diff --git a/csharp/src/Google.Protobuf/CodedOutputStream.cs b/csharp/src/Google.Protobuf/CodedOutputStream.cs
new file mode 100644
index 0000000..d6355f0
--- /dev/null
+++ b/csharp/src/Google.Protobuf/CodedOutputStream.cs
@@ -0,0 +1,708 @@
+#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 Google.Protobuf.Collections;
+using System;
+using System.IO;
+using System.Text;
+
+namespace Google.Protobuf
+{
+    /// <summary>
+    /// Encodes and writes protocol message fields.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This class is generally used by generated code to write appropriate
+    /// primitives to the stream. It effectively encapsulates the lowest
+    /// levels of protocol buffer format. Unlike some other implementations,
+    /// this does not include combined "write tag and value" methods. Generated
+    /// code knows the exact byte representations of the tags they're going to write,
+    /// so there's no need to re-encode them each time. Manually-written code calling
+    /// this class should just call one of the <c>WriteTag</c> overloads before each value.
+    /// </para>
+    /// <para>
+    /// Repeated fields and map fields are not handled by this class; use <c>RepeatedField&lt;T&gt;</c>
+    /// and <c>MapField&lt;TKey, TValue&gt;</c> to serialize such fields.
+    /// </para>
+    /// </remarks>
+    public sealed partial class CodedOutputStream
+    {
+        // "Local" copy of Encoding.UTF8, for efficiency. (Yes, it makes a difference.)
+        internal static readonly Encoding Utf8Encoding = Encoding.UTF8;
+
+        /// <summary>
+        /// The buffer size used by CreateInstance(Stream).
+        /// </summary>
+        public static readonly int DefaultBufferSize = 4096;
+
+        private readonly byte[] buffer;
+        private readonly int limit;
+        private int position;
+        private readonly Stream output;
+
+        #region Construction
+        /// <summary>
+        /// Creates a new CodedOutputStream that writes directly to the given
+        /// byte array. If more bytes are written than fit in the array,
+        /// OutOfSpaceException will be thrown.
+        /// </summary>
+        public CodedOutputStream(byte[] flatArray) : this(flatArray, 0, flatArray.Length)
+        {
+        }
+
+        /// <summary>
+        /// Creates a new CodedOutputStream that writes directly to the given
+        /// byte array slice. If more bytes are written than fit in the array,
+        /// OutOfSpaceException will be thrown.
+        /// </summary>
+        private CodedOutputStream(byte[] buffer, int offset, int length)
+        {
+            this.output = null;
+            this.buffer = buffer;
+            this.position = offset;
+            this.limit = offset + length;
+        }
+
+        private CodedOutputStream(Stream output, byte[] buffer)
+        {
+            this.output = output;
+            this.buffer = buffer;
+            this.position = 0;
+            this.limit = buffer.Length;
+        }
+
+        /// <summary>
+        /// Creates a new CodedOutputStream which write to the given stream.
+        /// </summary>
+        public CodedOutputStream(Stream output) : this(output, DefaultBufferSize)
+        {
+        }
+
+        /// <summary>
+        /// Creates a new CodedOutputStream which write to the given stream and uses
+        /// the specified buffer size.
+        /// </summary>
+        public CodedOutputStream(Stream output, int bufferSize) : this(output, new byte[bufferSize])
+        {
+        }    
+        #endregion
+
+        /// <summary>
+        /// Returns the current position in the stream, or the position in the output buffer
+        /// </summary>
+        public long Position
+        {
+            get
+            {
+                if (output != null)
+                {
+                    return output.Position + position;
+                }
+                return position;
+            }
+        }
+
+        #region Writing of values (not including tags)
+
+        /// <summary>
+        /// Writes a double field value, without a tag, to the stream.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteDouble(double value)
+        {
+            WriteRawLittleEndian64((ulong)BitConverter.DoubleToInt64Bits(value));
+        }
+
+        /// <summary>
+        /// Writes a float field value, without a tag, to the stream.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteFloat(float value)
+        {
+            byte[] rawBytes = BitConverter.GetBytes(value);
+            if (!BitConverter.IsLittleEndian)
+            {
+                ByteArray.Reverse(rawBytes);
+            }
+
+            if (limit - position >= 4)
+            {
+                buffer[position++] = rawBytes[0];
+                buffer[position++] = rawBytes[1];
+                buffer[position++] = rawBytes[2];
+                buffer[position++] = rawBytes[3];
+            }
+            else
+            {
+                WriteRawBytes(rawBytes, 0, 4);
+            }
+        }
+
+        /// <summary>
+        /// Writes a uint64 field value, without a tag, to the stream.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteUInt64(ulong value)
+        {
+            WriteRawVarint64(value);
+        }
+
+        /// <summary>
+        /// Writes an int64 field value, without a tag, to the stream.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteInt64(long value)
+        {
+            WriteRawVarint64((ulong) value);
+        }
+
+        /// <summary>
+        /// Writes an int32 field value, without a tag, to the stream.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteInt32(int value)
+        {
+            if (value >= 0)
+            {
+                WriteRawVarint32((uint) value);
+            }
+            else
+            {
+                // Must sign-extend.
+                WriteRawVarint64((ulong) value);
+            }
+        }
+
+        /// <summary>
+        /// Writes a fixed64 field value, without a tag, to the stream.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteFixed64(ulong value)
+        {
+            WriteRawLittleEndian64(value);
+        }
+
+        /// <summary>
+        /// Writes a fixed32 field value, without a tag, to the stream.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteFixed32(uint value)
+        {
+            WriteRawLittleEndian32(value);
+        }
+
+        /// <summary>
+        /// Writes a bool field value, without a tag, to the stream.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteBool(bool value)
+        {
+            WriteRawByte(value ? (byte) 1 : (byte) 0);
+        }
+
+        /// <summary>
+        /// Writes a string field value, without a tag, to the stream.
+        /// The data is length-prefixed.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteString(string value)
+        {
+            // Optimise the case where we have enough space to write
+            // the string directly to the buffer, which should be common.
+            int length = Utf8Encoding.GetByteCount(value);
+            WriteLength(length);
+            if (limit - position >= length)
+            {
+                if (length == value.Length) // Must be all ASCII...
+                {
+                    for (int i = 0; i < length; i++)
+                    {
+                        buffer[position + i] = (byte)value[i];
+                    }
+                }
+                else
+                {
+                    Utf8Encoding.GetBytes(value, 0, value.Length, buffer, position);
+                }
+                position += length;
+            }
+            else
+            {
+                byte[] bytes = Utf8Encoding.GetBytes(value);
+                WriteRawBytes(bytes);
+            }
+        }
+
+        /// <summary>
+        /// Writes a message, without a tag, to the stream.
+        /// The data is length-prefixed.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteMessage(IMessage value)
+        {
+            WriteLength(value.CalculateSize());
+            value.WriteTo(this);
+        }
+
+        /// <summary>
+        /// Write a byte string, without a tag, to the stream.
+        /// The data is length-prefixed.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteBytes(ByteString value)
+        {
+            WriteLength(value.Length);
+            value.WriteRawBytesTo(this);
+        }
+
+        /// <summary>
+        /// Writes a uint32 value, without a tag, to the stream.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteUInt32(uint value)
+        {
+            WriteRawVarint32(value);
+        }
+
+        /// <summary>
+        /// Writes an enum value, without a tag, to the stream.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteEnum(int value)
+        {
+            WriteInt32(value);
+        }
+
+        /// <summary>
+        /// Writes an sfixed32 value, without a tag, to the stream.
+        /// </summary>
+        /// <param name="value">The value to write.</param>
+        public void WriteSFixed32(int value)
+        {
+            WriteRawLittleEndian32((uint) value);
+        }
+
+        /// <summary>
+        /// Writes an sfixed64 value, without a tag, to the stream.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteSFixed64(long value)
+        {
+            WriteRawLittleEndian64((ulong) value);
+        }
+
+        /// <summary>
+        /// Writes an sint32 value, without a tag, to the stream.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteSInt32(int value)
+        {
+            WriteRawVarint32(EncodeZigZag32(value));
+        }
+
+        /// <summary>
+        /// Writes an sint64 value, without a tag, to the stream.
+        /// </summary>
+        /// <param name="value">The value to write</param>
+        public void WriteSInt64(long value)
+        {
+            WriteRawVarint64(EncodeZigZag64(value));
+        }
+
+        /// <summary>
+        /// Writes a length (in bytes) for length-delimited data.
+        /// </summary>
+        /// <remarks>
+        /// This method simply writes a rawint, but exists for clarity in calling code.
+        /// </remarks>
+        /// <param name="length">Length value, in bytes.</param>
+        public void WriteLength(int length)
+        {
+            WriteRawVarint32((uint) length);
+        }
+
+        #endregion
+
+        #region Raw tag writing
+        /// <summary>
+        /// Encodes and writes a tag.
+        /// </summary>
+        /// <param name="fieldNumber">The number of the field to write the tag for</param>
+        /// <param name="type">The wire format type of the tag to write</param>
+        public void WriteTag(int fieldNumber, WireFormat.WireType type)
+        {
+            WriteRawVarint32(WireFormat.MakeTag(fieldNumber, type));
+        }
+
+        /// <summary>
+        /// Writes an already-encoded tag.
+        /// </summary>
+        /// <param name="tag">The encoded tag</param>
+        public void WriteTag(uint tag)
+        {
+            WriteRawVarint32(tag);
+        }
+
+        /// <summary>
+        /// Writes the given single-byte tag directly to the stream.
+        /// </summary>
+        /// <param name="b1">The encoded tag</param>
+        public void WriteRawTag(byte b1)
+        {
+            WriteRawByte(b1);
+        }
+
+        /// <summary>
+        /// Writes the given two-byte tag directly to the stream.
+        /// </summary>
+        /// <param name="b1">The first byte of the encoded tag</param>
+        /// <param name="b2">The second byte of the encoded tag</param>
+        public void WriteRawTag(byte b1, byte b2)
+        {
+            WriteRawByte(b1);
+            WriteRawByte(b2);
+        }
+
+        /// <summary>
+        /// Writes the given three-byte tag directly to the stream.
+        /// </summary>
+        /// <param name="b1">The first byte of the encoded tag</param>
+        /// <param name="b2">The second byte of the encoded tag</param>
+        /// <param name="b3">The third byte of the encoded tag</param>
+        public void WriteRawTag(byte b1, byte b2, byte b3)
+        {
+            WriteRawByte(b1);
+            WriteRawByte(b2);
+            WriteRawByte(b3);
+        }
+
+        /// <summary>
+        /// Writes the given four-byte tag directly to the stream.
+        /// </summary>
+        /// <param name="b1">The first byte of the encoded tag</param>
+        /// <param name="b2">The second byte of the encoded tag</param>
+        /// <param name="b3">The third byte of the encoded tag</param>
+        /// <param name="b4">The fourth byte of the encoded tag</param>
+        public void WriteRawTag(byte b1, byte b2, byte b3, byte b4)
+        {
+            WriteRawByte(b1);
+            WriteRawByte(b2);
+            WriteRawByte(b3);
+            WriteRawByte(b4);
+        }
+
+        /// <summary>
+        /// Writes the given five-byte tag directly to the stream.
+        /// </summary>
+        /// <param name="b1">The first byte of the encoded tag</param>
+        /// <param name="b2">The second byte of the encoded tag</param>
+        /// <param name="b3">The third byte of the encoded tag</param>
+        /// <param name="b4">The fourth byte of the encoded tag</param>
+        /// <param name="b5">The fifth byte of the encoded tag</param>
+        public void WriteRawTag(byte b1, byte b2, byte b3, byte b4, byte b5)
+        {
+            WriteRawByte(b1);
+            WriteRawByte(b2);
+            WriteRawByte(b3);
+            WriteRawByte(b4);
+            WriteRawByte(b5);
+        }
+        #endregion
+
+        #region Underlying writing primitives
+        /// <summary>
+        /// Writes a 32 bit value as a varint. The fast route is taken when
+        /// there's enough buffer space left to whizz through without checking
+        /// for each byte; otherwise, we resort to calling WriteRawByte each time.
+        /// </summary>
+        internal void WriteRawVarint32(uint value)
+        {
+            // Optimize for the common case of a single byte value
+            if (value < 128 && position < limit)
+            {
+                buffer[position++] = (byte)value;
+                return;
+            }
+
+            while (value > 127 && position < limit)
+            {
+                buffer[position++] = (byte) ((value & 0x7F) | 0x80);
+                value >>= 7;
+            }
+            while (value > 127)
+            {
+                WriteRawByte((byte) ((value & 0x7F) | 0x80));
+                value >>= 7;
+            }
+            if (position < limit)
+            {
+                buffer[position++] = (byte) value;
+            }
+            else
+            {
+                WriteRawByte((byte) value);
+            }
+        }
+
+        internal void WriteRawVarint64(ulong value)
+        {
+            while (value > 127 && position < limit)
+            {
+                buffer[position++] = (byte) ((value & 0x7F) | 0x80);
+                value >>= 7;
+            }
+            while (value > 127)
+            {
+                WriteRawByte((byte) ((value & 0x7F) | 0x80));
+                value >>= 7;
+            }
+            if (position < limit)
+            {
+                buffer[position++] = (byte) value;
+            }
+            else
+            {
+                WriteRawByte((byte) value);
+            }
+        }
+
+        internal void WriteRawLittleEndian32(uint value)
+        {
+            if (position + 4 > limit)
+            {
+                WriteRawByte((byte) value);
+                WriteRawByte((byte) (value >> 8));
+                WriteRawByte((byte) (value >> 16));
+                WriteRawByte((byte) (value >> 24));
+            }
+            else
+            {
+                buffer[position++] = ((byte) value);
+                buffer[position++] = ((byte) (value >> 8));
+                buffer[position++] = ((byte) (value >> 16));
+                buffer[position++] = ((byte) (value >> 24));
+            }
+        }
+
+        internal void WriteRawLittleEndian64(ulong value)
+        {
+            if (position + 8 > limit)
+            {
+                WriteRawByte((byte) value);
+                WriteRawByte((byte) (value >> 8));
+                WriteRawByte((byte) (value >> 16));
+                WriteRawByte((byte) (value >> 24));
+                WriteRawByte((byte) (value >> 32));
+                WriteRawByte((byte) (value >> 40));
+                WriteRawByte((byte) (value >> 48));
+                WriteRawByte((byte) (value >> 56));
+            }
+            else
+            {
+                buffer[position++] = ((byte) value);
+                buffer[position++] = ((byte) (value >> 8));
+                buffer[position++] = ((byte) (value >> 16));
+                buffer[position++] = ((byte) (value >> 24));
+                buffer[position++] = ((byte) (value >> 32));
+                buffer[position++] = ((byte) (value >> 40));
+                buffer[position++] = ((byte) (value >> 48));
+                buffer[position++] = ((byte) (value >> 56));
+            }
+        }
+
+        internal void WriteRawByte(byte value)
+        {
+            if (position == limit)
+            {
+                RefreshBuffer();
+            }
+
+            buffer[position++] = value;
+        }
+
+        internal void WriteRawByte(uint value)
+        {
+            WriteRawByte((byte) value);
+        }
+
+        /// <summary>
+        /// Writes out an array of bytes.
+        /// </summary>
+        internal void WriteRawBytes(byte[] value)
+        {
+            WriteRawBytes(value, 0, value.Length);
+        }
+
+        /// <summary>
+        /// Writes out part of an array of bytes.
+        /// </summary>
+        internal void WriteRawBytes(byte[] value, int offset, int length)
+        {
+            if (limit - position >= length)
+            {
+                ByteArray.Copy(value, offset, buffer, position, length);
+                // We have room in the current buffer.
+                position += length;
+            }
+            else
+            {
+                // Write extends past current buffer.  Fill the rest of this buffer and
+                // flush.
+                int bytesWritten = limit - position;
+                ByteArray.Copy(value, offset, buffer, position, bytesWritten);
+                offset += bytesWritten;
+                length -= bytesWritten;
+                position = limit;
+                RefreshBuffer();
+
+                // Now deal with the rest.
+                // Since we have an output stream, this is our buffer
+                // and buffer offset == 0
+                if (length <= limit)
+                {
+                    // Fits in new buffer.
+                    ByteArray.Copy(value, offset, buffer, 0, length);
+                    position = length;
+                }
+                else
+                {
+                    // Write is very big.  Let's do it all at once.
+                    output.Write(value, offset, length);
+                }
+            }
+        }
+
+        #endregion
+
+        /// <summary>
+        /// Encode a 32-bit value with ZigZag encoding.
+        /// </summary>
+        /// <remarks>
+        /// ZigZag encodes signed integers into values that can be efficiently
+        /// encoded with varint.  (Otherwise, negative values must be 
+        /// sign-extended to 64 bits to be varint encoded, thus always taking
+        /// 10 bytes on the wire.)
+        /// </remarks>
+        internal static uint EncodeZigZag32(int n)
+        {
+            // Note:  the right-shift must be arithmetic
+            return (uint) ((n << 1) ^ (n >> 31));
+        }
+
+        /// <summary>
+        /// Encode a 64-bit value with ZigZag encoding.
+        /// </summary>
+        /// <remarks>
+        /// ZigZag encodes signed integers into values that can be efficiently
+        /// encoded with varint.  (Otherwise, negative values must be 
+        /// sign-extended to 64 bits to be varint encoded, thus always taking
+        /// 10 bytes on the wire.)
+        /// </remarks>
+        internal static ulong EncodeZigZag64(long n)
+        {
+            return (ulong) ((n << 1) ^ (n >> 63));
+        }
+
+        private void RefreshBuffer()
+        {
+            if (output == null)
+            {
+                // We're writing to a single buffer.
+                throw new OutOfSpaceException();
+            }
+
+            // Since we have an output stream, this is our buffer
+            // and buffer offset == 0
+            output.Write(buffer, 0, position);
+            position = 0;
+        }
+
+        /// <summary>
+        /// Indicates that a CodedOutputStream wrapping a flat byte array
+        /// ran out of space.
+        /// </summary>
+        public sealed class OutOfSpaceException : IOException
+        {
+            internal OutOfSpaceException()
+                : base("CodedOutputStream was writing to a flat byte array and ran out of space.")
+            {
+            }
+        }
+
+        /// <summary>
+        /// Flushes any buffered data to the underlying stream (if there is one).
+        /// </summary>
+        public void Flush()
+        {
+            if (output != null)
+            {
+                RefreshBuffer();
+            }
+        }
+
+        /// <summary>
+        /// Verifies that SpaceLeft returns zero. It's common to create a byte array
+        /// that is exactly big enough to hold a message, then write to it with
+        /// a CodedOutputStream. Calling CheckNoSpaceLeft after writing verifies that
+        /// the message was actually as big as expected, which can help bugs.
+        /// </summary>
+        public void CheckNoSpaceLeft()
+        {
+            if (SpaceLeft != 0)
+            {
+                throw new InvalidOperationException("Did not write as much data as expected.");
+            }
+        }
+
+        /// <summary>
+        /// If writing to a flat array, returns the space left in the array. Otherwise,
+        /// throws an InvalidOperationException.
+        /// </summary>
+        public int SpaceLeft
+        {
+            get
+            {
+                if (output == null)
+                {
+                    return limit - position;
+                }
+                else
+                {
+                    throw new InvalidOperationException(
+                        "SpaceLeft can only be called on CodedOutputStreams that are " +
+                        "writing to a flat array.");
+                }
+            }
+        }
+    }
+}
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