Imports System
Imports System.Net
Imports System.Net.Sockets
Imports System.Text
Imports System.Threading
Imports Microsoft.VisualBasic

' 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
    ' 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 ipHostInfo As IPHostEntry = Dns.Resolve(Dns.GetHostName())
        Dim ipAddress As IPAddress = ipHostInfo.AddressList(0)
        Dim localEndPoint As New IPEndPoint(ipAddress, 11000)

        ' Create 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.
        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 and processed before continuing.
            allDone.WaitOne()
        End While
    End Sub 'Main


    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 '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 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))

            ' 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. " + vbLf + " 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)
        ' 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()
        ' Signal the main thread to continue.
        allDone.Set()
    End Sub 'SendCallback
End Class 'AsynchronousSocketListener