Fixed CSHARP-225. ReplicaSetConnector.Connect now makes sure that the serverInstance it identifies as primary was connected to using the official replica set configured host name for that server. Also fixed a NullReferenceException in MongoServerInstance.Disconnet.

Driver/Core/MongoServerInstance.cs

@@ -239,8 +239,11 @@ namespace MongoDB.Driver {
         internal void Disconnect() {
             if (state != MongoServerState.Disconnected) {
                 try {
-                    connectionPool.Close();
-                    connectionPool = null;
+                    // if we fail during Connect the connectionPool field will still be null
+                    if (connectionPool != null) {
+                        connectionPool.Close();
+                        connectionPool = null;
+                    }
                 } finally {
                     State = MongoServerState.Disconnected;
                 }

Driver/Internal/ReplicaSetConnector.cs

@@ -68,9 +68,14 @@ namespace MongoDB.Driver.Internal {
                 // return as soon as we've connected to the primary
                 var serverInstance = response.ServerInstance;
                 if (serverInstance.State == MongoServerState.Connected && serverInstance.IsPrimary) {
-                    // process any additional responses in the background
-                    ThreadPool.QueueUserWorkItem(ProcessAdditionalResponsesWorkItem);
-                    return;
+                    // make sure this serverInstance is still an instance of this server
+                    // it might have been removed if the connection string used a DNS alias for the host
+                    // (in which case we have already queued a connect using the official host name and should see that response shortly)
+                    if (server.Instances.Contains(serverInstance)) {
+                        // process any additional responses in the background
+                        ThreadPool.QueueUserWorkItem(ProcessAdditionalResponsesWorkItem);
+                        return;
+                    }
                 }
             }