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I'm having an odd problem when trying to connect to my async tcpsocket via telnet.
Here's my code:

// State object for reading client data asynchronously
    public class StateObject
        // Client socket.
        public Socket workSocket = null;
        // Size of receive buffer.
        public const int BufferSize = 1024;
        // Receive buffer.
        public byte[] buffer = new byte[BufferSize];
        // Received data string.
        public StringBuilder sb = new StringBuilder();  

    public class AsynchronousSocketListener
        private static IPAddress localIPAddress
                if (!System.Net.NetworkInformation.NetworkInterface.GetIsNetworkAvailable())
                    return null;

                IPHostEntry host = Dns.GetHostEntry(Dns.GetHostName());

                return host
                    .FirstOrDefault(ip => ip.AddressFamily == AddressFamily.InterNetwork);
        private static short clientPort = 13228;

        // Thread signal.
        public static ManualResetEvent allDone = new ManualResetEvent(false);

        public AsynchronousSocketListener()

        public static void StartListening()
            // Data buffer for incoming data.
            byte[] bytes = new Byte[1024];

            // Establish the local endpoint for the socket.
            IPAddress ipAddress = localIPAddress;
            IPEndPoint localEndPoint = new IPEndPoint(ipAddress, clientPort);

            // Create a TCP/IP socket.
            Socket listener = new Socket(AddressFamily.InterNetwork,
                SocketType.Stream, ProtocolType.Tcp );

            // Bind the socket to the local endpoint and listen for incoming connections.

                while (true) {
                    // Set the event to nonsignaled state.

                    // Start an asynchronous socket to listen for connections.
                    Console.WriteLine("Waiting for a connection...");
                        new AsyncCallback(AcceptCallback),
                        listener );
                    // Wait until a connection is made before continuing.

            catch (Exception e)

            Console.WriteLine("\nPress ENTER to continue...");

        public static void AcceptCallback(IAsyncResult ar)
            // Signal the main thread to continue.

            // Get the socket that handles the client request.
            Socket listener = (Socket) ar.AsyncState;
            Socket handler = listener.EndAccept(ar);

            // Create the state object.
            StateObject state = new StateObject();
            state.workSocket = handler;
            handler.BeginReceive( state.buffer, 0, state.buffer.Length, 0,  new AsyncCallback(ReadCallback), state);

        public static void ReadCallback(IAsyncResult ar)
            String content = String.Empty;
            // Retrieve the state object and the handler socket
            // from the asynchronous state object.
            StateObject state = (StateObject) ar.AsyncState;
            Socket handler = state.workSocket;

            // Read data from the client socket. 
            int bytesRead = handler.EndReceive(ar);

            if (bytesRead > 0) {
                // There  might be more data, so store the data received so far.

                // Check for end-of-file tag. If it is not there, read 
                // more data.
                content =;
                if (content.IndexOf("<EOF>") > -1) {
                    // All the data has been read from the 
                    // client. Display it on the console.
                    Console.WriteLine("Read {0} bytes from socket. \n Data : {1}",
                        content.Length, content );
                    // Echo the data back to the client.
                    Send(handler, content);
                } else {
                    // Not all data received. Get more.
                    handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0,
                    new AsyncCallback(ReadCallback), state);
        private static void Send(Socket handler, String data)
            // Convert the string data to byte data using ASCII encoding.
            byte[] byteData = Encoding.ASCII.GetBytes(data);

            // Begin sending the data to the remote device.
            handler.BeginSend(byteData, 0, byteData.Length, 0,
                new AsyncCallback(SendCallback), handler);

        private static void SendCallback(IAsyncResult ar)
                // Retrieve the socket from the state object.
                Socket handler = (Socket) ar.AsyncState;

                // Complete sending the data to the remote device.
                int bytesSent = handler.EndSend(ar);
                Console.WriteLine("Sent {0} bytes to client.", bytesSent);

            catch (Exception e) {

        public static int Main(String[] args)
            return 0;

I start the application, which then waits for a client connection, then I start a telnet session and then use the command 'open 192.168.x.x 13228'.
I've set a breakpoint at the beginning of 'AcceptCallback' and it is hit as expected.
I step through until execution reaches 'handler.BeginReceive(...)' and that's where my problem starts.
BeginReceive is called, but the app will never reach the 'ReadCallback' method, which in turn causes the Telnet client to hang (= I see the message 'Establishing connection to 192.168.x.x...' indefinitely).

What drives me nuts, is that I have no idea what the reason of the problem exactly is, any hints are greatly appreciated.

I tested again and noticed that it doesn't matter if 'handler.BeginReceive(...)' is called or not, the TelnetClient still hangs.

Updated 11-Apr-13 5:13am
Sergey Alexandrovich Kryukov 11-Apr-13 12:24pm
May I ask you why async? Telnet is pure client-server: request/response. One thread (in you do UI or something else, it should be a separate thread on client side, at least two on server side, because there is a set of clients)...
Richard MacCutchan 11-Apr-13 12:32pm
Does this server fulfill all the requirements of the Telnet protocol?

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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