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(ByVal 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(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 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(ByVal handler As Socket, ByVal 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(ByVal 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

Public Overloads Shared Function Main(ByVal args() As [String]) As Integer
        StartListening()
        Return 0
    End Function 'Main
End Class 'AsynchronousSocketListener