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diff --git a/src/DotNetOpenAuth.Core/Messaging/IProtocolMessageWithExtensions.cs b/src/DotNetOpenAuth.Core/Messaging/IProtocolMessageWithExtensions.cs
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+++ b/src/DotNetOpenAuth.Core/Messaging/IProtocolMessageWithExtensions.cs
@@ -0,0 +1,116 @@
+//-----------------------------------------------------------------------
+// <copyright file="IProtocolMessageWithExtensions.cs" company="Andrew Arnott">
+//     Copyright (c) Andrew Arnott. All rights reserved.
+// </copyright>
+//-----------------------------------------------------------------------
+
+namespace DotNetOpenAuth.Messaging {
+	using System;
+	using System.Collections.Generic;
+	using System.Diagnostics.Contracts;
+
+	/// <summary>
+	/// A protocol message that supports adding extensions to the payload for transmission.
+	/// </summary>
+	[ContractClass(typeof(IProtocolMessageWithExtensionsContract))]
+	public interface IProtocolMessageWithExtensions : IProtocolMessage {
+		/// <summary>
+		/// Gets the list of extensions that are included with this message.
+		/// </summary>
+		/// <remarks>
+		/// Implementations of this interface should ensure that this property never returns null.
+		/// </remarks>
+		IList<IExtensionMessage> Extensions { get; }
+	}
+
+	/// <summary>
+	/// Code contract for the <see cref="IProtocolMessageWithExtensions"/> interface.
+	/// </summary>
+	[ContractClassFor(typeof(IProtocolMessageWithExtensions))]
+	internal abstract class IProtocolMessageWithExtensionsContract : IProtocolMessageWithExtensions {
+		/// <summary>
+		/// Prevents a default instance of the <see cref="IProtocolMessageWithExtensionsContract"/> class from being created.
+		/// </summary>
+		private IProtocolMessageWithExtensionsContract() {
+		}
+
+		#region IProtocolMessageWithExtensions Members
+
+		/// <summary>
+		/// Gets the list of extensions that are included with this message.
+		/// </summary>
+		/// <remarks>
+		/// Implementations of this interface should ensure that this property never returns null.
+		/// </remarks>
+		IList<IExtensionMessage> IProtocolMessageWithExtensions.Extensions {
+			get {
+				Contract.Ensures(Contract.Result<IList<IExtensionMessage>>() != null);
+				throw new NotImplementedException();
+			}
+		}
+
+		#endregion
+
+		#region IProtocolMessage Members
+
+		/// <summary>
+		/// Gets the level of protection this message requires.
+		/// </summary>
+		MessageProtections IProtocolMessage.RequiredProtection {
+			get { throw new NotImplementedException(); }
+		}
+
+		/// <summary>
+		/// Gets a value indicating whether this is a direct or indirect message.
+		/// </summary>
+		MessageTransport IProtocolMessage.Transport {
+			get { throw new NotImplementedException(); }
+		}
+
+		#endregion
+
+		#region IMessage Members
+
+		/// <summary>
+		/// Gets the version of the protocol or extension this message is prepared to implement.
+		/// </summary>
+		/// <remarks>
+		/// Implementations of this interface should ensure that this property never returns null.
+		/// </remarks>
+		Version IMessage.Version {
+			get {
+				throw new NotImplementedException();
+			}
+		}
+
+		/// <summary>
+		/// Gets the extra, non-standard Protocol parameters included in the message.
+		/// </summary>
+		/// <remarks>
+		/// Implementations of this interface should ensure that this property never returns null.
+		/// </remarks>
+		IDictionary<string, string> IMessage.ExtraData {
+			get {
+				throw new NotImplementedException();
+			}
+		}
+
+		/// <summary>
+		/// Checks the message state for conformity to the protocol specification
+		/// and throws an exception if the message is invalid.
+		/// </summary>
+		/// <remarks>
+		/// 	<para>Some messages have required fields, or combinations of fields that must relate to each other
+		/// in specialized ways.  After deserializing a message, this method checks the state of the
+		/// message to see if it conforms to the protocol.</para>
+		/// 	<para>Note that this property should <i>not</i> check signatures or perform any state checks
+		/// outside this scope of this particular message.</para>
+		/// </remarks>
+		/// <exception cref="ProtocolException">Thrown if the message is invalid.</exception>
+		void IMessage.EnsureValidMessage() {
+			throw new NotImplementedException();
+		}
+
+		#endregion
+	}
+}