Question

I’m working with a windows form application in C#. I’m using a socket client which is connecting in an asynchronous way to a server. I would like the socket to try reconnecting immediately to the server if the connection is broken for any reason. Which is the best design to approach the problem? Should I build a thread which is continuously checking if the connection is lost and tries to reconnect to the server?

Here is the code of my XcomClient class which is handling the socket communication:

        public void StartConnecting()
    {
        socketClient.BeginConnect(this.remoteEP, new AsyncCallback(ConnectCallback), this.socketClient);
    }

    private void ConnectCallback(IAsyncResult ar)
    {
        try
        {
            // Retrieve the socket from the state object.
            Socket client = (Socket)ar.AsyncState;

            // Complete the connection.
            client.EndConnect(ar);

            // Signal that the connection has been made.
            connectDone.Set();

            StartReceiving();

            NotifyClientStatusSubscribers(true);
        }
        catch(Exception e)
        {
            if (!this.socketClient.Connected)
                StartConnecting();
            else
            { 

            }
        }
    }

    public void StartReceiving()
    {
        StateObject state = new StateObject();
        state.workSocket = this.socketClient;
        socketClient.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(OnDataReceived), state);
    }

    private void OnDataReceived(IAsyncResult ar)
    {
        try
        {
            StateObject state = (StateObject)ar.AsyncState;
            Socket client = state.workSocket;

            // Read data from the remote device.
            int iReadBytes = client.EndReceive(ar);
            if (iReadBytes > 0)
            {
                byte[] bytesReceived = new byte[iReadBytes];
                Buffer.BlockCopy(state.buffer, 0, bytesReceived, 0, iReadBytes);
                this.responseList.Enqueue(bytesReceived);
                StartReceiving();
                receiveDone.Set();
            }
            else
            {
                NotifyClientStatusSubscribers(false);
            }
        }
        catch (SocketException e)
        {
            NotifyClientStatusSubscribers(false);
        }
    }

Today I try to catch a disconnection by checking the number of bytes received or catching a socket exception.

Answer 1

If your application only receives data on a socket, then in most cases, you will never detect a broken connection. If you don’t receive any data for a long time, you don’t know if it’s because the connection is broken or if the other end simply hasn’t sent any data. You will, of course, detect (as EOF on the socket) connections closed by the other end in the normal fashion despite this.

In order to detect a broken connection, you need a keepalive. You need to either:

• make the other end guarantee that it will send data on a set schedule, and you time out and close the connection if you don’t get it, or,
• send a probe to the other end once in a while. In this case the OS will take care of noticing a broken connection and you will get an error reading the socket if it’s broken, either promptly (connection reset by peer) or eventually (connection timed out).

Either way, you need a timer. Whether you implement the timer as an event in an event loop or as a thread that sleeps is up to you and the best solution probably depends on how the rest of your application is structured. If you have a main thread that runs an event loop then it’s probably best to hook in to that.

You can also enable the TCP keepalives option on the socket, but an application-layer keepalive is generally considered more robust.