TCPIP networking with C#

Question

HI everyone,

I'm going to be writing some code that has to listen for TCPIP messages coming from GSM mobile phones over GPRS. In the fullness of time, I see this as running on a Virtual Private Server, and it could well be processing multiple messages every second.

I'm a bit of a network programming virgin, so I've done a bit of research on the internet, and read a few tutorials. The approach I am considering at the moment is a windows service using sockets to monitor the port. If my understanding is correct, I need one socket to listen for connections from clients, and each time someone tries to connect with the port I will be passed another socket with which to communicate with them? Does this sound about right to more experienced ears?

I'm planning on using asynchronous communication, but on of the bigger design questions is whether to use threading or not. Threading isn't something I've really played with, and I am aware of a number of pitfalls - race conditions and debugging problems being but two.

If I avoid threads, I know I have to supply an object that acts as an identifier for a particular conversation. I was thinking GUIDs for this - any opinions?

Thanks in advance for any responses...

Martin

Answer 1

Starting from .net framework 2.0 SP1 there are some changings in socket libraries related to asyncronous sockets.

All multithreading used under the hood. We have no need to use multithreading manually (we don't need to use even ThreadPool explicitly). All what we do - using BeginAcceptSocket for starting accepting new connections, and using SocketAsyncEventArgs after accepting new connection .

Short implementation:

//In constructor or in method Start
var tcpServer = new TcpListener(IPAddress.Any, port);
tcpServer.Start();
tcpServer.BeginAcceptSocket(EndAcceptSocket, tcpServer);

//In EndAcceptSocket
Socket sock= lister.EndAcceptSocket(asyncResult);
var e = new SocketAsyncEventArgs();
e.Completed += ReceiveCompleted; //some data receive handle
e.SetBuffer(new byte[SocketBufferSize], 0, SocketBufferSize);
if (!sock.ReceiveAsync(e))
{//IO operation finished syncronously
  //handle received data
  ReceiveCompleted(sock, e);
}//IO operation finished syncronously
//Add sock to internal storage

Full implementation:

using System;
using System.Collections.Generic;
using System.Net;
using System.Net.Sockets;
using System.Runtime.InteropServices;

namespace Ample
{
    public class IPEndPointEventArgs : EventArgs
    {
        public IPEndPointEventArgs(IPEndPoint ipEndPoint)
        {
            IPEndPoint = ipEndPoint;
        }

        public IPEndPoint IPEndPoint { get; private set; }
    }

    public class DataReceivedEventArgs : EventArgs
    {
        public DataReceivedEventArgs(byte[] data, IPEndPoint ipEndPoint)
        {
            Data = data;
            IPEndPoint = ipEndPoint;
        }

        public byte[] Data { get; private set; }
        public IPEndPoint IPEndPoint { get; private set; }

    }
    /// <summary>
    /// TcpListner wrapper
    /// Encapsulates asyncronous communications using TCP/IP.
    /// </summary>
    public sealed class TcpServer : IDisposable
    {
        //----------------------------------------------------------------------
        //Construction, Destruction
        //----------------------------------------------------------------------
        /// <summary>
        /// Creating server socket
        /// </summary>
        /// <param name="port">Server port number</param>
        public TcpServer(int port)
        {
            connectedSockets = new Dictionary<IPEndPoint, Socket>();
            tcpServer = new TcpListener(IPAddress.Any, port);
            tcpServer.Start();
            tcpServer.BeginAcceptSocket(EndAcceptSocket, tcpServer);
        }
        ~TcpServer()
        {
            DisposeImpl(false);
        }
        public void Dispose()
        {
            DisposeImpl(true);
        }

        //----------------------------------------------------------------------
        //Public Methods
        //----------------------------------------------------------------------

        public void SendData(byte[] data, IPEndPoint endPoint)
        {
            Socket sock;
            lock (syncHandle)
            {
                if (!connectedSockets.ContainsKey(endPoint))
                    return;
                sock = connectedSockets[endPoint];
            }
            sock.Send(data);
        }

        //----------------------------------------------------------------------
        //Events
        //----------------------------------------------------------------------
        public event EventHandler<IPEndPointEventArgs> SocketConnected;
        public event EventHandler<IPEndPointEventArgs> SocketDisconnected;
        public event EventHandler<DataReceivedEventArgs> DataReceived;

        //----------------------------------------------------------------------
        //Private Functions
        //----------------------------------------------------------------------
        #region Private Functions
        //Обработка нового соединения
        private void Connected(Socket socket)
        {
            var endPoint = (IPEndPoint)socket.RemoteEndPoint;

            lock (connectedSocketsSyncHandle)
            {
                if (connectedSockets.ContainsKey(endPoint))
                {
                    theLog.Log.DebugFormat("TcpServer.Connected: Socket already connected! Removing from local storage! EndPoint: {0}", endPoint);
                    connectedSockets[endPoint].Close();
                }

                SetDesiredKeepAlive(socket);
                connectedSockets[endPoint] = socket;
            }

            OnSocketConnected(endPoint);
        }

        private static void SetDesiredKeepAlive(Socket socket)
        {
            socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, true);
            const uint time = 10000;
            const uint interval = 20000;
            SetKeepAlive(socket, true, time, interval);
        }
        static void SetKeepAlive(Socket s, bool on, uint time, uint interval)
        {
            /* the native structure
            struct tcp_keepalive {
            ULONG onoff;
            ULONG keepalivetime;
            ULONG keepaliveinterval;
            };
            */

            // marshal the equivalent of the native structure into a byte array
            uint dummy = 0;
            var inOptionValues = new byte[Marshal.SizeOf(dummy) * 3];
            BitConverter.GetBytes((uint)(on ? 1 : 0)).CopyTo(inOptionValues, 0);
            BitConverter.GetBytes((uint)time).CopyTo(inOptionValues, Marshal.SizeOf(dummy));
            BitConverter.GetBytes((uint)interval).CopyTo(inOptionValues, Marshal.SizeOf(dummy) * 2);
            // of course there are other ways to marshal up this byte array, this is just one way

            // call WSAIoctl via IOControl
            int ignore = s.IOControl(IOControlCode.KeepAliveValues, inOptionValues, null);

        }
        //socket disconnected handler
        private void Disconnect(Socket socket)
        {
            var endPoint = (IPEndPoint)socket.RemoteEndPoint;

            lock (connectedSocketsSyncHandle)
            {
                connectedSockets.Remove(endPoint);
            }

            socket.Close();

            OnSocketDisconnected(endPoint);
        }

        private void ReceiveData(byte[] data, IPEndPoint endPoint)
        {
            OnDataReceived(data, endPoint);
        }

        private void EndAcceptSocket(IAsyncResult asyncResult)
        {
            var lister = (TcpListener)asyncResult.AsyncState;
            theLog.Log.Debug("TcpServer.EndAcceptSocket");
            if (disposed)
            {
                theLog.Log.Debug("TcpServer.EndAcceptSocket: tcp server already disposed!");
                return;
            }

            try
            {
                Socket sock;
                try
                {
                    sock = lister.EndAcceptSocket(asyncResult);
                    theLog.Log.DebugFormat("TcpServer.EndAcceptSocket: remote end point: {0}", sock.RemoteEndPoint);
                    Connected(sock);
                }
                finally
                {
                    //EndAcceptSocket can failes, but in any case we want to accept 
new connections
                    lister.BeginAcceptSocket(EndAcceptSocket, lister);
                }

                //we can use this only from .net framework 2.0 SP1 and higher
                var e = new SocketAsyncEventArgs();
                e.Completed += ReceiveCompleted;
                e.SetBuffer(new byte[SocketBufferSize], 0, SocketBufferSize);
                BeginReceiveAsync(sock, e);

            }
            catch (SocketException ex)
            {
                theLog.Log.Error("TcpServer.EndAcceptSocket: failes!", ex);
            }
            catch (Exception ex)
            {
                theLog.Log.Error("TcpServer.EndAcceptSocket: failes!", ex);
            }
        }

        private void BeginReceiveAsync(Socket sock, SocketAsyncEventArgs e)
        {
            if (!sock.ReceiveAsync(e))
            {//IO operation finished syncronously
                //handle received data
                ReceiveCompleted(sock, e);
            }//IO operation finished syncronously
        }

        void ReceiveCompleted(object sender, SocketAsyncEventArgs e)
        {
            var sock = (Socket)sender;
            if (!sock.Connected)
                Disconnect(sock);
            try
            {

                int size = e.BytesTransferred;
                if (size == 0)
                {
                    //this implementation based on IO Completion ports, and in this case
                    //receiving zero bytes mean socket disconnection
                    Disconnect(sock);
                }
                else
                {
                    var buf = new byte[size];
                    Array.Copy(e.Buffer, buf, size);
                    ReceiveData(buf, (IPEndPoint)sock.RemoteEndPoint);
                    BeginReceiveAsync(sock, e);
                }
            }
            catch (SocketException ex)
            {
                //We can't truly handle this excpetion here, but unhandled
                //exception caused process termination.
                //You can add new event to notify observer
                theLog.Log.Error("TcpServer: receive data error!", ex);
            }
            catch (Exception ex)
            {
                theLog.Log.Error("TcpServer: receive data error!", ex);
            }
        }

        private void DisposeImpl(bool manualDispose)
        {
            if (manualDispose)
            {
                //We should manually close all connected sockets
                Exception error = null;
                try
                {
                    if (tcpServer != null)
                    {
                        disposed = true;
                        tcpServer.Stop();
                    }
                }
                catch (Exception ex)
                {
                    theLog.Log.Error("TcpServer: tcpServer.Stop() failes!", ex);
                    error = ex;
                }

                try
                {
                    foreach (var sock in connectedSockets.Values)
                    {
                        sock.Close();
                    }
                }
                catch (SocketException ex)
                {
                    //During one socket disconnected we can faced exception
                    theLog.Log.Error("TcpServer: close accepted socket failes!", ex);
                    error = ex;
                }
                if ( error != null )
                    throw error;
            }
        }


        private void OnSocketConnected(IPEndPoint ipEndPoint)
        {
            var handler = SocketConnected;
            if (handler != null)
                handler(this, new IPEndPointEventArgs(ipEndPoint));
        }

        private void OnSocketDisconnected(IPEndPoint ipEndPoint)
        {
            var handler = SocketDisconnected;
            if (handler != null)
                handler(this, new IPEndPointEventArgs(ipEndPoint));
        }
        private void OnDataReceived(byte[] data, IPEndPoint ipEndPoint)
        {
            var handler = DataReceived;
            if ( handler != null )
                handler(this, new DataReceivedEventArgs(data, ipEndPoint));
        }

        #endregion Private Functions

        //----------------------------------------------------------------------
        //Private Fields
        //----------------------------------------------------------------------
        #region Private Fields
        private const int SocketBufferSize = 1024;
        private readonly TcpListener tcpServer;
        private bool disposed;
        private readonly Dictionary<IPEndPoint, Socket> connectedSockets;
        private readonly object connectedSocketsSyncHandle = new object();
        #endregion Private Fields
    }
}