Re: Need Help with this Async code

Tech-Archive recommends: Repair Windows Errors & Optimize Windows Performance

From: Tom Shelton (tshelton_at_YOUKNOWTHEDRILLcomcast.net)
Date: 01/31/05 

Date: Sun, 30 Jan 2005 23:10:58 -0800

On 2005-01-30, Terry Olsen <tolsen64@hotmail.com> wrote:
> I'm trying to use Async Socket calls to create a tcp server. It goes into
> the listening correctly, but when I connect, it crashes with the following
> exception:
>
> An unhandled exception of type 'System.InvalidOperationException' occurred
> in system.dll
> Additional information: AcceptCallback
> There is no source code available for the current location.
>
> Here's the code. Hopefully someone can help me out.
> -----------------------------------------------------------
> Private sck As Socket 'Class level
>
> Private Sub btnStart_Click(ByVal sender As System.Object, _
> ByVal e As System.EventArgs) Handles btnStart.Click
> Dim addr As IPAddress = IPAddress.Parse("0.0.0.0")
> Dim ep As New IPEndPoint(addr, CInt(txtPort.Text))
> Dim sck As New Socket(AddressFamily.InterNetwork, _
> SocketType.Stream, ProtocolType.Tcp)
> sck.Bind(ep)
> sck.Listen(2)
> sck.BeginAccept(AddressOf AcceptCallback, Nothing)
> lblStatus.Text = "Listening"
> End Sub
>
> Private Sub AcceptCallback(ByVal ar As IAsyncResult)
> sck.EndAccept(ar) 'I believe this is my problem
> Dim bytes(4095) As Byte
> sck.BeginReceive(bytes, 0, bytes.Length, SocketFlags.None, _
> AddressOf ReceiveCallback, bytes)
> End Sub
>
> Private Sub ReceiveCallback(ByVal ar As IAsyncResult)
> Dim bytes() As Byte = CType(ar.AsyncState, Byte())
> Dim numbytes As Int32 = sck.EndReceive(ar)
> If numbytes = 0 Then
> Else
> End If
> End Sub
>
>

Terry...

Your trying to read off your server socket. When a socket connection is
accepted, the client is actually connected on a new socket so that the
server socket goes back to listening.

You should really be doing something like:

Private Sub AcceptCallback (ByVal ar As IAsyncResult)
        Dim clt As Socket = sck.EndAccept(ar)
        ...
        clt.BeginRecieve (....)
        sck.BeginAccept (...)
End Sub

You might try looking through the async socket examples here:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cpconsockets.asp

HTH 

-- 
Tom Shelton [MVP]

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