Imports System
Imports System.Net
Imports System.Net.Sockets
Imports System.Text
Imports System.Threading
 
Public Class Form1
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        Call AsynchronousSocketListener.Main()
    End Sub
End Class
'-------------------------------
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
'-------------------------------
Public Class AsynchronousSocketListener
    ' Thread signal.
    Public Shared allDone As New ManualResetEvent(False)
    ' This server waits for a connection and then uses  asychronous operations 
    ' to
    ' accept the connection, get data from the connected client, 
    ' echo that data back to the connected client.
    ' It then disconnects from the client and waits for another client. 
    Public Shared Sub Main()
        ' Data buffer for incoming data.
        Dim bytes() As Byte = New [Byte](1023) {}
        ' Establish the local endpoint for the socket.
        Dim localEndPoint As New IPEndPoint(Net.IPAddress.Any, 3000)
        ' Create a TCP/IP socket.
        'Dim listener As New Socket(AddressFamily.InterNetwork, 
        ' SocketType.Stream, ProtocolType.Tcp)
        Dim listener As New Socket(AddressFamily.InterNetwork, _
          SocketType.Stream, ProtocolType.Tcp)
        ' Bind the socket to the local endpoint and listen for incoming 
        ' connections.
        listener.Bind(localEndPoint)
        listener.Listen(1000)
        While True
            ' Set the event to nonsignaled state.
            allDone.Reset()
            ' Start an asynchronous socket to listen for connections.
            listener.BeginAccept(New AsyncCallback(AddressOf AcceptCallback), _
              listener)
            ' Wait until a connection is made and processed before continuing.
            allDone.WaitOne()
        End While
    End Sub
 
    Public Shared Sub AcceptCallback(ByVal ar As IAsyncResult)
        ' Get the socket that handles the client request.
        Dim listener As Socket = CType(ar.AsyncState, Socket)
        ' End the operation.
        Dim handler As Socket = listener.EndAccept(ar)
        ' Create the state object for the async receive.
        Dim state As New StateObject
        state.workSocket = handler
        handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, New _
          AsyncCallback(AddressOf ReadCallback), state)
    End Sub
 
    Public Shared Sub ReadCallback(ByVal 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 the 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))
            content = state.sb.ToString()
            Send(handler)
        End If
    End Sub
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