Sending a value from server to client with sockets

Question

I am using the following projects in order to create an asynchronous communication between server and client sockets. When I am running those projects I am sending a message from the client to the server thus I got the message:

Data : recording EOF, Sent 14 bytes to client.

What I want to achieve is to send a boolean variable from the server to the client with the sockets. Is it doable to do so, I am wondering since in the code I have the server which waiting and listens and the client which sends data, can I do the opposite? In general what I want is to send a boolean to several clients. Why am I need the End of File in order to send a string? Is it necessary to convert everything to string?

EDIT: In general what I want is to send a variable from one computer to two others in order a process to begin simultaneously in all computers. In fact to create a switcher that gives a signal to begin a process in 2-3 machines in the same time.

I tried to use the following code for server:

class Program
{
    const int PORT_NO = 2201;
    const string SERVER_IP = "127.0.0.1";
    static void Main(string[] args)
    {
        //---listen at the specified IP and port no.---
        IPAddress localAdd = IPAddress.Parse(SERVER_IP);
        TcpListener listener = new TcpListener(localAdd, PORT_NO);
        Console.WriteLine("Listening...");
        listener.Start();
        //---incoming client connected---
        TcpClient client = listener.AcceptTcpClient();
        //---get the incoming data through a network stream---
        NetworkStream nwStream = client.GetStream();
        byte[] buffer = new byte[client.ReceiveBufferSize];
        //---read incoming stream---
        int bytesRead = nwStream.Read(buffer, 0, client.ReceiveBufferSize);
        //---convert the data received into a string---
        string dataReceived = Encoding.ASCII.GetString(buffer, 0, bytesRead);
        Console.WriteLine("Received : " + dataReceived);
        //---write back the text to the client---
        Console.WriteLine("Sending back : " + dataReceived);
        nwStream.Write(buffer, 0, bytesRead);
        client.Close();
        listener.Stop();
        Console.ReadLine();
    }
}

and for client:

class Program
{
    const int PORT_NO = 2201;
    const string SERVER_IP = "127.0.0.1";
    static void Main(string[] args)
    {
        //---data to send to the server---
        string textToSend = DateTime.Now.ToString();
        //---create a TCPClient object at the IP and port no.---
        TcpClient client = new TcpClient(SERVER_IP, PORT_NO);
        NetworkStream nwStream = client.GetStream();
        byte[] bytesToSend = ASCIIEncoding.ASCII.GetBytes(textToSend);
        //---send the text---
        Console.WriteLine("Sending : " + textToSend);
        nwStream.Write(bytesToSend, 0, bytesToSend.Length);
        //---read back the text---
        byte[] bytesToRead = new byte[client.ReceiveBufferSize];
        int bytesRead = nwStream.Read(bytesToRead, 0, client.ReceiveBufferSize);
        Console.WriteLine("Received : " + Encoding.ASCII.GetString(bytesToRead, 0, bytesRead));
        Console.ReadLine();
        client.Close();
    }
}

Just in the case I am working in the same machine. I will have in total 4 machines and I want the one of them to give a singal to the rest of them to begin recording rgb stream. Thus the server should send signal to the clients to begin recording. What should I do to change the behavior of the server to send data and not listen. Is it possible to have several machines listening and waiting for a signal to be given?

EDIT:

private void mouseClick1(object sender, MouseEventArgs e)
    {

        Thread thread = new Thread(() => StartServer());
        thread.Start();

        if (e.Button == MouseButtons.Left)
        {
            button5.Enabled = false;
            button3.Enabled = true;

            try
            {
                obj = new Capturer();
            }
            catch (Exception e1)
            {
                Console.WriteLine("The process failed: {0}", e1.ToString());
            }
        }
    }

    private void mouseClick2(object sender, MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Right)
        {
            obj.flag2 = true;
        }
    }

My code as it is now with left click calls startServer() function with a new thread which is the main code in @Ians implementation and afterthat I call my object. When I click right click I change a flag and the capturer stops. How can I stop also the server or pause in order to open it again with the left click?

Answer 1

On page https://msdn.microsoft.com/en-us/library/fx6588te%28v=vs.110%29.aspx see method AcceptCallback (it's called when client connects). There is line Socket handler = listener.EndAccept(ar);. This happens for each client when it connects, so keep these Socket instances to some list. When you want to send data to clients, use Send method from that same example with each socket from list (or selectively if you want to send only to some clients).

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);
}

Answer 2

Answers to the questions first:

Q: Is it necessary to convert everything to string?...In general what I want is to send a variable from one computer to two others in order a process to begin simultaneously in all computers.

A: No, it is not necessary to convert everything to string when sending using Socket. You may send byte[] which you most probably want.

Q: What I want to achieve is to send a boolean variable from the server to the client with the sockets

A: Do you mean boolean or byte? Because the basic variable type which you will get from the Socket is byte. You could always change byte to bool from the sender/receiver side by doing like

bool val = byteToCheck > 0;

A2: And since your server is Console Application, I recommend to take a look on hex string to byte[] conversion. This way, you could write something in string but interprets it as byte[]. Check this. The whole idea here is pretty simple. That is: you type in string, but it will be sent as byte[]. And since it is byte[] you can have any value in it.

And here I present my solution to handle your (1) multiple clients, (2) Async connect & accept & receive, but having (3) send sync, as well as (4) conversion from hex string to byte[] (the structure & idea), and last but not least (5) working code with user input (for you to change this part) for testing!

I would solve such problem using simple Socket class, since it is the solution I am most familiar with. But you could always do similarly if you use your TcpListener.Server (which is the underlying network of Socket class). And, as you desire, I would do that with ASync.

There are several steps needed to achieve what you want in both your server and your client:

---

Server

1. Make your Socket as class field rather than method field, since you will use if everywhere and you need multiple methods to achieve what you want. And initialize it as soon as you started your main routine.

   const int PORT_NO = 2201;
   const string SERVER_IP = "127.0.0.1";
   static Socket serverSocket; //put here as static
   static void Main(string[] args) {
       //---listen at the specified IP and port no.---
       Console.WriteLine("Listening...");
       serverSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
       serverSocket.Bind(new IPEndPoint(IPAddress.Any, PORT_NO));
       serverSocket.Listen(4); //the maximum pending client, define as you wish
       //your next main routine
   }
   

2. Since the server will serve many clients, I will recommend you to use ASync rather than Sync for the process. Initialize your Socket by using BeginAccept rather than using Accept, put acceptCallback in your BeginAccept

   static void Main(string[] args) {
       //---listen at the specified IP and port no.---
       Console.WriteLine("Listening...");
       serverSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
       serverSocket.Bind(new IPEndPoint(IPAddress.Any, PORT_NO));
       serverSocket.Listen(4); //the maximum pending client, define as you wish
       serverSocket.BeginAccept(new AsyncCallback(acceptCallback), null);      
       //other stuffs
   }
   

3. Define acceptCallback, which is where you will go when you accept a Socket. Put EndAccept there.

   private void acceptCallback(IAsyncResult result) { //if the buffer is old, then there might already be something there...
       System.Net.Sockets.Socket socket = null;
       try {
           socket = serverSocket.EndAccept(result); // To get your client socket
           //do something later
       } catch (Exception e) { // this exception will happen when "this" is be disposed...        
           //do something later
       }
   }
   

4. I would typically list my client sockets, and do something on client disposal (that is unlisted it) - but this depends on the need. In this case, you seem to need it. And don't forget to create buffers, etc... This is for buffering the incoming data.

5. Start to accept something received from the client, using another ASync BeginReceive on the client Socket (and now you need receiveCallback). Then, very important, repeat your BeginAccept to accept other clients!

   private const int BUFFER_SIZE = 4096;
   private static byte[] buffer = new byte[BUFFER_SIZE]; //buffer size is limited to BUFFER_SIZE per message
   private static List<Socket> clientSockets = new List<Socket>(); //may be needed by you
   private static void acceptCallback(IAsyncResult result) { //if the buffer is old, then there might already be something there...
       Socket socket = null;
       try {
           socket = serverSocket.EndAccept(result); // The objectDisposedException will come here... thus, it is to be expected!
           //Do something as you see it needs on client acceptance such as listing
           clientSockets.Add(socket); //may be needed later
           socket.BeginReceive(buffer, 0, buffer.Length, SocketFlags.None, new AsyncCallback(receiveCallback), socket);
           serverSocket.BeginAccept(new AsyncCallback(acceptCallback), null); //to receive another client
       } catch (Exception e) { // this exception will happen when "this" is be disposed...        
           //Do something here
           Console.WriteLine(e.ToString());
       }
   }
   

6. Define your receiveCallback, that is, when you receive something from your client. This part could be quite tricky because of failures! But basically, what you need for now is simply EndReceive and again, very important, to repeat the BeginReceive from the same client such that you can receive its next message!

   const int MAX_RECEIVE_ATTEMPT = 10;
   static int receiveAttempt = 0; //this is not fool proof, obviously, since actually you must have multiple of this for multiple clients, but for the sake of simplicity I put this
   private static void receiveCallback(IAsyncResult result) {
       Socket socket = null;
       try {
           socket = (Socket)result.AsyncState; //this is to get the sender
           if (socket.Connected) { //simple checking
               int received = socket.EndReceive(result);
               if (received > 0) {
                   byte[] data = new byte[received]; //the data is in the byte[] format, not string!
                   Buffer.BlockCopy(buffer, 0, data, 0, data.Length); //There are several way to do this according to https://stackoverflow.com/questions/5099604/any-faster-way-of-copying-arrays-in-c in general, System.Buffer.memcpyimpl is the fastest
                   //DO SOMETHING ON THE DATA IN byte[] data!! Yihaa!!
                   Console.WriteLine(Encoding.UTF8.GetString(data)); //Here I just print it, but you need to do something else
                   receiveAttempt = 0; //reset receive attempt
                   socket.BeginReceive(buffer, 0, buffer.Length, SocketFlags.None, new AsyncCallback(receiveCallback), socket); //repeat beginReceive
               } else if (receiveAttempt < MAX_RECEIVE_ATTEMPT) { //fail but not exceeding max attempt, repeats
                   ++receiveAttempt; //increase receive attempt;
                   socket.BeginReceive(buffer, 0, buffer.Length, SocketFlags.None, new AsyncCallback(receiveCallback), socket); //repeat beginReceive
               } else { //completely fails!
                   Console.WriteLine("receiveCallback fails!"); //don't repeat beginReceive
                   receiveAttempt = 0; //reset this for the next connection
               }
           }
       } catch (Exception e) { // this exception will happen when "this" is be disposed...
           Console.WriteLine("receiveCallback fails with exception! " + e.ToString());
       }
   }
   

7. And suppose you want to reply your sender after you receive the message, simply do this in the if (received > 0) part:

   if (received > 0) {
       byte[] data = new byte[received]; //the data is in the byte[] format, not string!
       //DO SOMETHING ON THE DATA int byte[]!! Yihaa!!
       Console.WriteLine(Encoding.UTF8.GetString(data)); //Here I just print it, but you need to do something else                     
   
       //Message retrieval part
       //Suppose you only want to declare that you receive data from a client to that client
       string msg = "I receive your message on: " + DateTime.Now;
       socket.Send(Encoding.ASCII.GetBytes(msg)); //Note that you actually send data in byte[]
       Console.WriteLine("I sent this message to the client: " + msg);
   
       receiveAttempt = 0; //reset receive attempt
       socket.BeginReceive(buffer, 0, buffer.Length, SocketFlags.None, new AsyncCallback(receiveCallback), socket); //repeat beginReceive
   }
   

8. And after putting a little more things in your main routine, you are done(!) - IF you do not ask for sending to client as byte[]

9. And now, if you want to send something to all your clients as byte[] you simply need to list all your client (see step 4-5). See this and convert the result string above (remember to type it in hex string format as required) to byte[] then send it to all the clients using your client socket list (here is where it is needed!):

   static void Main(string[] args) {
       //---listen at the specified IP and port no.---
       Console.WriteLine("Listening...");
       serverSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
       serverSocket.Bind(new IPEndPoint(IPAddress.Any, PORT_NO));
       serverSocket.Listen(4); //the maximum pending client, define as you wish
       serverSocket.BeginAccept(new AsyncCallback(acceptCallback), null);      
   
       //normally, there isn't anything else needed here
       string result = "";
       do {
           result = Console.ReadLine();
           if (result.ToLower().Trim() != "exit") {
               byte[] bytes = null;
               //you can use `result` and change it to `bytes` by any mechanism which you want
               //the mechanism which suits you is probably the hex string to byte[]
               //this is the reason why you may want to list the client sockets
               foreach(Socket socket in clientSockets)
                   socket.Send(bytes); //send everything to all clients as bytes
           }
       } while (result.ToLower().Trim() != "exit");
   }
   

And here, you are more or less done with your server. Next is your client

---

Client:

1. Similarly, put the Socket class in the class context rather than method context and initialize it as soon as you start your program

   const int PORT_NO = 2201;
   const string SERVER_IP = "127.0.0.1";
   static Socket clientSocket; //put here
   static void Main(string[] args) {
       //Similarly, start defining your client socket as soon as you start. 
       clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
   
       //your other main routines
   }
   

2. Then start to connect by ASync BeginConnect. I would normally go further by LoopConnect just for failure handling like this.

   static void loopConnect(int noOfRetry, int attemptPeriodInSeconds) {
       int attempts = 0;
       while (!clientSocket.Connected && attempts < noOfRetry) {
           try {
               ++attempts;
               IAsyncResult result = clientSocket.BeginConnect(IPAddress.Parse(SERVER_IP), PORT_NO, endConnect, null);
               result.AsyncWaitHandle.WaitOne(TimeSpan.FromSeconds(attemptPeriodInSeconds));
               System.Threading.Thread.Sleep(attemptPeriodInSeconds * 1000);
           } catch (Exception e) {
               Console.WriteLine("Error: " + e.ToString());
           }
       }
       if (!clientSocket.Connected) {
           Console.WriteLine("Connection attempt is unsuccessful!");
           return;
       }
   }
   

3. Similar concept to what you do to the server BeginAccept you need to define endConnectCallback for the ASync BeginConnect you use. But here, unlike server which needs to re-calling BeginAccept, once you are connected, you do not need to do any new BeginConnect since you only need to be connected once.

4. You may want to declare buffer etc. Then, after you connect, don't forget the next ASync BeginReceive to handle the message retrieval part (similar with the server)

   private const int BUFFER_SIZE = 4096;
   private static byte[] buffer = new byte[BUFFER_SIZE]; //buffer size is limited to BUFFER_SIZE per message
   private static void endConnectCallback(IAsyncResult ar) {
       try {
           clientSocket.EndConnect(ar);
           if (clientSocket.Connected) {
               clientSocket.BeginReceive(buffer, 0, buffer.Length, SocketFlags.None, new AsyncCallback(receiveCallback), clientSocket);
           } else {
               Console.WriteLine("End of connection attempt, fail to connect...");
           }
       } catch (Exception e) {
           Console.WriteLine("End-connection attempt is unsuccessful! " + e.ToString());
       }
   }
   

5. Naturally, you need to define your receiveCallback, just like what you did for the server. And yes, it is as you have guessed, it is almost identical to what you did for the server!

6. You can do anything you want with your data. Note that the data you receive is actually in byte[], not string. So you can do anything with it. But for example's sake, I will just use string to display.

   const int MAX_RECEIVE_ATTEMPT = 10;
   static int receiveAttempt = 0;
   private static void receiveCallback(IAsyncResult result) {
       System.Net.Sockets.Socket socket = null;
       try {
           socket = (System.Net.Sockets.Socket)result.AsyncState;
           if (socket.Connected) {
               int received = socket.EndReceive(result);
               if (received > 0) {
                   receiveAttempt = 0;
                   byte[] data = new byte[received];
                   Buffer.BlockCopy(buffer, 0, data, 0, data.Length); //copy the data from your buffer
                   //DO ANYTHING THAT YOU WANT WITH data, IT IS THE RECEIVED PACKET!
                   //Notice that your data is not string! It is actually byte[]
                   //For now I will just print it out
                   Console.WriteLine("Server: " + Encoding.UTF8.GetString(data));
                   socket.BeginReceive(buffer, 0, buffer.Length, SocketFlags.None, new AsyncCallback(receiveCallback), socket);
               } else if (receiveAttempt < MAX_RECEIVE_ATTEMPT) { //not exceeding the max attempt, try again
                   ++receiveAttempt;
                   socket.BeginReceive(buffer, 0, buffer.Length, SocketFlags.None, new AsyncCallback(receiveCallback), socket);
               } else { //completely fails!
                   Console.WriteLine("receiveCallback is failed!");
                   receiveAttempt = 0;
                   clientSocket.Close();
               }
           }
       } catch (Exception e) { // this exception will happen when "this" is be disposed...
           Console.WriteLine("receiveCallback is failed! " + e.ToString());
       }
   }
   

7. And at the very very last... Yes, again, as you have already guessed, you just need to do something on your main routine - suppose you want to use it to send data. Because you use Console but you want it to send things as byte[], you need to do the conversion (see the explanation in server 9.). And afterwards you are completely done!!

   static void Main(string[] args) {
       //Similarly, start defining your client socket as soon as you start. 
       clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
       loopConnect(3, 3); //for failure handling
       string result = "";
       do {
           result = Console.ReadLine(); //you need to change this part
           if (result.ToLower().Trim() != "exit") {
               byte[] bytes = Encoding.ASCII.GetBytes(result); //Again, note that your data is actually of byte[], not string
               //do something on bytes by using the reference such that you can type in HEX STRING but sending thing in bytes
               clientSocket.Send(bytes);
           }
       } while (result.ToLower().Trim() != "exit");
   }
   

---

Results:

Here you go! I tested it by sending string for display, but I already put up what is needed when you want to change it to byte[]

---

Code for your test:

Server

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

namespace TcpListenerConsoleApplication {
    class Program {
        const int PORT_NO = 2201;
        const string SERVER_IP = "127.0.0.1";
        static Socket serverSocket;
        static void Main(string[] args) {
            //---listen at the specified IP and port no.---
            Console.WriteLine("Listening...");
            serverSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            serverSocket.Bind(new IPEndPoint(IPAddress.Any, PORT_NO));
            serverSocket.Listen(4); //the maximum pending client, define as you wish
            serverSocket.BeginAccept(new AsyncCallback(acceptCallback), null);      
            string result = "";
            do {
                result = Console.ReadLine();
            } while (result.ToLower().Trim() != "exit");
        }

        private const int BUFFER_SIZE = 4096;
        private static byte[] buffer = new byte[BUFFER_SIZE]; //buffer size is limited to BUFFER_SIZE per message
        private static void acceptCallback(IAsyncResult result) { //if the buffer is old, then there might already be something there...
            Socket socket = null;
            try {
                socket = serverSocket.EndAccept(result); // The objectDisposedException will come here... thus, it is to be expected!
                //Do something as you see it needs on client acceptance
                socket.BeginReceive(buffer, 0, buffer.Length, SocketFlags.None, new AsyncCallback(receiveCallback), socket);
                serverSocket.BeginAccept(new AsyncCallback(acceptCallback), null); //to receive another client
            } catch (Exception e) { // this exception will happen when "this" is be disposed...        
                //Do something here             
                Console.WriteLine(e.ToString());
            }
        }

        const int MAX_RECEIVE_ATTEMPT = 10;
        static int receiveAttempt = 0; //this is not fool proof, obviously, since actually you must have multiple of this for multiple clients, but for the sake of simplicity I put this
        private static void receiveCallback(IAsyncResult result) {
            Socket socket = null;
            try {
                socket = (Socket)result.AsyncState; //this is to get the sender
                if (socket.Connected) { //simple checking
                    int received = socket.EndReceive(result);
                    if (received > 0) {
                        byte[] data = new byte[received]; //the data is in the byte[] format, not string!
                        Buffer.BlockCopy(buffer, 0, data, 0, data.Length); //There are several way to do this according to https://stackoverflow.com/questions/5099604/any-faster-way-of-copying-arrays-in-c in general, System.Buffer.memcpyimpl is the fastest
                        //DO SOMETHING ON THE DATA int byte[]!! Yihaa!!
                        Console.WriteLine(Encoding.UTF8.GetString(data)); //Here I just print it, but you need to do something else                     

                        //Message retrieval part
                        //Suppose you only want to declare that you receive data from a client to that client
                        string msg = "I receive your message on: " + DateTime.Now;                      
                        socket.Send(Encoding.ASCII.GetBytes(msg)); //Note that you actually send data in byte[]
                        Console.WriteLine("I sent this message to the client: " + msg);

                        receiveAttempt = 0; //reset receive attempt
                        socket.BeginReceive(buffer, 0, buffer.Length, SocketFlags.None, new AsyncCallback(receiveCallback), socket); //repeat beginReceive
                    } else if (receiveAttempt < MAX_RECEIVE_ATTEMPT) { //fail but not exceeding max attempt, repeats
                        ++receiveAttempt; //increase receive attempt;
                        socket.BeginReceive(buffer, 0, buffer.Length, SocketFlags.None, new AsyncCallback(receiveCallback), socket); //repeat beginReceive
                    } else { //completely fails!
                        Console.WriteLine("receiveCallback fails!"); //don't repeat beginReceive
                        receiveAttempt = 0; //reset this for the next connection
                    }
                }
            } catch (Exception e) { // this exception will happen when "this" is be disposed...
                Console.WriteLine("receiveCallback fails with exception! " + e.ToString());
            }
        }

    }
}

Client

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

namespace TcpClientConsoleApplication {
    class Program {
        const int PORT_NO = 2201;
        const string SERVER_IP = "127.0.0.1";
        static Socket clientSocket; //put here
        static void Main(string[] args) {
            //Similarly, start defining your client socket as soon as you start. 
            clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            loopConnect(3, 3); //for failure handling
            string result = "";
            do {
                result = Console.ReadLine(); //you need to change this part
                if (result.ToLower().Trim() != "exit") {
                    byte[] bytes = Encoding.ASCII.GetBytes(result); //Again, note that your data is actually of byte[], not string
                    //do something on bytes by using the reference such that you can type in HEX STRING but sending thing in bytes
                    clientSocket.Send(bytes);
                }
            } while (result.ToLower().Trim() != "exit");
        }

        static void loopConnect(int noOfRetry, int attemptPeriodInSeconds) {
            int attempts = 0;
            while (!clientSocket.Connected && attempts < noOfRetry) {
                try {
                    ++attempts;
                    IAsyncResult result = clientSocket.BeginConnect(IPAddress.Parse(SERVER_IP), PORT_NO, endConnectCallback, null);
                    result.AsyncWaitHandle.WaitOne(TimeSpan.FromSeconds(attemptPeriodInSeconds));
                    System.Threading.Thread.Sleep(attemptPeriodInSeconds * 1000);
                } catch (Exception e) {
                    Console.WriteLine("Error: " + e.ToString());
                }
            }
            if (!clientSocket.Connected) {
                Console.WriteLine("Connection attempt is unsuccessful!");
                return;
            }
        }

        private const int BUFFER_SIZE = 4096;
        private static byte[] buffer = new byte[BUFFER_SIZE]; //buffer size is limited to BUFFER_SIZE per message
        private static void endConnectCallback(IAsyncResult ar) {
            try {
                clientSocket.EndConnect(ar);
                if (clientSocket.Connected) {
                    clientSocket.BeginReceive(buffer, 0, buffer.Length, SocketFlags.None, new AsyncCallback(receiveCallback), clientSocket);
                } else {
                    Console.WriteLine("End of connection attempt, fail to connect...");
                }
            } catch (Exception e) {
                Console.WriteLine("End-connection attempt is unsuccessful! " + e.ToString());
            }
        }

        const int MAX_RECEIVE_ATTEMPT = 10;
        static int receiveAttempt = 0;
        private static void receiveCallback(IAsyncResult result) {
            System.Net.Sockets.Socket socket = null;
            try {
                socket = (System.Net.Sockets.Socket)result.AsyncState;
                if (socket.Connected) {
                    int received = socket.EndReceive(result);
                    if (received > 0) {
                        receiveAttempt = 0;
                        byte[] data = new byte[received];
                        Buffer.BlockCopy(buffer, 0, data, 0, data.Length); //There are several way to do this according to https://stackoverflow.com/questions/5099604/any-faster-way-of-copying-arrays-in-c in general, System.Buffer.memcpyimpl is the fastest
                        //DO ANYTHING THAT YOU WANT WITH data, IT IS THE RECEIVED PACKET!
                        //Notice that your data is not string! It is actually byte[]
                        //For now I will just print it out
                        Console.WriteLine("Server: " + Encoding.UTF8.GetString(data));
                        socket.BeginReceive(buffer, 0, buffer.Length, SocketFlags.None, new AsyncCallback(receiveCallback), socket);
                    } else if (receiveAttempt < MAX_RECEIVE_ATTEMPT) { //not exceeding the max attempt, try again
                        ++receiveAttempt;
                        socket.BeginReceive(buffer, 0, buffer.Length, SocketFlags.None, new AsyncCallback(receiveCallback), socket);
                    } else { //completely fails!
                        Console.WriteLine("receiveCallback is failed!");
                        receiveAttempt = 0;
                        clientSocket.Close();
                    }
                }
            } catch (Exception e) { // this exception will happen when "this" is be disposed...
                Console.WriteLine("receiveCallback is failed! " + e.ToString());
            }
        }
    }
}

---

Last Remarks (Edit)

Since the code above is run using Console Application it must be run with static main void keyword. And thus client Socket defined above is of static type. This may prevent the client Socket to be defined multiple times as each time it is "defined", since it is of the same class named Program, it will refer to the same Socket (though this may not always the case, at least according to the OP's experiment: he can run multiple clients successfully in the same computer).

Nevertheless, to overcome this is not that hard. Simply port the client application to the platform which is non-initiated as static class (such as WinForms) and all the above code would still run as per normal. Alternatively, if it must be run using the Console Applications, and the problem occurs, simply copy the client application and re-define it using different namespace or different class name to avoid defining identical Socket due to identical namespace or class.

But the most important part on this problem solving is the use of Async and Sync wisely to solve the given issue.

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Continuation of this topic can be found here

Answer 3

You can send data to a client, and it can be achieved similarly to how you do so client -> server (unless your sockets are receive only, if so switch them to send and receive). Sending a Boolean requires conversion to a byte, you can achieve this via the BitConverter class.

Answer 4

Why don't you make your life easier and use SignalR?
Below you can see a simple example where the server and the clients are console apps

Client (Run this .exe many times)

using System;
using Microsoft.AspNet.SignalR.Client;

namespace SignalRClient
{
    class Program
    {
        private static IHubProxy HubProxy { get; set; }
        const string ServerURI = "http://localhost:1234/signalr";
        private static HubConnection Connection { get; set; }

        static void Main(string[] args)
        {
            Connection = new HubConnection(ServerURI);
            HubProxy = Connection.CreateHubProxy("MyHub");

            HubProxy.On<string, string>("SendMessage", (name, message) => Console.WriteLine(name + ":" + message));
            Connection.Start().Wait();

            Console.WriteLine("Press Enter to stop client");
            Console.ReadLine();
        }
    }
}

Server (Run this .exe before you run the clients)

using System;
using System.Threading.Tasks;
using Microsoft.AspNet.SignalR;
using Microsoft.AspNet.SignalR.Client;
using Microsoft.Owin.Hosting;
using Owin;

namespace SignalRServer
{
    class Program
    {
        static private IDisposable SignalR { get; set; }
        const string ServerURI = "http://localhost:1234";
        private static IHubProxy HubProxy { get; set; }
        private static HubConnection Connection { get; set; }
        static void Main(string[] args)
        {
            SignalR = WebApp.Start(ServerURI);
            Console.WriteLine("Server running at " + ServerURI);

            Connection = new HubConnection(ServerURI);
            HubProxy = Connection.CreateHubProxy("MyHub");
            HubProxy.On<string, string>("SendMessage", (name, message) => Console.WriteLine(name + ":" + message));
            Connection.Start().Wait();

            string messageToSentToClients;
            do
            {
                Console.WriteLine("Type someting to send to clients and press enter");
                messageToSentToClients = Console.ReadLine();
               HubProxy.Invoke("Send", "Server", messageToSentToClients);
           } while (messageToSentToClients != "exit");

       }
    }

    public class MyHub : Hub
    {
        public void Send(string name, string message) { Clients.All.sendMessage(name, message); }
    }

    class Startup
    {
        public void Configuration(IAppBuilder app) { app.MapSignalR(); }
    }
}

In order for the above to work you need the following NuGet packages:

Client:
Microsoft.AspNet.SignalR.Client
Server
Microsoft.AspNet.SignalR
Microsoft.AspNet.SignalR.Client
Microsoft.AspNet.SignalR.SelfHost
Microsoft.Owin.Host.HttpListener

If you want the server/clients to be on different machines all you have to do is change the ServeURI properties in both projects:

//Clients
const string ServerURI = "http://SERVER_IP:PORT/signalr";
//Server
const string ServerURI = "http://SERVER_IP:PORT"; 

You can find another similar example in WinForms here:
https://code.msdn.microsoft.com/windowsdesktop/Using-SignalR-in-WinForms-f1ec847b