examples of zeromq pub/sub with C# winform

Question

I'm trying to create a C# Winform application that uses ZeroMQ (clrzmq .net bindings (x86) via nuget) in a pub/sub model.

After much searching, I can only find standalone C# examples where the code uses a while statement to process new messages indefinitely. When I try to use these examples, I don't know where to put the code, and it just blocks the gui and everything else.

I don't know if it's impossible to do without using another thread, but I was under the impression that ZeroMQ's asynchronous behaviors could work without coding extra threads. Perhaps I just don't know where to put the zeromq code, or perhaps I really do need another thread.

If someone could provide a simple pub/sub example with directions of where to actually insert the code into a default C# winform application it would be very appreciated.

Answer 1

I am assuming you are using the clrzmq ZeroMq wrapper in your project. As far as I know it is not possible to receive message non-blocking in a simple loop using clrzmq, it will block either indefinitely, for a specific amount of time (by supplying a timeout to the receive method) or until you receive a message.

However, it is fairly trivial to set up a thread to poll the socket periodically and push incoming messages to onto a Queue. You may then use for example a simple WinForms Timer to periodically dequeue any pending messages from that (shared) Queue. Here is a working example of a threaded subscriber:

public class ZeroMqSubscriber
{
    private readonly ZmqContext _zmqContext;
    private readonly ZmqSocket _zmqSocket;
    private readonly Thread _workerThread;
    private readonly ManualResetEvent _stopEvent = new ManualResetEvent(false);
    private readonly object _locker = new object();
    private readonly Queue<string> _queue = new Queue<string>();

    public ZeroMqSubscriber(string endPoint)
    {
        _zmqContext = ZmqContext.Create();
        _zmqSocket = _zmqContext.CreateSocket(SocketType.SUB);
        _zmqSocket.Connect(endPoint);
        _zmqSocket.SubscribeAll();

        _workerThread = new Thread(ReceiveData);
        _workerThread.Start();
    }

    public string[] GetMessages()
    {
        lock (_locker)
        {
            var messages = _queue.ToArray();
            _queue.Clear();
            return messages;
        }
    }

    public void Stop()
    {
        _stopEvent.Set();
        _workerThread.Join();
    }

    private void ReceiveData()
    {
         try
         {
             while (!_stopEvent.WaitOne(0))
             {
                 var message = _zmqSocket.Receive(Encoding.UTF8, 
                               new TimeSpan(0, 0, 0, 1));
                 if (string.IsNullOrEmpty(message))
                     continue;

                 lock (_locker)
                     _queue.Enqueue(message);
             }
         }
         finally
         {
             _zmqSocket.Dispose();
             _zmqContext.Dispose();
         }
    }
}

From the Form you simply poll the Queue periodically (this example uses a Forms Timer and simply appends the message data to a Textbox):

private readonly ZeroMqSubscriber _zeroMqSubscriber = 
        new ZeroMqSubscriber("tcp://127.0.0.1:5000");

void ReceiveTimerTick(object sender, EventArgs e)
{
    var messages = _zeroMqSubscriber.GetMessages();
    foreach (var message in messages)
        _textbox.AppendText(message + Environment.NewLine);
}