Crossthreading not working

Question

1.67/5 (3 votes)

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Hi,

This is my code:

VB

Private Delegate Sub UpdateListInvoker(ByVal WhatToWrite As String)
  Private Sub UpdateListBox(ByVal WhatToWrite As String)
      If Form1.ListBox2.InvokeRequired Then
          Form1.ListBox2.Invoke(New UpdateListInvoker(AddressOf UpdateListBox), WhatToWrite)
      Else
          Form1.ListBox2.Items.Add(WhatToWrite)
      End If
  End Sub

To use: UpdateListBox("Hello")

It just isn't doing anything. Why?

-Rixterz

Posted 12-Jun-14 6:24am

Rixterz123

Updated 15-Jun-14 7:01am

v2

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Comments

OriginalGriff 12-Jun-14 12:31pm

Are you calling it?
What happens in the debugger?

[no name] 12-Jun-14 12:35pm

When the form loads, there is Receiver.RunWorkerAsync(), where Receiver is a backgroundworker.
This is Receiver's DoWork sub:

Private Sub Receiver_DoWork(sender As System.Object, e As System.ComponentModel.DoWorkEventArgs) Handles Receiver.DoWork
Try
Dim TCPListener As New TcpListener(IncomingPort)
TCPListener.Start()
While True
Dim client As New ClientConnection(TCPListener.AcceptTcpClient)
End While
Catch ex As Exception

End Try
End Sub

And in the ClientConnection class, this code calls the update:

Public Sub ReceiveMessage(ByVal ar As IAsyncResult)
Dim bytesRead As Integer
Try
SyncLock TCPClient.GetStream
bytesRead = TCPClient.GetStream.EndRead(ar)
End SyncLock
If bytesRead < 1 Then
AllClients.Remove(ClientIP)
UpdateListBox("Disconnected")
Exit Sub
Else
Dim MessageReceived As String = System.Text.Encoding.ASCII.GetString(data, 0, bytesRead)
If MessageReceived.StartsWith("PCDetails") Then
Dim MachineNameAndUserName As String = MessageReceived.Split(":")(1)
UpdateListBox("Details received: " & MachineNameAndUserName)
End If
End If
SyncLock TCPClient.GetStream
TCPClient.GetStream.BeginRead(data, 0, CInt(TCPClient.ReceiveBufferSize), AddressOf ReceiveMessage, Nothing)
End SyncLock
Catch ex As Exception
AllClients.Remove(ClientIP)
UpdateListBox("Disconnected")
End Try
End Sub

Duncan Edwards Jones 16-Jun-14 8:01am

This loop :-

While True
Dim client As New ClientConnection(TCPListener.AcceptTcpClient)
End While

Doesn't seem to do anything with the received client connection. Is there something in the constructor that does or..?

[no name] 16-Jun-14 14:57pm

It calls the New() sub in the class ClientConnection. The communications are fine, but the listbox won't update when a message is received.

[no name] 12-Jun-14 12:39pm

I forgot to say that nothing shows up in the debugger.

OriginalGriff 12-Jun-14 14:22pm

Which means what? Does the method get executed by both threads? One? None?

[no name] 12-Jun-14 14:42pm

In the clientconnection class, you can see UpdateListBox("Disconnected"),
UpdateListBox("Details received: " & MachineNameAndUserName),
and UpdateListBox("Disconnected") again. This is the only place they're called.

Dave Kreskowiak 12-Jun-14 15:42pm

Put a breakpoint on the if statement in the UpdateListBox code. Does the breakpoint ever get hit? If not, the problem is not the invoke but the code that's calling the UpdateListBox method.

[no name] 13-Jun-14 12:37pm

Actually, I can't use breakpoints because other things that this program does requires Administrator privileges so it elevates itself, thus not being under control of the debugger. I've established that the UpdateListBox does get called because I put a MsgBox in there, and also that the listbox update doesn't even work if done in a BackgroundWorker.

[no name] 15-Jun-14 13:00pm

BUMP

1 solution

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pdoxtader · Accepted Answer · 2014-06-17T03:54:00

Solution 1

Ok, so it looks like your trying to invoke a delegate on form1 from a different form in your project, and when I tested THAT (as opposed to a thread started from form1), I had the same problem you did.

You need to make a refrence to form1 available to the background thread (or from form2). This is how I got it to work (calling a delegate on form1 from form2) in my quick test app:

Try this:

VB

' Form1 code:

' Define your delegate:
Public Delegate Sub UpdateListInvoker(ByVal whatToWrite As String)

' Create your reference object:
Public UpdateListBox As UpdateListInvoker

' Your callback:
Public Sub ListBoxUpdater(ByVal WhatToWrite As String)
   If Me.InvokeRequired Then
        Me.Invoke(UpdateListBox, WhatToWrite)
   Else
        Me.ListBox2.Items.Add(WhatToWrite)
    End If
End Sub

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    UpdateListBox = AddressOf ListBoxUpdater
    Form2.f1 = Me
    Form2.Show()
End Sub


' Form2 code:
Public f1 As Form1

Private Sub bgThread()
    ' Elsewhere in your code...
    f1.UpdateListBox("what you're adding to the listbox")
End Sub

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Dim t As New Threading.Thread(AddressOf bgThread)
    t.Start()
End Sub

You see that in form2, I'm starting a background thread when I click a button, and then from the background thread I'm updating a listbox on form1 by using it's reference (f1).

This should do the trick for you.

-Pete

Posted 17-Jun-14 3:54am

pdoxtader

Updated 18-Jun-14 3:41am

v9

Comments

[no name] 17-Jun-14 14:07pm

Nope. I recorded the application in action here: https://hostr.co/aTp4uOx2tZgh

pdoxtader 18-Jun-14 7:41am

Ok, well this IS the correct way to use delegates. I've done it many times. I'm going to put together a quick and dirty test app to verify that I haven't forgotten anything here... but again: this IS the correct way to do it. If this isn't working for you, then there's something else wrong in your code that we can't see here given what you've posted.

pdoxtader 18-Jun-14 8:17am

Updated the solution. I think this is what you need.

[no name] 18-Jun-14 13:47pm

I'm afraid it's not as easy as that. You'll only understand if you view my solution here: https://hostr.co/mU0XJvlbqJJ8

pdoxtader 18-Jun-14 14:30pm

Here ya go: https://hostr.co/Z9xq2ggnIknt

Let me know if you have any trouble.

[no name] 18-Jun-14 15:09pm

Still not working. Please run the server before the other application: https://hostr.co/l0AWrOvYRQcD

pdoxtader 18-Jun-14 15:44pm

The problem this time was that we were using client before it was finished being created. Change ClientConnection's constructor to look like this:

Public Sub New(ByVal Client As TcpClient, ByRef _callback As UpdateListInvoker)
UpdateListBox = _callback
TCPClient = Client
ClientIP = Client.Client.RemoteEndPoint.ToString
AllClients.Add(ClientIP, Me)
Dim notice As New Threading.Thread(AddressOf SendConnectedNotice)
notice.Start

ReDim data(TCPClient.ReceiveBufferSize)
TCPClient.GetStream.BeginRead(data, 0, CInt(TCPClient.ReceiveBufferSize), AddressOf ReceiveMessage, Nothing)
End Sub

Private Sub SendConnectedNotice()
Threading.Thread.Sleep(100)
UpdateListBox("Connected: " & ClientIP)
End Sub

And that should be it. I have it working on my machine.

Oh, and the last line of code in the constructor should be "notice.start"... that way by the time we try to use client in form1, it's finished being created. Meaning I think you should move the "TCPClient.GetStream.BeginRead..." out of the constructor. Maybe put it in it's own function.

-Pete

[no name] 18-Jun-14 16:29pm

Wow! It works! I got an appcrash first of all because of a NullReferenceException on the line:
Me.Invoke(client.UpdateListBox, WhatToWrite)
because Dim client As ClientConnection left it empty and:
Dim client = New ClientConnection(TCPListener.AcceptTcpClient, AddressOf ListBoxUpdater)
didn't change it to anything. So I removed the "Dim" and it worked great. My project was at a standstill but now you've kickstarted it again-thanks very much!

pdoxtader 18-Jun-14 16:39pm

Oh... yeah. I forgot to mention that client isn't a local variable anymore. Yep - getting rid of that Dim was exactly the right thing to do.

Awesome. I'm glad it worked out for you. Your welcome... and if You click "accept solution", I'd really appreciate it.

Good luck with the rest of the project.

-Pete

[no name] 19-Jun-14 16:18pm

Hi,

I have another little problem with this project - the progressbar will not update because of the error "This operation has already had OperationCompleted called on it and further calls are illegal." The new project is here: https://hostr.co/Tam43xZQcVAv

-Rixterz