sockets - why does beginRecieve block?

Question

Ok, So I am using the MSDN example for asynchronous client sockets. The code I am using for the client is below. I'm just testing it right now by connecting to a telnet server installed on my computer as well as by connecting to the telnet port on a couple of cisco switches. The results are same for connecting to any of those telnet servers. The receiveCallback routine executes... If bytesRead > 0 comes up as true, which then causes client.BeginReceive to be called again. At this point the program hangs for a long time 30 secs or more (I'm assuming it hangs until the telnet server closes the connection). While it is hung the windows form is not responsive (can't even move it around the screen). Eventally, the the ReceiveCallback routine is called again, this time bytesRead > 0 is false and the program unhangs.

I get that I haven't built in logic to deal with messages etc, but why does beginReceive cause the ui to become unresponsive.

code:

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

Imports System

Imports System.Threading

Public Class Form2


    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        AsynchronousClient.Main()
    End Sub

End Class





' State object for receiving data from remote device.

Public Class StateObject
    ' Client socket.
    Public workSocket As Socket = Nothing
    ' Size of receive buffer.
    Public Const BufferSize As Integer = 256
    ' Receive buffer.
    Public buffer(BufferSize) As Byte
    ' Received data string.
    Public sb As New StringBuilder
End Class 'StateObject


Public Class AsynchronousClient
    ' The port number for the remote device.
    Private Const port As Integer = 23

    ' ManualResetEvent instances signal completion.
    Private Shared connectDone As New ManualResetEvent(False)
    Private Shared sendDone As New ManualResetEvent(False)
    Private Shared receiveDone As New ManualResetEvent(False)

    ' The response from the remote device.
    Private Shared response As String = String.Empty


    Public Shared Sub Main()
        'http://msdn.microsoft.com/en-us/library/bew39x2a.aspx
        'http://msdn.microsoft.com/en-us/library/bbx2eya8.aspx



        ' Establish the remote endpoint for the socket.
        ' For this example use local machine.
        '  Dim ipHostInfo As IPHostEntry = Dns.Resolve(Dns.GetHostName())
        Dim ipHostInfo As IPHostEntry = Dns.Resolve("127.0.0.1")

        Dim ipAddress As IPAddress = ipHostInfo.AddressList(0)
        Dim remoteEP As New IPEndPoint(ipAddress, port)

        ' Create a TCP/IP socket.
        Dim client As New Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp)

        ' Connect to the remote endpoint.
        client.BeginConnect(remoteEP, New AsyncCallback(AddressOf ConnectCallback), client)

        ' Wait for connect.
        connectDone.WaitOne()

        ' Send test data to the remote device.
        Send(client, "This is a test")
        ' Send(client, "GET")
        sendDone.WaitOne()

        ' Receive the response from the remote device.
        Receive(client)
        receiveDone.WaitOne()
        Debug.Print("got here")
        ' Write the response to the console.
        Debug.Print("Response received : " & response)

        ' Release the socket.
        client.Shutdown(SocketShutdown.Both)
        client.Close()
    End Sub 'Main


    Private Shared Sub ConnectCallback(ByVal ar As IAsyncResult)
        ' Retrieve the socket from the state object.
        Dim client As Socket = CType(ar.AsyncState, Socket)

        ' Complete the connection.
        client.EndConnect(ar)

        Debug.Print("Socket connected to " & client.RemoteEndPoint.ToString())

        ' Signal that the connection has been made.
        connectDone.Set()
    End Sub 'ConnectCallback


    Private Shared Sub Receive(ByVal client As Socket)

        ' Create the state object.
        Dim state As New StateObject
        state.workSocket = client

        ' Begin receiving the data from the remote device.
        client.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, New AsyncCallback(AddressOf ReceiveCallback), state)
    End Sub 'Receive


    Private Shared Sub ReceiveCallback(ByVal ar As IAsyncResult)

        ' Retrieve the state object and the client socket 
        ' from the asynchronous state object.
        Dim state As StateObject = CType(ar.AsyncState, StateObject)
        Dim client As Socket = state.workSocket

        ' Read data from the remote device.
        Dim bytesRead As Integer = client.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))

            'Get the rest of the data.
            client.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, New AsyncCallback(AddressOf ReceiveCallback), state)
        Else
            ' All the data has arrived; put it in response.
            If state.sb.Length > 1 Then
                response = state.sb.ToString()
            End If
            ' Signal that all bytes have been received.
            receiveDone.Set()
        End If
    End Sub 'ReceiveCallback


    Private Shared Sub Send(ByVal client 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.
        client.BeginSend(byteData, 0, byteData.Length, 0, New AsyncCallback(AddressOf SendCallback), client)
    End Sub 'Send


    Private Shared Sub SendCallback(ByVal ar As IAsyncResult)
        ' Retrieve the socket from the state object.
        Dim client As Socket = CType(ar.AsyncState, Socket)

        ' Complete sending the data to the remote device.
        Dim bytesSent As Integer = client.EndSend(ar)
        Debug.Print("Sent " & bytesSent & " bytes to server.")

        ' Signal that all bytes have been sent.
        sendDone.Set()
    End Sub 'SendCallback
End Class 'AsynchronousClient

sockets - why does beginRecieve block?

Answers (1)

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