diff --git a/csharp/src/Google.Protobuf/ICustomDiagnosticMessage.cs b/csharp/src/Google.Protobuf/ICustomDiagnosticMessage.cs
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+++ b/csharp/src/Google.Protobuf/ICustomDiagnosticMessage.cs
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+﻿#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2016 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
+
+namespace Google.Protobuf
+{
+    /// <summary>
+    /// A message type that has a custom string format for diagnostic purposes.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Calling <see cref="object.ToString"/> on a generated message type normally
+    /// returns the JSON representation. If a message type implements this interface,
+    /// then the <see cref="ToDiagnosticString"/> method will be called instead of the regular
+    /// JSON formatting code, but only when <c>ToString()</c> is called either on the message itself
+    /// or on another message which contains it. This does not affect the normal JSON formatting of
+    /// the message.
+    /// </para>
+    /// <para>
+    /// For example, if you create a proto message representing a GUID, the internal
+    /// representation may be a <c>bytes</c> field or four <c>fixed32</c> fields. However, when debugging
+    /// it may be more convenient to see a result in the same format as <see cref="System.Guid"/> provides.
+    /// </para>
+    /// <para>This interface extends <see cref="IMessage"/> to avoid it accidentally being implemented
+    /// on types other than messages, where it would not be used by anything in the framework.</para>
+    /// </remarks>
+    public interface ICustomDiagnosticMessage : IMessage
+    {
+        /// <summary>
+        /// Returns a string representation of this object, for diagnostic purposes.
+        /// </summary>
+        /// <remarks>
+        /// This method is called when a message is formatted as part of a <see cref="object.ToString"/>
+        /// call. It does not affect the JSON representation used by <see cref="JsonFormatter"/> other than
+        /// in calls to <see cref="JsonFormatter.ToDiagnosticString(IMessage)"/>. While it is recommended
+        /// that the result is valid JSON, this is never assumed by the Protobuf library.
+        /// </remarks>
+        /// <returns>A string representation of this object, for diagnostic purposes.</returns>
+        string ToDiagnosticString();
+    }
+}