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 :)

csharp/src/Google.Protobuf.Test/Google.Protobuf.Test.csproj

-<?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" />

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

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]);

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

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>
@@ -135,14 +136,27 @@ 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
         {
             get { return fields; }
         }
 
+        /// <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>
@@ -164,13 +178,6 @@ namespace Google.Protobuf.Reflection
             get { return oneofs; }
         }
 
-        /// <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>
@@ -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