Cancel blocking AcceptTcpClient call

Question

As everyone may already know, the simplest way to accept incoming TCP connections in C# is by looping over TcpListener.AcceptTcpClient(). Additionally this way will block code execution until a connection is obtained. This is extremely limiting to a GUI, so I want to listen for connections in either a seperate thread or task.

I have been told, that threads have several disadvantages, however nobody explained me what these are. So instead of using threads, I used tasks. This works great, however since the AcceptTcpClient method is blocking execution, I can't find any way of handling a task cancellation.

Currently the code looks like this, but I have no idea how I would want to cancel the task when I want the program to stop listening for connections.

First off the function executed in the task:

static void Listen () {
// Create listener object
TcpListener serverSocket = new TcpListener ( serverAddr, serverPort );

// Begin listening for connections
while ( true ) {
    try {
        serverSocket.Start ();
    } catch ( SocketException ) {
        MessageBox.Show ( "Another server is currently listening at port " + serverPort );
    }

    // Block and wait for incoming connection
    if ( serverSocket.Pending() ) {
        TcpClient serverClient = serverSocket.AcceptTcpClient ();
        // Retrieve data from network stream
        NetworkStream serverStream = serverClient.GetStream ();
        serverStream.Read ( data, 0, data.Length );
        string serverMsg = ascii.GetString ( data );
        MessageBox.Show ( "Message recieved: " + serverMsg );

        // Close stream and TcpClient connection
        serverClient.Close ();
        serverStream.Close ();

        // Empty buffer
        data = new Byte[256];
        serverMsg = null;
    }
}

Second, the functions starting and stopping the listening service:

private void btnListen_Click (object sender, EventArgs e) {
    btnListen.Enabled = false;
    btnStop.Enabled = true;
    Task listenTask = new Task ( Listen );
    listenTask.Start();
}

private void btnStop_Click ( object sender, EventArgs e ) {
    btnListen.Enabled = true;
    btnStop.Enabled = false;
    //listenTask.Abort();
}

I just need something to replace the listenTask.Abort() call (Which I commented out because the method doesn't exist)

Answer 1

The following code will close/abort AcceptTcpClient when isRunning variable becomes false

public static bool isRunning;

delegate void mThread(ref book isRunning);
delegate void AccptTcpClnt(ref TcpClient client, TcpListener listener);

public static main()
{
   isRunning = true;
   mThread t = new mThread(StartListening);
   Thread masterThread = new Thread(() => t(this, ref isRunning));
   masterThread.IsBackground = true; //better to run it as a background thread
   masterThread.Start();
}

public static void AccptClnt(ref TcpClient client, TcpListener listener)
{
  if(client == null)
    client = listener.AcceptTcpClient(); 
}

public static void StartListening(ref bool isRunning)
{
  TcpListener listener = new TcpListener(new IPEndPoint(IPAddress.Any, portNum));

  try
  {
     listener.Start();

     TcpClient handler = null;
     while (isRunning)
     {
        AccptTcpClnt t = new AccptTcpClnt(AccptClnt);

        Thread tt = new Thread(() => t(ref handler, listener));
        tt.IsBackground = true;
        // the AcceptTcpClient() is a blocking method, so we are invoking it
        // in a separate dedicated thread 
        tt.Start(); 
        while (isRunning && tt.IsAlive && handler == null) 
        Thread.Sleep(500); //change the time as you prefer


        if (handler != null)
        {
           //handle the accepted connection here
        }        
        // as was suggested in comments, aborting the thread this way
        // is not a good practice. so we can omit the else if block
        // else if (!isRunning && tt.IsAlive)
        // {
        //   tt.Abort();
        //}                   
     }
     // when isRunning is set to false, the code exits the while(isRunning)
     // and listner.Stop() is called which throws SocketException 
     listener.Stop();           
  }
  // catching the SocketException as was suggested by the most
  // voted answer
  catch (SocketException e)
  {

  }

}