Asynchronous Socket Server in C#, client to client communication through socket server

Question

I am trying to develop server/client Asynchronous sockets using c#. I have followed guide on MSDN Link. In my case, socket server listening on particular endpoint, many clients can connect to server at a time, clients can talk to server and server can talk to clients. lets say client 1 and client 2 connected with server, client 1 can send message to server and server can send to client 1, same for client 2 case. Now i want clients should be able to communicate with each other through server. For example;client 2 want to communicate with client 1, for that client 2 will send message to server (this message will contain some preset characters;), then server will receive text from client 2 and will get the handler for client 1 and send this message to client 1, client 1 will response the server, Now i want to send the response of client 1 against that message to client 2, but i do not know how to do that because client 1 communicates through its own handler with server, I am struck here, help will be highly appreciated!! my code is given below:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Net;
using System.Net.Sockets;



namespace SocketServer
{
    // 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 int clientNumber;
    }
    public class AsyncSocketServer
    {
        public static ManualResetEvent allDone = new ManualResetEvent(false);

        public static Dictionary<int, StateObject> Clients = new Dictionary<int, StateObject>();

        public static int connectedClient = 0;




        public AsyncSocketServer()
        {


        } 
        public static void startListening() {

            Byte[] bytes = new Byte[1024];
            int Port = 1122;

            IPAddress IP = IPAddress.Parse("127.0.0.1");
            IPEndPoint EP = new IPEndPoint(IP, Port);
            Socket listner = new Socket(IP.AddressFamily, SocketType.Stream, ProtocolType.Tcp);



            try
            {
                listner.Bind(EP);
                listner.Listen(100);

                while (true)
                {
                   allDone.Reset();

                    Console.WriteLine("Waiting for the Connection......");

                    listner.BeginAccept(new AsyncCallback(AcceptCallBack), listner);

                    allDone.WaitOne();
                }


            }
            catch(Exception e)
            {
                Console.WriteLine("Exception Occured ! in start listening method "+e.ToString());
            }

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

        }

        public static void AcceptCallBack(IAsyncResult ar)
        {
            connectedClient++;
            Console.WriteLine("client number " + connectedClient);
            allDone.Set();



            Socket listner = (Socket)  ar.AsyncState;
            Socket handler = listner.EndAccept(ar);

            StateObject state = new StateObject();
            state.clientNumber = connectedClient;

            Clients.Add(connectedClient, state);
           Console.WriteLine("total clients {0}",Clients.Count());

            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;



        // Retrieve the state object and the handler socket  
        // from the asynchronous state object.  
        try { 
        StateObject state = (StateObject) ar.AsyncState;
        state.sb.Clear();
        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.  
            state.sb.Append(Encoding.ASCII.GetString(  
                state.buffer,0,bytesRead));  

            // Check for end-of-file tag. If it is not there, read   
            // more data.  

            content = state.sb.ToString();

            if (content.Substring(0, 3) == "cmd") {
                foreach (StateObject Client in Clients.Values) {
                    if (Client.clientNumber == 1) { 
                        Console.WriteLine("value is "+Client.clientNumber);
                        if (isClientConnected(Client.workSocket)){
                            Send(Client.workSocket, "did you receive my message");
                            //now client number 1 will response through its own handler, but i want to get response of 
                            //client number 1 and return this response to client number 2

                        }
                        else {
                            string responsemsg = "client number " + Client.clientNumber + " is disconnected !";
                            Console.WriteLine(responsemsg);
                            Send(handler,responsemsg);
                        }
                    }

                }
            }

            Console.WriteLine("Read {0} bytes from client {1} socket. \n Data : {2}",
                    content.Length, state.clientNumber,content);
            // Echo the data back to the client.  

            if (isClientConnected(handler))
            {
                Send(handler, content);
            }
            handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReadCallBack), state);

        }
        }
        catch (SocketException e)
        {
            //once if any client disconnected then control will come into this block
            Console.WriteLine("Socket Exception Occured in Read Call Back : " + e.Message.ToString());

        }
        catch (Exception e)
        {
            //once if any client disconnected then control will come into this block
            Console.WriteLine("Exception Occured in Read Call Back : " + e.Message.ToString());

        }
        }
        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)
        {
            try
            {
                // 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);



                //handler.Shutdown(SocketShutdown.Both);
                //handler.Close();

            }
            catch (Exception e)
            {
                Console.WriteLine(e.ToString());
            }
        }

        public static bool isClientConnected(Socket handler){

            return handler.Connected;
        }
        public static int Main(string[] args)
        {

            startListening();


            return 0;


        }
    }
}

Answer 1

For any complex socket-based application, I would recommend using a socket library like DotNetty to abstract the transport layer and allow you to focus on your application logic. Check out their SecureChat example, it may be pretty similar to what you're trying to achieve.

I've thrown together a quick example of a DotNetty server that would allow you to send commands between clients by having the clients register with the server, and then having the server route messages between the clients.

using System;
using System.Collections.Concurrent;
using System.Threading;
using System.Threading.Tasks;
using DotNetty.Transport.Channels;
using Newtonsoft.Json;
using System.IO;

namespace MultiClientSocketExample
{
    public enum Command
    {
        Register = 1,  // Register a new client
        SendToClient = 2, // Send a message from one client to antoher
        DoClientAction = 3 // Replace this with your client-to-client command
    }

    // Envelope for all messages handled by the server
    public class Message
    {
        public string ClientId { get; set; }
        public Command Command { get; set; }
        public string Data { get; set; }
    }

    // Command for seding a message from one client to antoher.   
    // This would be serialized as JSON and stored in the 'Data' member of the Message object.
    public class SendToClientCommand
    {
        public string DestinationClientId { get; set; }  // The client to receive the message

        public Command ClientCommand { get; set; } // The command for the destination client to execute

        public string Data { get; set; } // The payload for the destination client
    }

    // An object for storing unhandled messages in a queue to be processed asynchronously
    // This allows us to process messages and respond to the appropriate client,
    // without having to do everything in the ChannelRead0 method and block the main thread
    public class UnhandledMessage
    {
        private readonly Message message;
        private readonly IChannelHandlerContext context;

        public UnhandledMessage(Message message, IChannelHandlerContext context)
        {
            this.message = message;
            this.context = context;
        }

        public Message Message => message;
        public IChannelHandlerContext Context => context;

        public Command Command => message.Command;
        public string ClientId => message.ClientId;
        public string Data => message.Data;
    }

    // A representation of the connected Clients on the server.  
    // Note:  This is not the 'Client' class that would be used to communicate with the server.
    public class Client
    {
        private readonly string clientId;
        private readonly IChannelHandlerContext context;

        public Client(string clientId, IChannelHandlerContext context)
        {
            this.clientId = clientId;
            this.context = context;
        }

        public string ClientId => clientId;
        public IChannelHandlerContext Context => context;
    }

    // The socket server, using DotNetty's SimpleChannelInboundHandler
    // The ChannelRead0 method is called for each Message received
    public class Server : SimpleChannelInboundHandler<Message>, IDisposable
    {
        private readonly ConcurrentDictionary<string, Client> clients;
        private readonly ConcurrentQueue<UnhandledMessage> unhandledMessages;
        private readonly CancellationTokenSource cancellation;
        private readonly AutoResetEvent newMessage;

        public Server(CancellationToken cancellation)
        {
            this.clients = new ConcurrentDictionary<string, Client>();
            this.newMessage = new AutoResetEvent(false);
            this.cancellation = CancellationTokenSource.CreateLinkedTokenSource(cancellation);
        }

        // The start method should be called when the server is bound to a port.
        // Messages will be received, but will not be processed unless/until the Start method is called
        public Task Start()
        {
            // Start a dedicated thread to process messages so that the ChannelRead operation does not block
            return Task.Run(() =>
            {
                var serializer = JsonSerializer.CreateDefault();  // This will be used to deserialize the Data member of the messages

                while (!cancellation.IsCancellationRequested)
                {
                    UnhandledMessage message;
                    var messageEnqueued = newMessage.WaitOne(100);  // Sleep until a new message arrives

                    while (unhandledMessages.TryDequeue(out message))  // Process each message in the queue, then sleep until new messages arrive
                    {
                        if (message != null)
                        {
                            // Note: This part could be sent to the thread pool if you want to process messages in parallel
                            switch (message.Command)
                            {
                                case Command.Register:
                                    // Register a new client, or update an existing client with a new Context
                                    var client = new Client(message.ClientId, message.Context);
                                    clients.AddOrUpdate(message.ClientId, client, (_,__) => client);
                                    break;
                                case Command.SendToClient:
                                    Client destinationClient;
                                    using (var reader = new JsonTextReader(new StringReader(message.Data)))
                                    {
                                        var sendToClientCommand = serializer.Deserialize<SendToClientCommand>(reader);
                                        if (clients.TryGetValue(sendToClientCommand.DestinationClientId, out destinationClient))
                                        {
                                            var clientMessage = new Message { ClientId = message.ClientId, Command = sendToClientCommand.ClientCommand, Data = sendToClientCommand.Data };
                                            destinationClient.Context.Channel.WriteAndFlushAsync(clientMessage);
                                        }
                                    }
                                    break;
                            }
                        }
                    }
                }
            }, cancellation.Token);
        }

        // Receive each message from the clients and enqueue them to be procesed by the dedicated thread
        protected override void ChannelRead0(IChannelHandlerContext context, Message message)
        {
            unhandledMessages.Enqueue(new UnhandledMessage(message, context));
            newMessage.Set(); // Trigger an event so that the thread processing messages wakes up when a new message arrives
        }

        // Flush the channel once the Read operation has completed
        public override void ChannelReadComplete(IChannelHandlerContext context)
        {
            context.Flush();
            base.ChannelReadComplete(context);
        }

        // Automatically stop the message-processing thread when this object is disposed
        public void Dispose()
        {
            cancellation.Cancel();
        }
    }
}

Answer 2

I have tried to do the same, also based on the same code of MSDN a possible solution is to use a list of sockets:

List<Socket> clients = new List<Socket>();

then, when a client connects, add the client to the list:

public void AcceptCallback(IAsyncResult ar)
    {
        // Signal the main thread to continue.  
        allDone.Set();

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

        clients.Add(handler);
        ...
    }

You have to know the id ( handle )of each client connected, and then you can send some message to a specific client:

 public void SendToOne(string id,string message)
    {
        foreach (Socket s in clients)
        {
            if (s.Handle.ToString() == id)
            {
                Send(s, message);
            }
        }

    }