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eliminate cross thread errors in vb.net

Posted on 2013-10-28
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Last Modified: 2013-10-28
Hi,

I have a windows form app in VB.net, whereby the main form instantiates a class, Rtu, that has events. I added events to Rtu in order to get a MainForm textbox to display messages from the Rtu class.  Not sure if there's a better way send messages.

In any event, in the Rtu init I raise an event which is handled by the mainform class method messageHandler.  I added it like this:
            AddHandler rtu.broadcast, _
             AddressOf Me.messageHandler

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This worked fine on init, but later down the road I encountered a problem and got this error in the messageHandler callback:

System.InvalidOperationException was unhandled
  Message=Cross-thread operation not valid: Control 'displayTextbox' accessed from a thread other than the thread it was created on.
Private Sub messageHandler(ByVal e As String)
    Try
        displayTextbox.Text = e
    Catch ex As Exception
        displayTextbox.Text = "Command execution failed "
    End Try
End Sub

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I looked up the error and found what I thought was a solution here:
http://msdn.microsoft.com/en-us/library/ms171728(v=vs.85).aspx
which talks about thread-safe ways to access components and gives code, but
there was better source code here which I used:
http://www.vbforums.com/showthread.php?575929-Type-SetTextCallback-is-not-defined

The problem is that when the line
Me.Invoke(d, New Object() {msg}) 

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is executed, the program just hangs.

How can I fix this?
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Question by:ugeb
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8 Comments
 
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Expert Comment

by:Éric Moreau
ID: 39605230
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LVL 85

Expert Comment

by:Mike Tomlinson
ID: 39605429
"the program just hangs"

A hang on Invoke() indicates deadlock, which means the main UI thread is unable to service the Invoke() request.  This can happen if you've improperly used SyncLock(), or possibly if you've got a polling loop running in the main UI thread.  There are other possibilities...we'd need to see more code to diagnose.
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Author Comment

by:ugeb
ID: 39606335
I do check invokeRequired, which is why I have an invoke statement later on.  This is the code block from mainform:
    Private Delegate Sub SetTextCallback(ByVal [text] As String)

    Private Sub messageHandler(ByVal msg As String)
        Try
            If Me.displayTextbox.InvokeRequired Then
                Dim d As New SetTextCallback(AddressOf messageHandler)
                Me.Invoke(d, New Object() {msg})
            Else
                Me.displayTextbox.Text = msg
            End If
        Catch ex As Exception
            displayTextbox.Text = "Command execution failed "
        End Try
    End Sub

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I have a DDE (Ndde library) system running in the rtu instance which raises the event every time there's a call back.  It runs asynchronously and calls a method in rtu (whenever it has data) which in turn raises the event.
I left the catch block as invalid code on purpose while I'm debugging.  It never gets called.
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LVL 85

Accepted Solution

by:
Mike Tomlinson earned 500 total points
ID: 39606379
There's nothing wrong with that code.  You can try switching to BeginInvoke() to see if that makes a difference.

If not, then either you have code somewhere else that is mis-behaving (see my previous comment), or the Ndde library is the bad apple.
0
 
LVL 11

Author Comment

by:ugeb
ID: 39606513
Yes, BeginInvoke seemed to eliminate the deadlock.  Is there any housekeeping I need to do by using BeginInvoke instead of Invoke?  Will there be memory or other issues?
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LVL 85

Expert Comment

by:Mike Tomlinson
ID: 39606540
None that I can think of.  Invoke() is synchronous, while BeginInvoke() is asynchronous.  Other than that, they're the same...
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Author Closing Comment

by:ugeb
ID: 39606612
Thank you, that did the trick!
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