WCF Duplex Channel: Check if callback channel is still available

Question

I have the following problem. I'm writing chat software. The client/server mechanism is based on DualHttpBinding of WCF. This means that if a user sends a message, all clients that are in the room where the message has been sent, are notified by the server.

I want to ensure, that if a client's application crashes (whyever), the client object is removed from the rooms' lists.

Is there a possibility to check the callback channel's state before calling a callback operation? The problem is, that if i call an operation on a client which is not anymore connected (because of an unexpected crash), the service will hang.

 public YagzResult SendMessage(Message message)
    {
        foreach (ChatNodeAddress chatNodeAddress in message.Destination)
        {
            ChatNode chatNode = chatProvider.FindChatNode(chatNodeAddress);
            if (chatNode != null)
            {
                User currentUser = CurrentUser;
                foreach (User user in chatNode)
                {
                    //Don't notify the current client. Deadlock!
                    if (!user.Equals(currentUser))
                    {
                        //Get the callback channel here
                        IYagzClient client = GetClientByUser(user);

                        if (client != null)
                        {
                            //--> If the client here called is not any more available,
                            //the service will hang <---
                            client.OnChatMessageReceived(message);
                        }
                    }
                }
            }
            else
            {
                return YagzResult.ChatNodeNotFound;
            }
        }
        return YagzResult.Ok;
    }

How can i check if a client is still listening? BTW, the operations called on the client are all declared OneWay and the ConcurrencyMode is set to "Multiple".

Thank you all!

Greets,

Simon

Answer 1

You can cast callback contract to ICommunicationObject and then check for the channel state.

Answer 2

There are events on a CommunicationObject (i.e. callback channel) for Closed and Faulted. You may want to add handlers for these and track which clients still have a valid channel available.

You can also take a look at the IChannelInitializer class to implement tracking of clients.