public class StateObject
    {
        public Socket workSocket = null;
        public const int BufferSize = 1024;
        public byte[] buffer = new byte[BufferSize];
        public StringBuilder sb = new StringBuilder();
    }

    public class AsynchronousSocketListener
    {
        public static ManualResetEvent allDone = new ManualResetEvent(false);
        public AsynchronousSocketListener()
        {
        }
        public static void StartListening()
        {
            IPHostEntry ipHostInfo = Dns.GetHostEntry(Dns.GetHostName());
            IPAddress ipAddress = ipHostInfo.AddressList[0];
            IPEndPoint localEndPoint = new IPEndPoint(ipAddress, 11000);

            Socket listener = new Socket(ipAddress.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
         try
            {
                listener.Bind(localEndPoint);
                listener.Listen(100);
                while (true)
                {
                    allDone.Reset();
                    listener.BeginAccept(new AsyncCallback(AcceptCallback), listener);
                    allDone.WaitOne();
                }
            }
       catch (Exception p)
            {
                Console.WriteLine("Error..... " + p.StackTrace);
            }
        }
        public static void AcceptCallback(IAsyncResult ar)
        {
            allDone.Set();
            Socket listener = (Socket)ar.AsyncState;
            Socket handler = listener.EndAccept(ar);
            StateObject state = new StateObject();
            state.workSocket = handler;
            handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReadCallback), state);
        }
        public static void ReadCallback(IAsyncResult ar)
        {
            String content = String.Empty;
            StateObject state = (StateObject)ar.AsyncState;
            Socket handler = state.workSocket;
            int bytesRead = handler.EndReceive(ar);
            if (bytesRead > 0)
            {
                state.sb.Append(Encoding.ASCII.GetString(state.buffer, 0, bytesRead));
                content = state.sb.ToString();
                content = "x";
                Send(handler, content);
            }
        }

        private static void Send(Socket handler, String data)
        {
            byte[] byteData = Encoding.ASCII.GetBytes(data);
            handler.BeginSend(byteData, 0, byteData.Length, 0, new AsyncCallback(SendCallback), handler);
        }
        private static void SendCallback(IAsyncResult ar)
        {
            try
            {
                Socket handler = (Socket)ar.AsyncState;
                int bytesSent = handler.EndSend(ar);
                handler.Shutdown(SocketShutdown.Both);
                handler.Close();
            }
            catch (Exception p)
            {
                Console.WriteLine("Error..... " + p.StackTrace);
            }
        }
    }