C# TcpListener Gets Stuck When Called For 2nd Time

Question

I have this code to connect to a panel.

    private void conn_init()
    {
        bwConnector = new BackgroundWorker(); 
        //the int "anarg" does nothing other than avoid a compiler error.
        //I do "anang=0" during Form_Load and I never use it, so you can ignore it.
        bwConnector.DoWork += new DoWorkEventHandler((sender, args) => conn_function(anarg));
        if (bwConnector.IsBusy != true)
        {
            bwConnector.RunWorkerAsync();
        }
    }

And the conn_function that my backgroundworker runs is:

    void conn_function(int a_param)
    {
        try
        {
            Int32 port = 7050;
            server = new TcpListener(IPAddress.Any, port);
            server.Start(); 

            Byte[] bytes = new Byte[256];
            String data = "";
            while (true)
            {
                data = "";
                int i;
                i = 1;
                client = server.AcceptTcpClient();
                stream = client.GetStream();
                while (i != 0)
                {
                    data = "";                        
                    try
                    {                               
                        i = stream.Read(bytes, 0, bytes.Length);
                        stream.Flush();
                    }
                    catch (SocketException e)
                    {
                        richTextBox1.Invoke(new EventHandler(delegate
                        {
                            richTextBox1.Text = e.Message;
                        }));
                    }

                    data = System.Text.Encoding.UTF8.GetString(bytes, 0, i);
                    //stuff that deals with the data
                }
            }
        }
    }

I also have a timer that checks whether the connection is dead, and when it is dead, I run this code.

server.Stop();
conn_init();

My problem is, when I reset my device, my code can detect the disconnect and then it can reconnect, no issues. But when I reset it once more, I get the below error on this line:

//I get the error here and it says this:
//Unable to read data from the transport connection : An existing connection was 
//forcibly closed by the remote host
i = stream.Read(bytes, 0, bytes.Length);

I am pretty sure I am missing something very simple, yet too technical for a beginner in connection coding like me. Because it works after the first reset, it just fails after the second. I thank you in advance for any assistance you might provide.

Charlieface · Answer

You have a number of issue with your code:

You don't appear to be disposing the stream and client after it ends.
BackgroundWorker is effectively deprecated, you should use Task and await instead. Use a CancellationToken to stop the server.
Your server loop cannot handle multiple clients because it is not handing off the client to another thread/task.
You don't need to call GetStream inside the loop.
Do not swallow catch (SocketException e), it usually leaves the socket in an unrecoverable state. Instead, display the message and shutdown the socket.

async Task RunServerLoop(int a_param, CancellationToken token)
{
    TcpListener server = null;
    try
    {
        Int32 port = 7050;
        server = new TcpListener(IPAddress.Any, port);
        server.Start(); 

        while (!token.IsCancellationRequested)
        {
            client = await server.AcceptTcpClientAsync(token);
            Task.Run(() => HandleClient(client, token));
        }
    }
    catch (OperationCanceledException)
    { //
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex.Message);
    }
    finally
    {
        if (server?.Active == true)
            server.Shutdown();
    }
}

async Task HandleClient(CancellationToken token)
{
    try
    {                               
        using client;
        using var stream = client.GetStream();
        Byte[] bytes = new Byte[256];
        int i;
        while ((i = await stream.ReadAsync(bytes, 0, bytes.Length, token)) > 0)
        {
            var data = System.Text.Encoding.UTF8.GetString(bytes, 0, i);
            //stuff that deals with the data
        }
    }
    catch (OperationCanceledException)
    { //
    }
    catch (SocketException e)
    {
        richTextBox1.Invoke(new EventHandler(delegate
        {
            richTextBox1.Text = e.Message;
        }));
    }
}

You would call it like this:

CancellationTokenSource _cancel;

private async void conn_init()
{
    _cancel = new CancellationTokenSource();
    await RunServerLoop(somevalue, _cancel.Token);
}

I note that you don't appear to be using any kind of framing mechanism. Do not assume that a single Send from one side will necessarily be received in a single Read on the other. Read may or may not fill the buffer, and may or may not contain data from different messages from the other side.

C# TcpListener Gets Stuck When Called For 2nd Time

1 Answers