Question

With the code below I’m getting: “The name ‘listener’ does not exist in the current context”

Really? Why?

static void ReceiveSocketMsgs()
{
    try
    {
        TcpListener listener;
        listener = new TcpListener(IPAddress.Any, MainForm.GOHRFTrackerMainForm.socketPortNum);
        listener.Start();
        using (TcpClient c = listener.AcceptTcpClient())
        {
            using (NetworkStream n = c.GetStream())
            {
                string msg = new BinaryReader(n).ReadString();
                BinaryWriter w = new BinaryWriter(n);
                w.Write(msg + " received");
                w.Flush(); // Must call Flush because we're not disposing the writer. 
            }
        }
    }
    catch (Exception ex)
    {
        //some exception (if you close the app, it will be "threadabort")
    }
    finally
    {
        listener.Stop();
    }
}

Answer 1

That’s just how C# scoping works. It does get in the way in lock statements and try/catch clauses. Just move the declaration outside:

static void ReceiveSocketMsgs()
{
    TcpListener listener = null;
    try
    {
        listener = new TcpListener(IPAddress.Any, MainForm.GOHRFTrackerMainForm.socketPortNum);
        ...
    }
    catch (Exception ex)
    {
        //some exception (if you close the app, it will be "threadabort")
    }
    finally
    {
        if (listener != null)
            listener.Stop();
    }
}

To keep the listener initialization inside the try block, initialize the variable to null and check that before calling Stop.

Fixed the initialisation. Well spotted BoltClock.