Stopping a TcpListener after calling BeginAcceptTcpClient

Question

I have this code...

internal static void Start()
{
    TcpListener listenerSocket = new TcpListener(IPAddress.Any, 32599);
    listenerSocket.Start();
    listenerSocket.BeginAcceptTcpClient(new AsyncCallback(AcceptClient), null);
}

Then my call back function looks like this...

private static void AcceptClient(IAsyncResult asyncResult)
{
    MessageHandler handler = new MessageHandler(listenerSocket.EndAcceptTcpClient(asyncResult));
    ThreadPool.QueueUserWorkItem((object state) => handler.Process());
    listenerSocket.BeginAcceptTcpClient(new AsyncCallback(AcceptClient), null);
}

Now, I call BeginAcceptTcpClient, then some time later I want to stop the server. To do this I have been calling TcpListener.Stop(), or TcpListener.Server.Close(). Both of these however execute my AcceptClient function. This then throws an exception when I call EndAcceptTcpClient. What is the best practice way around this? I could just put a flag in to stop the execution of AcceptClient once I have called stop, but I wonder if I am missing something.

Update 1

Currently I have patched it by changing the code to look like this.

private static void AcceptClient(IAsyncResult asyncResult)
{
     if (!shutdown)
     {
          MessageHandler handler = new MessageHandler(listenerSocket.EndAcceptTcpClient(asyncResult));
          ThreadPool.QueueUserWorkItem((object state) => handler.Process());
          listenerSocket.BeginAcceptTcpClient(new AsyncCallback(AcceptClient), null);
     }
}

private static bool shutdown = false;
internal static void Stop()
{
     shutdown = true;
     listenerSocket.Stop();
}

Update 2

I changed it to impliment the answer from Spencer Ruport.

private static void AcceptClient(IAsyncResult asyncResult)
{
    if (listenerSocket.Server.IsBound)
    {
            MessageHandler handler = new MessageHandler(listenerSocket.EndAcceptTcpClient(asyncResult));
            ThreadPool.QueueUserWorkItem((object state) => handler.Process());
            listenerSocket.BeginAcceptTcpClient(new AsyncCallback(AcceptClient), null);
    }
}

Answer 1

I just ran into this issue myself, and I believe your current solution is incomplete/incorrect. There is no guarantee of atomicity between the check for IsBound and the subsequent call to EndAcceptTcpClient(). You can still get an exception if the listener is Stop()'d between those two statements. You didn't say what exception you're getting but I assume it's the same one I'm getting, ObjectDisposedException (complaining that the underlying socket has already been disposed).

You should be able to check this by simulating the thread scheduling:

• Set a breakpoint on the line after the IsBound check in your callback
• Freeze the thread that hits the breakpoint (Threads window -> right click, "Freeze")
• Run/trigger the code that calls TcpListener.Stop()
• Break in and step through the EndAcceptTcpClient() call. You should see the ObjectDisposedException.

IMO the ideal solution would be for Microsoft to throw a different exception from EndAcceptTcpClient in this case, e.g. ListenCanceledException or something like that.

As it is, we have to infer what's happening from the ObjectDisposedException. Just catch the exception and behave accordingly. In my code I silently eat the exception, since I have code elsewhere that's doing the real shutdown work (i.e. the code that called TcpListener.Stop() in the first place). You should already have exception handling in that area anyway, since you can get various SocketExceptions. This is just tacking another catch handler onto that try block.

I admit I'm uncomfortable with this approach since in principle the catch could be a false positive, with a genuine "bad" object access in there. But on the other hand there aren't too many object accesses in the EndAcceptTcpClient() call that could otherwise trigger this exception. I hope.

Here's my code. This is early/prototype stuff, ignore the Console calls.

    private void OnAccept(IAsyncResult iar)
    {
        TcpListener l = (TcpListener) iar.AsyncState;
        TcpClient c;
        try
        {
            c = l.EndAcceptTcpClient(iar);
            // keep listening
            l.BeginAcceptTcpClient(new AsyncCallback(OnAccept), l);
        }
        catch (SocketException ex)
        {
            Console.WriteLine("Error accepting TCP connection: {0}", ex.Message);

            // unrecoverable
            _doneEvent.Set();
            return;
        }
        catch (ObjectDisposedException)
        {
            // The listener was Stop()'d, disposing the underlying socket and
            // triggering the completion of the callback. We're already exiting,
            // so just return.
            Console.WriteLine("Listen canceled.");
            return;
        }

        // meanwhile...
        SslStream s = new SslStream(c.GetStream());
        Console.WriteLine("Authenticating...");
        s.BeginAuthenticateAsServer(_cert, new AsyncCallback(OnAuthenticate), s);
    }