diff --git a/csharp/src/Google.Protobuf/ByteString.cs b/csharp/src/Google.Protobuf/ByteString.cs
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+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#endregion
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.IO;
+using System.Text;
+
+namespace Google.Protobuf
+{
+    /// <summary>
+    /// Immutable array of bytes.
+    /// </summary>
+    public sealed class ByteString : IEnumerable<byte>, IEquatable<ByteString>
+    {
+        private static readonly ByteString empty = new ByteString(new byte[0]);
+
+        private readonly byte[] bytes;
+
+        /// <summary>
+        /// Unsafe operations that can cause IO Failure and/or other catestrophic side-effects.
+        /// </summary>
+        internal static class Unsafe
+        {
+            /// <summary>
+            /// Constructs a new ByteString from the given byte array. The array is
+            /// *not* copied, and must not be modified after this constructor is called.
+            /// </summary>
+            internal static ByteString FromBytes(byte[] bytes)
+            {
+                return new ByteString(bytes);
+            }
+
+            /// <summary>
+            /// Provides direct, unrestricted access to the bytes contained in this instance.
+            /// You must not modify or resize the byte array returned by this method.
+            /// </summary>
+            internal static byte[] GetBuffer(ByteString bytes)
+            {
+                return bytes.bytes;
+            }
+        }
+
+        /// <summary>
+        /// Internal use only.  Ensure that the provided array is not mutated and belongs to this instance.
+        /// </summary>
+        internal static ByteString AttachBytes(byte[] bytes)
+        {
+            return new ByteString(bytes);
+        }
+
+        /// <summary>
+        /// Constructs a new ByteString from the given byte array. The array is
+        /// *not* copied, and must not be modified after this constructor is called.
+        /// </summary>
+        private ByteString(byte[] bytes)
+        {
+            this.bytes = bytes;
+        }
+
+        /// <summary>
+        /// Returns an empty ByteString.
+        /// </summary>
+        public static ByteString Empty
+        {
+            get { return empty; }
+        }
+
+        /// <summary>
+        /// Returns the length of this ByteString in bytes.
+        /// </summary>
+        public int Length
+        {
+            get { return bytes.Length; }
+        }
+
+        /// <summary>
+        /// Returns <c>true</c> if this byte string is empty, <c>false</c> otherwise.
+        /// </summary>
+        public bool IsEmpty
+        {
+            get { return Length == 0; }
+        }
+
+        /// <summary>
+        /// Converts this <see cref="ByteString"/> into a byte array.
+        /// </summary>
+        /// <remarks>The data is copied - changes to the returned array will not be reflected in this <c>ByteString</c>.</remarks>
+        /// <returns>A byte array with the same data as this <c>ByteString</c>.</returns>
+        public byte[] ToByteArray()
+        {
+            return (byte[]) bytes.Clone();
+        }
+
+        /// <summary>
+        /// Converts this <see cref="ByteString"/> into a standard base64 representation.
+        /// </summary>
+        /// <returns>A base64 representation of this <c>ByteString</c>.</returns>
+        public string ToBase64()
+        {
+            return Convert.ToBase64String(bytes);
+        }
+
+        /// <summary>
+        /// Constructs a <see cref="ByteString" /> from the Base64 Encoded String.
+        /// </summary>
+        public static ByteString FromBase64(string bytes)
+        {
+            // By handling the empty string explicitly, we not only optimize but we fix a
+            // problem on CF 2.0. See issue 61 for details.
+            return bytes == "" ? Empty : new ByteString(Convert.FromBase64String(bytes));
+        }
+
+        /// <summary>
+        /// Constructs a <see cref="ByteString" /> from the given array. The contents
+        /// are copied, so further modifications to the array will not
+        /// be reflected in the returned ByteString.
+        /// This method can also be invoked in <c>ByteString.CopyFrom(0xaa, 0xbb, ...)</c> form
+        /// which is primarily useful for testing.
+        /// </summary>
+        public static ByteString CopyFrom(params byte[] bytes)
+        {
+            return new ByteString((byte[]) bytes.Clone());
+        }
+
+        /// <summary>
+        /// Constructs a <see cref="ByteString" /> from a portion of a byte array.
+        /// </summary>
+        public static ByteString CopyFrom(byte[] bytes, int offset, int count)
+        {
+            byte[] portion = new byte[count];
+            ByteArray.Copy(bytes, offset, portion, 0, count);
+            return new ByteString(portion);
+        }
+
+        /// <summary>
+        /// Creates a new <see cref="ByteString" /> by encoding the specified text with
+        /// the given encoding.
+        /// </summary>
+        public static ByteString CopyFrom(string text, Encoding encoding)
+        {
+            return new ByteString(encoding.GetBytes(text));
+        }
+
+        /// <summary>
+        /// Creates a new <see cref="ByteString" /> by encoding the specified text in UTF-8.
+        /// </summary>
+        public static ByteString CopyFromUtf8(string text)
+        {
+            return CopyFrom(text, Encoding.UTF8);
+        }
+
+        /// <summary>
+        /// Retuns the byte at the given index.
+        /// </summary>
+        public byte this[int index]
+        {
+            get { return bytes[index]; }
+        }
+
+        /// <summary>
+        /// Converts this <see cref="ByteString"/> into a string by applying the given encoding.
+        /// </summary>
+        /// <remarks>
+        /// This method should only be used to convert binary data which was the result of encoding
+        /// text with the given encoding.
+        /// </remarks>
+        /// <param name="encoding">The encoding to use to decode the binary data into text.</param>
+        /// <returns>The result of decoding the binary data with the given decoding.</returns>
+        public string ToString(Encoding encoding)
+        {
+            return encoding.GetString(bytes, 0, bytes.Length);
+        }
+
+        /// <summary>
+        /// Converts this <see cref="ByteString"/> into a string by applying the UTF-8 encoding.
+        /// </summary>
+        /// <remarks>
+        /// This method should only be used to convert binary data which was the result of encoding
+        /// text with UTF-8.
+        /// </remarks>
+        /// <returns>The result of decoding the binary data with the given decoding.</returns>
+        public string ToStringUtf8()
+        {
+            return ToString(Encoding.UTF8);
+        }
+
+        /// <summary>
+        /// Returns an iterator over the bytes in this <see cref="ByteString"/>.
+        /// </summary>
+        /// <returns>An iterator over the bytes in this object.</returns>
+        public IEnumerator<byte> GetEnumerator()
+        {
+            return ((IEnumerable<byte>) bytes).GetEnumerator();
+        }
+
+        /// <summary>
+        /// Returns an iterator over the bytes in this <see cref="ByteString"/>.
+        /// </summary>
+        /// <returns>An iterator over the bytes in this object.</returns>
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return GetEnumerator();
+        }
+
+        /// <summary>
+        /// Creates a CodedInputStream from this ByteString's data.
+        /// </summary>
+        public CodedInputStream CreateCodedInput()
+        {
+            // We trust CodedInputStream not to reveal the provided byte array or modify it
+            return new CodedInputStream(bytes);
+        }
+
+        /// <summary>
+        /// Compares two byte strings for equality.
+        /// </summary>
+        /// <param name="lhs">The first byte string to compare.</param>
+        /// <param name="rhs">The second byte string to compare.</param>
+        /// <returns><c>true</c> if the byte strings are equal; false otherwise.</returns>
+        public static bool operator ==(ByteString lhs, ByteString rhs)
+        {
+            if (ReferenceEquals(lhs, rhs))
+            {
+                return true;
+            }
+            if (ReferenceEquals(lhs, null) || ReferenceEquals(rhs, null))
+            {
+                return false;
+            }
+            if (lhs.bytes.Length != rhs.bytes.Length)
+            {
+                return false;
+            }
+            for (int i = 0; i < lhs.Length; i++)
+            {
+                if (rhs.bytes[i] != lhs.bytes[i])
+                {
+                    return false;
+                }
+            }
+            return true;
+        }
+
+        /// <summary>
+        /// Compares two byte strings for inequality.
+        /// </summary>
+        /// <param name="lhs">The first byte string to compare.</param>
+        /// <param name="rhs">The second byte string to compare.</param>
+        /// <returns><c>false</c> if the byte strings are equal; true otherwise.</returns>
+        public static bool operator !=(ByteString lhs, ByteString rhs)
+        {
+            return !(lhs == rhs);
+        }
+
+        /// <summary>
+        /// Compares this byte string with another object.
+        /// </summary>
+        /// <param name="obj">The object to compare this with.</param>
+        /// <returns><c>true</c> if <paramref name="obj"/> refers to an equal <see cref="ByteString"/>; <c>false</c> otherwise.</returns>
+        public override bool Equals(object obj)
+        {
+            return this == (obj as ByteString);
+        }
+
+        /// <summary>
+        /// Returns a hash code for this object. Two equal byte strings
+        /// will return the same hash code.
+        /// </summary>
+        /// <returns>A hash code for this object.</returns>
+        public override int GetHashCode()
+        {
+            int ret = 23;
+            foreach (byte b in bytes)
+            {
+                ret = (ret << 8) | b;
+            }
+            return ret;
+        }
+
+        /// <summary>
+        /// Compares this byte string with another.
+        /// </summary>
+        /// <param name="other">The <see cref="ByteString"/> to compare this with.</param>
+        /// <returns><c>true</c> if <paramref name="other"/> refers to an equal byte string; <c>false</c> otherwise.</returns>
+        public bool Equals(ByteString other)
+        {
+            return this == other;
+        }
+
+        /// <summary>
+        /// Used internally by CodedOutputStream to avoid creating a copy for the write
+        /// </summary>
+        internal void WriteRawBytesTo(CodedOutputStream outputStream)
+        {
+            outputStream.WriteRawBytes(bytes, 0, bytes.Length);
+        }
+
+        /// <summary>
+        /// Copies the entire byte array to the destination array provided at the offset specified.
+        /// </summary>
+        public void CopyTo(byte[] array, int position)
+        {
+            ByteArray.Copy(bytes, 0, array, position, bytes.Length);
+        }
+
+        /// <summary>
+        /// Writes the entire byte array to the provided stream
+        /// </summary>
+        public void WriteTo(Stream outputStream)
+        {
+            outputStream.Write(bytes, 0, bytes.Length);
+        }
+    }
+}
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