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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.

Imports System
Imports System.Net
Imports System.Net.Sockets
Imports System.Text
Imports System.Threading


' State object for reading client data asynchronously
Public Class StateObject
    ' Client  socket.
    Public workSocket As Socket = Nothing
    ' Size of receive buffer.
    Public Const BufferSize As Integer = 1024
    ' Receive buffer.
    Public buffer(BufferSize) As Byte
    ' Received data string.
    Public sb As New StringBuilder() 
End Class 'StateObject

Public Class AsynchronousSocketListener
    
    ' Incoming data from the client.
    Public Shared data As String = Nothing
    
    ' Thread signal.
    Public Shared allDone As New ManualResetEvent(False)
    
    
    Public Sub New()
    End Sub 'New
    
    
    Public Shared Sub StartListening()
        ' Data buffer for incoming data.
        Dim bytes() As Byte = New [Byte](1024) {}
        
        ' Establish the local endpoint for the socket.
        ' The DNS name of the computer
        ' running the listener is "host.contoso.com".
        Dim ipHostInfo As IPHostEntry = Dns.Resolve(Dns.GetHostName())
        Dim ipAddress As IPAddress = ipHostInfo.AddressList(0)
        Dim localEndPoint As New IPEndPoint(ipAddress, 11000)
        
        ' Intializes a TCP/IP socket.
        Dim listener As New Socket(AddressFamily.InterNetwork, _
            SocketType.Stream, ProtocolType.Tcp)
        
        ' Bind the socket to the local endpoint and listen for incoming 
        ' connections.
        Try
            listener.Bind(localEndPoint)
            listener.Listen(100)
            
            While True
                ' Set the event to nonsignaled state.
                allDone.Reset()
                
                ' Start an asynchronous socket to listen for connections.
                Console.WriteLine("Waiting for a connection...")
                listener.BeginAccept(New AsyncCallback(AddressOf AcceptCallback), _
                listener)
                
                ' Wait until a connection is made before continuing.
                allDone.WaitOne()
            End While
        
        Catch e As Exception
            Console.WriteLine(e.ToString())
        End Try
        
        Console.WriteLine(ControlChars.Cr + "Press ENTER to continue...")
        Console.Read()
    End Sub 'StartListening
    
    
    Public Shared Sub AcceptCallback(ar As IAsyncResult)
        ' Signal the main thread to continue.
        allDone.Set()
        
        ' Get the socket that handles the client request.
        Dim listener As Socket = CType(ar.AsyncState, Socket)
        Dim handler As Socket = listener.EndAccept(ar)
        
        ' Create the state object.
        Dim state As New StateObject()
        state.workSocket = handler
        handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, _
            New AsyncCallback(AddressOf ReadCallback), state)
    End Sub 'AcceptCallback
    
    
    Public Shared Sub ReadCallback(ar As IAsyncResult)
        Dim content As [String] = [String].Empty
        
        ' Retrieve the state object and the handler socket
        ' from the asynchronous state object.
        Dim state As StateObject = CType(ar.AsyncState, StateObject)
        Dim handler As Socket = state.workSocket
        
        ' Read data from client socket. 
        Dim bytesRead As Integer = handler.EndReceive(ar)
        
        If bytesRead > 0 Then
            ' 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.IndexOf("<EOF>") > - 1 Then
                ' All the data has been read from the 
                ' client. Display it on the console.
                Console.WriteLine("Read {0} bytes from socket. " + _
                    ControlChars.Cr + " 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(AddressOf ReadCallback), state)
            End If
        End If
    End Sub 'ReadCallback
    
    
    Private Shared Sub Send(handler As Socket, data As [String])
        ' Convert the string data to byte data using ASCII encoding.
        Dim byteData As Byte() = Encoding.ASCII.GetBytes(data)
        
        ' Begin sending the data to the remote device.
        handler.BeginSend(byteData, 0, byteData.Length, 0, _
            New AsyncCallback(AddressOf SendCallback), handler)
    End Sub 'Send
    
    
    Private Shared Sub SendCallback(ar As IAsyncResult)
        Try
            ' Retrieve the socket from the state object.
            Dim handler As Socket = CType(ar.AsyncState, Socket)
            
            ' Complete sending the data to the remote device.
            Dim bytesSent As Integer = handler.EndSend(ar)
            Console.WriteLine("Sent {0} bytes to client.", bytesSent)
            
            handler.Shutdown(SocketShutdown.Both)
            handler.Close()
        
        Catch e As Exception
            Console.WriteLine(e.ToString())
        End Try
    End Sub 'SendCallback
    
    'Entry point that delegates to C-style main Private Function.
    Public Overloads Shared Sub Main()
        System.Environment.ExitCode = _
        Main(System.Environment.GetCommandLineArgs())
    End Sub
    
    Overloads Public Shared Function Main(args() As [String]) As Integer
        StartListening()
        Return 0
    End Function 'Main
End Class 'AsynchronousSocketListener
[C#]
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 {
    
    // Incoming data from client.
    public static string data = null;

    // 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 "host.contoso.com".
        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 {
            listener.Bind(localEndPoint);
            listener.Listen(100);

            while (true) {
                // Set the event to nonsignaled state.
                allDone.Reset();

                // Start an asynchronous socket to listen for connections.
                Console.WriteLine("Waiting for a connection...");
                listener.BeginAccept( 
                    new AsyncCallback(AcceptCallback),
                    listener );

                // Wait until a connection is made before continuing.
                allDone.WaitOne();
            }

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

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

    public static 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);

        // 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.
            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.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);

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

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


    public static int Main(String[] args) {
        StartListening();
        return 0;
    }
}

See Also

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

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