(0, mapMessage.MapStringString.Count);
+        }
+
+        [Test]
+        public void SetValue_SingleFields()
+        {
+            // Just a sample (primitives, messages, enums, strings, byte strings)
+            var message = SampleMessages.CreateFullTestAllTypes();
+            var fields = TestAllTypes.Descriptor.FieldAccessors;
+            fields[TestAllTypes.SingleBoolFieldNumber].SetValue(message, false);
+            fields[TestAllTypes.SingleInt32FieldNumber].SetValue(message, 500);
+            fields[TestAllTypes.SingleStringFieldNumber].SetValue(message, "It's a string");
+            fields[TestAllTypes.SingleBytesFieldNumber].SetValue(message, ByteString.CopyFrom(99, 98, 97));
+            fields[TestAllTypes.SingleForeignEnumFieldNumber].SetValue(message, ForeignEnum.FOREIGN_FOO);
+            fields[TestAllTypes.SingleForeignMessageFieldNumber].SetValue(message, new ForeignMessage { C = 12345 });
+            fields[TestAllTypes.SingleDoubleFieldNumber].SetValue(message, 20150701.5);
+
+            var expected = new TestAllTypes(SampleMessages.CreateFullTestAllTypes())
+            {
+                SingleBool = false,
+                SingleInt32 = 500,
+                SingleString = "It's a string",
+                SingleBytes = ByteString.CopyFrom(99, 98, 97),
+                SingleForeignEnum = ForeignEnum.FOREIGN_FOO,
+                SingleForeignMessage = new ForeignMessage { C = 12345 },
+                SingleDouble = 20150701.5
+            };
+
+            Assert.AreEqual(expected, message);
+        }
+
+        [Test]
+        public void SetValue_SingleFields_WrongType()
+        {
+            IMessage message = SampleMessages.CreateFullTestAllTypes();
+            var fields = message.Descriptor.FieldAccessors;
+            Assert.Throws<InvalidCastException>(() => fields[TestAllTypes.SingleBoolFieldNumber].SetValue(message, "This isn't a bool"));
+        }
+
+        [Test]
+        public void SetValue_MapFields()
+        {
+            IMessage message = new TestMap();
+            var fields = message.Descriptor.FieldAccessors;
+            Assert.Throws<InvalidOperationException>(() => fields[TestMap.MapStringStringFieldNumber].SetValue(message, new Dictionary<string, string>()));
+        }
+
+        [Test]
+        public void SetValue_RepeatedFields()
+        {
+            IMessage message = SampleMessages.CreateFullTestAllTypes();
+            var fields = message.Descriptor.FieldAccessors;
+            Assert.Throws<InvalidOperationException>(() => fields[TestAllTypes.RepeatedDoubleFieldNumber].SetValue(message, new double[10]));
+        }
+
+        [Test]
+        public void GetValue_IncorrectType()
+        {
+            IMessage message = SampleMessages.CreateFullTestAllTypes();
+            var fields = message.Descriptor.FieldAccessors;
+            Assert.Throws<InvalidCastException>(() => fields[TestAllTypes.SingleBoolFieldNumber].GetValue(new TestMap()));
+        }
+
+        [Test]
+        public void Oneof()
+        {
+            var message = new TestAllTypes();
+            var descriptor = TestAllTypes.Descriptor;
+            Assert.AreEqual(1, descriptor.Oneofs.Count);
+            var oneof = descriptor.Oneofs[0];
+            Assert.AreEqual("oneof_field", oneof.Name);
+            Assert.IsNull(oneof.Accessor.GetCaseFieldDescriptor(message));
+
+            message.OneofString = "foo";
+            Assert.AreSame(descriptor.FieldAccessors[TestAllTypes.OneofStringFieldNumber].Descriptor, oneof.Accessor.GetCaseFieldDescriptor(message));
+
+            message.OneofUint32 = 10;
+            Assert.AreSame(descriptor.FieldAccessors[TestAllTypes.OneofUint32FieldNumber].Descriptor, oneof.Accessor.GetCaseFieldDescriptor(message));
+
+            oneof.Accessor.Clear(message);
+            Assert.AreEqual(TestAllTypes.OneofFieldOneofCase.None, message.OneofFieldCase);
+        }
+
+        [Test]
+        public void FieldAccessor_ByName()
+        {
+            var descriptor = TestAllTypes.Descriptor;
+            Assert.AreSame(
+                descriptor.FieldAccessors[TestAllTypes.SingleBoolFieldNumber],
+                descriptor.FieldAccessors["single_bool"]);
+        }
+
+        [Test]
+        public void FieldAccessor_NotFound()
+        {
+            var descriptor = TestAllTypes.Descriptor;
+            Assert.Throws<KeyNotFoundException>(() => descriptor.FieldAccessors[999999].ToString());
+            Assert.Throws<KeyNotFoundException>(() => descriptor.FieldAccessors["not found"].ToString());
+        }
+    }
+}
diff --git a/csharp/src/Google.Protobuf.Test/WellKnownTypes/WrappersTest.cs b/csharp/src/Google.Protobuf.Test/WellKnownTypes/WrappersTest.cs
index c617db36..a8288483 100644
--- a/csharp/src/Google.Protobuf.Test/WellKnownTypes/WrappersTest.cs
+++ b/csharp/src/Google.Protobuf.Test/WellKnownTypes/WrappersTest.cs
@@ -192,7 +192,7 @@ namespace Google.Protobuf.WellKnownTypes
                 Uint32Field = 3,
                 Uint64Field = 4
             };
-            var fields = TestWellKnownTypes.Descriptor.FieldAccessorsByFieldNumber;
+            var fields = TestWellKnownTypes.Descriptor.FieldAccessors;
 
             Assert.AreEqual("x", fields[TestWellKnownTypes.StringFieldFieldNumber].GetValue(message));
             Assert.AreEqual(ByteString.CopyFrom(1, 2, 3), fields[TestWellKnownTypes.BytesFieldFieldNumber].GetValue(message));
@@ -216,7 +216,7 @@ namespace Google.Protobuf.WellKnownTypes
         {
             // Just a single example... note that we can't have a null value here
             var message = new RepeatedWellKnownTypes { Int32Field = { 1, 2 } };
-            var fields = RepeatedWellKnownTypes.Descriptor.FieldAccessorsByFieldNumber;
+            var fields = RepeatedWellKnownTypes.Descriptor.FieldAccessors;
             var list = (IList) fields[RepeatedWellKnownTypes.Int32FieldFieldNumber].GetValue(message);
             CollectionAssert.AreEqual(new[] { 1, 2 }, list);
         }
@@ -226,7 +226,7 @@ namespace Google.Protobuf.WellKnownTypes
         {
             // Just a single example... note that we can't have a null value here
             var message = new MapWellKnownTypes { Int32Field = { { 1, 2 }, { 3, null } } };
-            var fields = MapWellKnownTypes.Descriptor.FieldAccessorsByFieldNumber;
+            var fields = MapWellKnownTypes.Descriptor.FieldAccessors;
             var dictionary = (IDictionary) fields[MapWellKnownTypes.Int32FieldFieldNumber].GetValue(message);
             Assert.AreEqual(2, dictionary[1]);
             Assert.IsNull(dictionary[3]);
diff --git a/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
index 041d4711..718c4797 100644
--- a/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
@@ -231,7 +231,7 @@ namespace Google.Protobuf.Reflection
         /// 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>
+        /// <typeparam name="T">The type of descriptor to look for</typeparam>
         /// <returns>The type's descriptor, or null if not found.</returns>
         public T FindTypeByName<T>(String name)
             where T : class, IDescriptor
diff --git a/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
index b29b4b20..909a31ff 100644
--- a/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
@@ -61,10 +61,10 @@ namespace Google.Protobuf.Reflection
         private readonly IList<MessageDescriptor> nestedTypes;
         private readonly IList<EnumDescriptor> enumTypes;
         private readonly IList<FieldDescriptor> fields;
+        private readonly FieldAccessorCollection fieldAccessors;
         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, GeneratedCodeInfo generatedCodeInfo)
             : base(file, file.ComputeFullName(parent, proto.Name), typeIndex)
@@ -94,6 +94,7 @@ namespace Google.Protobuf.Reflection
                 (field, index) =>
                 new FieldDescriptor(field, file, this, index, generatedCodeInfo == null ? null : generatedCodeInfo.PropertyNames[index]));
             file.DescriptorPool.AddSymbol(this);
+            fieldAccessors = new FieldAccessorCollection(this);
         }
                 
         /// <summary>
@@ -135,8 +136,13 @@ namespace Google.Protobuf.Reflection
             get { return containingType; }
         }
 
+        // TODO: It's confusing that FieldAccessors[x] doesn't retrieve the accessor
+        // for Fields[x]. We should think about this further... how often does a user really
+        // want the fields in declaration order?
+
         /// <value>
-        /// An unmodifiable list of this message type's fields.
+        /// An unmodifiable list of this message type's fields, in the declaration order
+        /// within the .proto file.
         /// </value>
         public IList<FieldDescriptor> Fields
         {
@@ -144,6 +150,14 @@ namespace Google.Protobuf.Reflection
         }
 
         /// <value>
+        /// A collection of accessors, which can be retrieved by name or field number.
+        /// </value>
+        public FieldAccessorCollection FieldAccessors
+        {
+            get { return fieldAccessors; }
+        }
+
+        /// <value>
         /// An unmodifiable list of this message type's nested types.
         /// </value>
         public IList<MessageDescriptor> NestedTypes
@@ -165,13 +179,6 @@ 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>
@@ -222,8 +229,61 @@ namespace Google.Protobuf.Reflection
             {
                 oneof.CrossLink();
             }
+        }
+
+        /// <summary>
+        /// A collection to simplify retrieving the field accessor for a particular field.
+        /// </summary>
+        public sealed class FieldAccessorCollection
+        {
+            private readonly MessageDescriptor messageDescriptor;
+
+            internal FieldAccessorCollection(MessageDescriptor messageDescriptor)
+            {
+                this.messageDescriptor = messageDescriptor;
+            }
 
-            fieldAccessorsByFieldNumber = new ReadOnlyDictionary<int, IFieldAccessor>(fields.ToDictionary(field => field.FieldNumber, field => field.Accessor));
+            /// <summary>
+            /// Retrieves the accessor for the field with the given number.
+            /// </summary>
+            /// <param name="number">Number of the field to retrieve the accessor for</param>
+            /// <returns>The accessor for the given field, or null if reflective field access is
+            /// not supported for the field.</returns>
+            /// <exception cref="KeyNotFoundException">The message descriptor does not contain a field
+            /// with the given number</exception>
+            public IFieldAccessor this[int number]
+            {
+                get
+                {
+                    var fieldDescriptor = messageDescriptor.FindFieldByNumber(number);
+                    if (fieldDescriptor == null)
+                    {
+                        throw new KeyNotFoundException("No such field number");
+                    }
+                    return fieldDescriptor.Accessor;
+                }
+            }
+
+            /// <summary>
+            /// Retrieves the accessor for the field with the given name.
+            /// </summary>
+            /// <param name="number">Number of the field to retrieve the accessor for</param>
+            /// <returns>The accessor for the given field, or null if reflective field access is
+            /// not supported for the field.</returns>
+            /// <exception cref="KeyNotFoundException">The message descriptor does not contain a field
+            /// with the given name</exception>
+            public IFieldAccessor this[string name]
+            {
+                get
+                {
+                    var fieldDescriptor = messageDescriptor.FindFieldByName(name);
+                    if (fieldDescriptor == null)
+                    {
+                        throw new KeyNotFoundException("No such field name");
+                    }
+                    return fieldDescriptor.Accessor;
+                }
+            }
         }
     }
 }
\ No newline at end of file