Asynchronous Server Socket Example


The following example program creates a server that receives connection requests from clients. The server is built with an asynchronous socket, so execution of the server application is not suspended while it waits for a connection from a client. The application receives a string from the client, displays the string on the console, and then echoes the string back to the client. The string from the client must contain the string "<EOF>" to signal the end of the message.

using System;  
using System.Net;  
using System.Net.Sockets;  
using System.Text;  
using System.Threading;  
// 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 {  
    // 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.  
        // The DNS name of the computer  
        // running the listener is "".  
        IPHostEntry ipHostInfo = Dns.Resolve(Dns.GetHostName());  
        IPAddress ipAddress = ipHostInfo.AddressList[0];  
        IPEndPoint localEndPoint = new IPEndPoint(ipAddress, 11000);  
        // 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.  
        try {  
            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, 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.  
        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) {  
        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);  
        } catch (Exception e) {  
    public static int Main(String[] args) {  
        return 0;  

Asynchronous Client Socket Example
Using an Asynchronous Server Socket
Socket Code Examples