diff --git a/csharp/src/Google.Protobuf/FieldCodec.cs b/csharp/src/Google.Protobuf/FieldCodec.cs
new file mode 100644
index 0000000..9831308
--- /dev/null
+++ b/csharp/src/Google.Protobuf/FieldCodec.cs
@@ -0,0 +1,473 @@
+﻿#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.Compatibility;
+using Google.Protobuf.WellKnownTypes;
+using System;
+using System.Collections.Generic;
+
+namespace Google.Protobuf
+{
+    /// <summary>
+    /// Factory methods for <see cref="FieldCodec{T}"/>.
+    /// </summary>
+    public static class FieldCodec
+    {
+        // TODO: Avoid the "dual hit" of lambda expressions: create open delegates instead. (At least test...)
+
+        /// <summary>
+        /// Retrieves a codec suitable for a string field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<string> ForString(uint tag)
+        {
+            return new FieldCodec<string>(input => input.ReadString(), (output, value) => output.WriteString(value), CodedOutputStream.ComputeStringSize, tag);
+        }
+
+        /// <summary>
+        /// Retrieves a codec suitable for a bytes field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<ByteString> ForBytes(uint tag)
+        {
+            return new FieldCodec<ByteString>(input => input.ReadBytes(), (output, value) => output.WriteBytes(value), CodedOutputStream.ComputeBytesSize, tag);
+        }
+
+        /// <summary>
+        /// Retrieves a codec suitable for a bool field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<bool> ForBool(uint tag)
+        {
+            return new FieldCodec<bool>(input => input.ReadBool(), (output, value) => output.WriteBool(value), CodedOutputStream.ComputeBoolSize, tag);
+        }
+
+        /// <summary>
+        /// Retrieves a codec suitable for an int32 field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<int> ForInt32(uint tag)
+        {
+            return new FieldCodec<int>(input => input.ReadInt32(), (output, value) => output.WriteInt32(value), CodedOutputStream.ComputeInt32Size, tag);
+        }
+
+        /// <summary>
+        /// Retrieves a codec suitable for an sint32 field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<int> ForSInt32(uint tag)
+        {
+            return new FieldCodec<int>(input => input.ReadSInt32(), (output, value) => output.WriteSInt32(value), CodedOutputStream.ComputeSInt32Size, tag);
+        }
+
+        /// <summary>
+        /// Retrieves a codec suitable for a fixed32 field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<uint> ForFixed32(uint tag)
+        {
+            return new FieldCodec<uint>(input => input.ReadFixed32(), (output, value) => output.WriteFixed32(value), 4, tag);
+        }
+
+        /// <summary>
+        /// Retrieves a codec suitable for an sfixed32 field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<int> ForSFixed32(uint tag)
+        {
+            return new FieldCodec<int>(input => input.ReadSFixed32(), (output, value) => output.WriteSFixed32(value), 4, tag);
+        }
+
+        /// <summary>
+        /// Retrieves a codec suitable for a uint32 field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<uint> ForUInt32(uint tag)
+        {
+            return new FieldCodec<uint>(input => input.ReadUInt32(), (output, value) => output.WriteUInt32(value), CodedOutputStream.ComputeUInt32Size, tag);
+        }
+
+        /// <summary>
+        /// Retrieves a codec suitable for an int64 field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<long> ForInt64(uint tag)
+        {
+            return new FieldCodec<long>(input => input.ReadInt64(), (output, value) => output.WriteInt64(value), CodedOutputStream.ComputeInt64Size, tag);
+        }
+
+        /// <summary>
+        /// Retrieves a codec suitable for an sint64 field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<long> ForSInt64(uint tag)
+        {
+            return new FieldCodec<long>(input => input.ReadSInt64(), (output, value) => output.WriteSInt64(value), CodedOutputStream.ComputeSInt64Size, tag);
+        }
+
+        /// <summary>
+        /// Retrieves a codec suitable for a fixed64 field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<ulong> ForFixed64(uint tag)
+        {
+            return new FieldCodec<ulong>(input => input.ReadFixed64(), (output, value) => output.WriteFixed64(value), 8, tag);
+        }
+
+        /// <summary>
+        /// Retrieves a codec suitable for an sfixed64 field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<long> ForSFixed64(uint tag)
+        {
+            return new FieldCodec<long>(input => input.ReadSFixed64(), (output, value) => output.WriteSFixed64(value), 8, tag);
+        }
+
+        /// <summary>
+        /// Retrieves a codec suitable for a uint64 field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<ulong> ForUInt64(uint tag)
+        {
+            return new FieldCodec<ulong>(input => input.ReadUInt64(), (output, value) => output.WriteUInt64(value), CodedOutputStream.ComputeUInt64Size, tag);
+        }
+
+        /// <summary>
+        /// Retrieves a codec suitable for a float field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<float> ForFloat(uint tag)
+        {
+            return new FieldCodec<float>(input => input.ReadFloat(), (output, value) => output.WriteFloat(value), CodedOutputStream.ComputeFloatSize, tag);
+        }
+
+        /// <summary>
+        /// Retrieves a codec suitable for a double field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<double> ForDouble(uint tag)
+        {
+            return new FieldCodec<double>(input => input.ReadDouble(), (output, value) => output.WriteDouble(value), CodedOutputStream.ComputeDoubleSize, tag);
+        }
+
+        // Enums are tricky. We can probably use expression trees to build these delegates automatically,
+        // but it's easy to generate the code for it.
+
+        /// <summary>
+        /// Retrieves a codec suitable for an enum field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <param name="toInt32">A conversion function from <see cref="Int32"/> to the enum type.</param>
+        /// <param name="fromInt32">A conversion function from the enum type to <see cref="Int32"/>.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<T> ForEnum<T>(uint tag, Func<T, int> toInt32, Func<int, T> fromInt32)
+        {
+            return new FieldCodec<T>(input => fromInt32(
+                input.ReadEnum()),
+                (output, value) => output.WriteEnum(toInt32(value)),
+                value => CodedOutputStream.ComputeEnumSize(toInt32(value)), tag);
+        }
+
+        /// <summary>
+        /// Retrieves a codec suitable for a message field with the given tag.
+        /// </summary>
+        /// <param name="tag">The tag.</param>
+        /// <param name="parser">A parser to use for the message type.</param>
+        /// <returns>A codec for the given tag.</returns>
+        public static FieldCodec<T> ForMessage<T>(uint tag, MessageParser<T> parser) where T : IMessage<T>
+        {
+            return new FieldCodec<T>(input => { T message = parser.CreateTemplate(); input.ReadMessage(message); return message; },
+                (output, value) => output.WriteMessage(value), message => CodedOutputStream.ComputeMessageSize(message), tag);
+        }
+
+        /// <summary>
+        /// Creates a codec for a wrapper type of a class - which must be string or ByteString.
+        /// </summary>
+        public static FieldCodec<T> ForClassWrapper<T>(uint tag) where T : class
+        {
+            var nestedCodec = WrapperCodecs.GetCodec<T>();
+            return new FieldCodec<T>(
+                input => WrapperCodecs.Read<T>(input, nestedCodec),
+                (output, value) => WrapperCodecs.Write<T>(output, value, nestedCodec),
+                value => WrapperCodecs.CalculateSize<T>(value, nestedCodec),
+                tag,
+                null); // Default value for the wrapper
+        }
+
+        /// <summary>
+        /// Creates a codec for a wrapper type of a struct - which must be Int32, Int64, UInt32, UInt64,
+        /// Bool, Single or Double.
+        /// </summary>
+        public static FieldCodec<T?> ForStructWrapper<T>(uint tag) where T : struct
+        {
+            var nestedCodec = WrapperCodecs.GetCodec<T>();
+            return new FieldCodec<T?>(
+                input => WrapperCodecs.Read<T>(input, nestedCodec),
+                (output, value) => WrapperCodecs.Write<T>(output, value.Value, nestedCodec),
+                value => value == null ? 0 : WrapperCodecs.CalculateSize<T>(value.Value, nestedCodec),
+                tag,
+                null); // Default value for the wrapper
+        }
+
+        /// <summary>
+        /// Helper code to create codecs for wrapper types.
+        /// </summary>
+        /// <remarks>
+        /// Somewhat ugly with all the static methods, but the conversions involved to/from nullable types make it
+        /// slightly tricky to improve. So long as we keep the public API (ForClassWrapper, ForStructWrapper) in place,
+        /// we can refactor later if we come up with something cleaner.
+        /// </remarks>
+        private static class WrapperCodecs
+        {
+            private static readonly Dictionary<System.Type, object> Codecs = new Dictionary<System.Type, object>
+            {
+                { typeof(bool), ForBool(WireFormat.MakeTag(WrappersReflection.WrapperValueFieldNumber, WireFormat.WireType.Varint)) },
+                { typeof(int), ForInt32(WireFormat.MakeTag(WrappersReflection.WrapperValueFieldNumber, WireFormat.WireType.Varint)) },
+                { typeof(long), ForInt64(WireFormat.MakeTag(WrappersReflection.WrapperValueFieldNumber, WireFormat.WireType.Varint)) },
+                { typeof(uint), ForUInt32(WireFormat.MakeTag(WrappersReflection.WrapperValueFieldNumber, WireFormat.WireType.Varint)) },
+                { typeof(ulong), ForUInt64(WireFormat.MakeTag(WrappersReflection.WrapperValueFieldNumber, WireFormat.WireType.Varint)) },
+                { typeof(float), ForFloat(WireFormat.MakeTag(WrappersReflection.WrapperValueFieldNumber, WireFormat.WireType.Fixed32)) },
+                { typeof(double), ForDouble(WireFormat.MakeTag(WrappersReflection.WrapperValueFieldNumber, WireFormat.WireType.Fixed64)) },
+                { typeof(string), ForString(WireFormat.MakeTag(WrappersReflection.WrapperValueFieldNumber, WireFormat.WireType.LengthDelimited)) },
+                { typeof(ByteString), ForBytes(WireFormat.MakeTag(WrappersReflection.WrapperValueFieldNumber, WireFormat.WireType.LengthDelimited)) }
+            };
+
+            /// <summary>
+            /// Returns a field codec which effectively wraps a value of type T in a message.
+            /// 
+            /// </summary>
+            internal static FieldCodec<T> GetCodec<T>()
+            {
+                object value;
+                if (!Codecs.TryGetValue(typeof(T), out value))
+                {
+                    throw new InvalidOperationException("Invalid type argument requested for wrapper codec: " + typeof(T));
+                }
+                return (FieldCodec<T>) value;
+            }
+
+            internal static T Read<T>(CodedInputStream input, FieldCodec<T> codec)
+            {
+                int length = input.ReadLength();
+                int oldLimit = input.PushLimit(length);
+
+                uint tag;
+                T value = codec.DefaultValue;
+                while ((tag = input.ReadTag()) != 0)
+                {
+                    if (tag == codec.Tag)
+                    {
+                        value = codec.Read(input);
+                    }
+                    else
+                    {
+                        input.SkipLastField();
+                    }
+
+                }
+                input.CheckReadEndOfStreamTag();
+                input.PopLimit(oldLimit);
+
+                return value;
+            }
+
+            internal static void Write<T>(CodedOutputStream output, T value, FieldCodec<T> codec)
+            {
+                output.WriteLength(codec.CalculateSizeWithTag(value));
+                codec.WriteTagAndValue(output, value);
+            }
+
+            internal  static int CalculateSize<T>(T value, FieldCodec<T> codec)
+            {
+                int fieldLength = codec.CalculateSizeWithTag(value);
+                return CodedOutputStream.ComputeLengthSize(fieldLength) + fieldLength;
+            }
+        }
+    }
+
+    /// <summary>
+    /// <para>
+    /// An encode/decode pair for a single field. This effectively encapsulates
+    /// all the information needed to read or write the field value from/to a coded
+    /// stream.
+    /// </para>
+    /// <para>
+    /// This class is public and has to be as it is used by generated code, but its public
+    /// API is very limited - just what the generated code needs to call directly.
+    /// </para>
+    /// </summary>
+    /// <remarks>
+    /// This never writes default values to the stream, and does not address "packedness"
+    /// in repeated fields itself, other than to know whether or not the field *should* be packed.
+    /// </remarks>
+    public sealed class FieldCodec<T>
+    {
+        private static readonly T DefaultDefault;
+        private static readonly bool TypeSupportsPacking = typeof(T).IsValueType() && Nullable.GetUnderlyingType(typeof(T)) == null;
+
+        static FieldCodec()
+        {
+            if (typeof(T) == typeof(string))
+            {
+                DefaultDefault = (T)(object)"";
+            }
+            else if (typeof(T) == typeof(ByteString))
+            {
+                DefaultDefault = (T)(object)ByteString.Empty;
+            }
+            // Otherwise it's the default value of the CLR type
+        }
+
+        internal static bool IsPackedRepeatedField(uint tag) =>
+            TypeSupportsPacking && WireFormat.GetTagWireType(tag) == WireFormat.WireType.LengthDelimited;
+
+        internal bool PackedRepeatedField { get; }
+
+        /// <summary>
+        /// Returns a delegate to write a value (unconditionally) to a coded output stream.
+        /// </summary>
+        internal Action<CodedOutputStream, T> ValueWriter { get; }
+
+        /// <summary>
+        /// Returns the size calculator for just a value.
+        /// </summary>
+        internal Func<T, int> ValueSizeCalculator { get; }
+
+        /// <summary>
+        /// Returns a delegate to read a value from a coded input stream. It is assumed that
+        /// the stream is already positioned on the appropriate tag.
+        /// </summary>
+        internal Func<CodedInputStream, T> ValueReader { get; }
+
+        /// <summary>
+        /// Returns the fixed size for an entry, or 0 if sizes vary.
+        /// </summary>
+        internal int FixedSize { get; }
+
+        /// <summary>
+        /// Gets the tag of the codec.
+        /// </summary>
+        /// <value>
+        /// The tag of the codec.
+        /// </value>
+        internal uint Tag { get; }
+
+        /// <summary>
+        /// Default value for this codec. Usually the same for every instance of the same type, but
+        /// for string/ByteString wrapper fields the codec's default value is null, whereas for
+        /// other string/ByteString fields it's "" or ByteString.Empty.
+        /// </summary>
+        /// <value>
+        /// The default value of the codec's type.
+        /// </value>
+        internal T DefaultValue { get; }
+
+        private readonly int tagSize;
+        
+        internal FieldCodec(
+                Func<CodedInputStream, T> reader,
+                Action<CodedOutputStream, T> writer,
+                int fixedSize,
+                uint tag) : this(reader, writer, _ => fixedSize, tag)
+        {
+            FixedSize = fixedSize;
+        }
+
+        internal FieldCodec(
+            Func<CodedInputStream, T> reader,
+            Action<CodedOutputStream, T> writer,
+            Func<T, int> sizeCalculator,
+            uint tag) : this(reader, writer, sizeCalculator, tag, DefaultDefault)
+        {
+        }
+
+        internal FieldCodec(
+            Func<CodedInputStream, T> reader,
+            Action<CodedOutputStream, T> writer,
+            Func<T, int> sizeCalculator,
+            uint tag,
+            T defaultValue)
+        {
+            ValueReader = reader;
+            ValueWriter = writer;
+            ValueSizeCalculator = sizeCalculator;
+            FixedSize = 0;
+            Tag = tag;
+            DefaultValue = defaultValue;
+            tagSize = CodedOutputStream.ComputeRawVarint32Size(tag);
+            // Detect packed-ness once, so we can check for it within RepeatedField<T>.
+            PackedRepeatedField = IsPackedRepeatedField(tag);
+        }
+
+        /// <summary>
+        /// Write a tag and the given value, *if* the value is not the default.
+        /// </summary>
+        public void WriteTagAndValue(CodedOutputStream output, T value)
+        {
+            if (!IsDefault(value))
+            {
+                output.WriteTag(Tag);
+                ValueWriter(output, value);
+            }
+        }
+
+        /// <summary>
+        /// Reads a value of the codec type from the given <see cref="CodedInputStream"/>.
+        /// </summary>
+        /// <param name="input">The input stream to read from.</param>
+        /// <returns>The value read from the stream.</returns>
+        public T Read(CodedInputStream input) => ValueReader(input);
+
+        /// <summary>
+        /// Calculates the size required to write the given value, with a tag,
+        /// if the value is not the default.
+        /// </summary>
+        public int CalculateSizeWithTag(T value) => IsDefault(value) ? 0 : ValueSizeCalculator(value) + tagSize;
+
+        private bool IsDefault(T value) => EqualityComparer<T>.Default.Equals(value, DefaultValue);
+    }
+}