How to crossthread over forms

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Hello,

I have some server code to accept a TCP connection from another computer and to display a message in a listbox on the server.

"Receiver" is the name of a backgroundworker.

I've only displayed relevant code here:

VB

Private Sub Receiver_DoWork(ParametersWereHere) Handles Receiver.DoWork
       Try
           Dim TCPListener As New TcpListener(IncomingPort)
           TCPListener.Start()
           While True
               client = New ClientConnection(TCPListener.AcceptTcpClient, AddressOf ListBoxUpdater)
           End While
       Catch ex As Exception
           MsgBox("3. " & ex.Message)
       End Try
   End Sub
   Private Sub ListBoxUpdater(ByVal WhatToWrite As String)
       If Me.InvokeRequired Then
     ***** Me.Invoke(client.UpdateListBox, WhatToWrite) *****
       Else
           Me.ListBox2.Items.Add(WhatToWrite)
       End If
   End Sub

And in the class ClientConnection:

VB

Public Delegate Sub UpdateListInvoker(ByVal WhatToWrite As String)
    Public UpdateListBox As UpdateListInvoker
    Public Sub New(ByVal Client As TcpClient, ByRef _callback As UpdateListInvoker)
        UpdateListBox = _callback
        TCPClient = Client
        ClientIP = Client.Client.RemoteEndPoint.ToString
        AllClients.Add(ClientIP, Me)
        UpdateListBox("Connected: " & ClientIP)
        ReDim data(TCPClient.ReceiveBufferSize)
        TCPClient.GetStream.BeginRead(data, 0, CInt(TCPClient.ReceiveBufferSize), AddressOf ReceiveMessage, Nothing)
    End Sub

The problem I'm facing is that on the starred line in the first code block, I get the error

object reference not set to an instance of an object

when I comment out the "try... catch... end try" statement in Receiver_DoWork().

The Watch window showed that "client" was empty, but it should have a ClientConnection assigned to it.

Can anyone see what's wrong here?

-Rixterz

Posted 8-Nov-14 10:49am

Ricky Fisher

Updated 8-Nov-14 10:59am

v5

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Comments

Sergey Alexandrovich Kryukov 8-Nov-14 21:36pm

There is no need in InvokeRequired. It will always return true, because you always call it in the non-UI thread (so, invoke is always required).
Where exactly the exception is thrown?
—SA

[no name] 9-Nov-14 6:35am

On the starred line

Sergey Alexandrovich Kryukov 9-Nov-14 13:08pm

I see, thank you. Use the debugger and inspect what object which are not supposed to be null appears null. Such problem is one of the easiest to debug and fix.
—SA

[no name] 9-Nov-14 14:22pm

I saw that client was empty.

Sergey Alexandrovich Kryukov 9-Nov-14 14:38pm

You are facing null problem, not empty...
—SA

[no name] 9-Nov-14 16:09pm

Sergey look where the TCPListener is declared.

Sergey Alexandrovich Kryukov 9-Nov-14 16:17pm

Yee... In a loop; amazing gibberish.
—SA

[no name] 9-Nov-14 16:28pm

More or less, but the main issue is that the TCP listeners TCP client collection of connections will never be accessible in its context because the declaration is inside the event which also runs on its own thread. thus making the client null or nothing since its inaccessible. Coverage of BackGround Worker .DoWork threading is mentioned in the CodeProject Article in my comment below or This Link

Sergey Alexandrovich Kryukov 9-Nov-14 16:47pm

I actually did not even took the code seriously, advised to use the debugger, but you right.
—SA

[no name] 9-Nov-14 18:02pm

Thanks. I normally don't give these chat app topics much interest either due to OP inexperience and lack of research. However, using the debugger here would give the OP some experience.

Sergey Alexandrovich Kryukov 9-Nov-14 20:13pm

Right idea for OP: server side needs at least two communication threads: one listens for new connection, and another thread reads/writes from/to network stream.
—SA

[no name] 9-Nov-14 20:51pm

I agree, and the delegates on the client class and server class should be set off on new threads. The declarations for the Listener/TCPClient should be declared at class level.

@OP (Declarations inside subroutines, makes them only accessible to the sub structures that they reside in. Also note: If a thread terminates, you can't get access to its resources.) So move them to class level.

[no name] 9-Nov-14 9:55am

Maybe this might help you with some pointers when dealing with threads.

http://www.codeproject.com/Articles/23517/How-to-Properly-Handle-Cross-thread-Events-and-Upd

And here is a .NET example:

http://tech.xster.net/tips/invoke-ui-changes-across-threads-on-vb-net/

[no name] 9-Nov-14 10:45am

But that's not the problem. I need to figure out why client is empty.

[no name] 9-Nov-14 12:50pm

That is one of your problems because your use of the delegate here is wrong. See the examples I posted, and you can see where you are going wrong, and each client should be declared New, and not inside the subs. (This is why its important to build a reusable structure when writing code.) And you shouldn't be working in your UI threads either. That's what multi threading is for; hence, why I provided the links for you.

[no name] 9-Nov-14 14:22pm

Again, that's not the problem.

[no name] 9-Nov-14 16:06pm

You seem to know what's not wrong, so why don't you know what's not right?

If you were so smart or understood basic structures, you would know that your TCPListner is declared at a non class level. So you can never access the collection of clients it accepts. Move that declaration to class level for a start. Or take the hard road of cross-threading the multi threaded avenue by invoking a delegate sub on a new thread to access the TCP client collection of connections. (Not advised and more complicated.)

Secondly, If you are handling your client connections in a background worker thread, you should have a basic understanding of multi threading in order to make proper use of a delegate sub to invoke controls/variables in cross thread operations since the BackGround Worker operates on its own thread.

So please tell me how that's not the problem with your use of delegates on your client and server classes?

I think you need to rethink your structuring to make use of reusable code.

DoWork is run on its own thread. See: Threading VS DoWork

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