Handling Disconnects with WCF

I've been working on an application which relies heavily on WCF to communicate. I have a very simple interface (contract) that the server supports to connect and disconnect: it exposes the methods Register() and Unregister(). When the client starts up and connects, it calls the Register() method, passing in some identification info. I save this information in a List<T> so that it can be used by the server to provide callbacks. When the client disconnects, it calls Unregister() which then removes the client from the collection.

That works well enough, right up until a client abruptly disconnects (ex. a connection error, network line goes down, etc.) Then suddenly I end up with a reference to a disconnected client on the server. If I attempt to use this connection to send a message back to the client an exception is thrown (which still works OK). It would be nice to know immediately as soon as a client disconnects.

If you are using TCP (full duplex) as the communication channel, you can do this pretty easily - note: this doesn't work for some of the other communication modes, like HTTP duplex. For those, you might have to rely on the various timeout settings in the .config file.

Here's what that ends up looking like (I happen to be taking advantage of LINQ here, but you can easily adjust that code if you're using an older vesion of the framework):

  203 private void IServer.Register(string systemName)

  204 {

  205     IClientCallback remoteMachine = OperationContext.Current.GetCallbackChannel<IClientCallback>();

  206     // Hook up events to let us know if the client disconnects w/o telling us.

  207     OperationContext.Current.Channel.Faulted += new EventHandler(ClientFaulted);

  208     OperationContext.Current.Channel.Closed += new EventHandler(ClientClosed);

  209 

  210     // Get object reference to figure out the IP address of the connected client

  211     MessageProperties prop = OperationContext.Current.IncomingMessageProperties;

  212     RemoteEndpointMessageProperty endpoint = prop[RemoteEndpointMessageProperty.Name] as RemoteEndpointMessageProperty;

  213 

  214     // (threading/locking code removed)

  215 

  216     client = m_callbackList.Find(c => c.SystemName == systemName);

  217     if (client == null)

  218     {

  219         // It's not already on our list, add it

  220         m_callbackList.Add(new RegisteredClient(systemName, endpoint.Address, remoteMachine));

  221     }

  222 }

 

When the client disconnects, the ClientFaulted/ClientClosed events will be fired. Honestly, at this point, the Unregister method isn't really needed, assuming you follow the same disconnection process in both cases.