Initial commit carried over from private repo. This is V2.

code/MessengerApi.Contracts/Client/IMessengerClient.cs

@@ -0,0 +1,30 @@
+namespace MessengerApi.Contracts
+{
+    /// <summary>
+    /// Exists for mocking reason. This is implemented by <see cref="MessengerClient"/>.
+    /// </summary>
+    public interface IMessengerClient
+    {
+        /// <summary>
+        /// Receives pending messages from the messenger API.
+        /// </summary>
+        /// <param name="credentials">Credentials to the API.</param>
+        IEnumerable<InboxMessage> GetMessages();
+
+        /// <summary>
+        /// Acknowledges message reception to the server.
+        /// </summary>
+        void AckMessage(InboxMessage message);
+
+        /// <summary>
+        /// Sends a message.
+        /// </summary>
+        /// <param name="credentials">Credentials to the API.</param>
+        void SendMessage(OutboxMessage outboxMessage);
+
+        /// <summary>
+        /// Returns user ids for allowed message recipients.
+        /// </summary>
+        Contact[] GetYellowPages();
+    }
+}