Backgroundworker and Clipboard.Gettext()

[Beanoid]

i have a function that i can call from the main thread that works fine, i want to add it to a background worker as it is part of a long running loop

   Private Sub BackgroundWorker1_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
    lots of procedures

dim clipboardtext as string = clipboard.gettext()

lots of procedures
    End Sub

the problem is now the clipboard always returns empty, i assumed it is a threading issue so i have tried

   Private Sub BackgroundWorker1_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
    lots of precedures

        Me.Invoke(Sub()
                          GettingClipBoardText()
                      End Sub)



lots of procedures
    End Sub

and added to the main form

    Public Function GettingClipBoardText()
        ClipBoardText = Clipboard.GetText
    End Function

System.InvalidOperationException: 'Invoke or BeginInvoke cannot be called on a control until the window handle has been created.

any ideas?

[jmcilhinney]

That error message seems to indicate that that code is being executed before the form is displayed. The Invoke method invokes a method on the thread that owns the control's window handle. If the form hasn't been displayed then the handle hasn't been created, so there's no thread that owns it. I'm not sure why your form hasn't been shown at that stage but, if you were going to do it that way, you might need to start your BackgroundWorker from the Shown event handler of the form, to ensure that it was already displayed.

That said, you shouldn't do it that way. Why would you? What's the point of starting a BackgroundWorker to execute on a background thread and then immediately invoking to the UI thread? Just get the Clipboard text first, before starting the BackgroundWorker. Pass the text as an argument to RunWorkerAsync and then get it back in the DoWork event handler from the e.Argument property.

[jdc20181]

The forum logged me out before I could get my response done, while multitasking other things... You should not run background threads on UI threads and vice verse, instead you should pass one to another.

e.g.

 
    Private Sub BackgroundWorker1_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork


        ' Access the clipboard text directly from the background thread
        GettingClipBoardText()

        ' ... lots of procedures
    End Sub

    Public Sub GettingClipBoardText()
        ' Access the clipboard text directly from the background thread
  
        clipboardtext = Clipboard.GetText()
    End Sub
End Class

It is possible that its another piece of the code that is causing the overall function to fail. And without more than a small tidbit there is not much more anyone can do but guess.

[Beanoid]

i have altered my code not to rely on clipboard.gettext as it seems to have issues when called continuously from a loop

[.paul.]

You could hook the clipboard and only retrieve the contents when it has changed…
Would that be helpful?

[.paul.]

https://www.vbforums.com/showthread....=1#post3914485

[vbdotnut]

Assuming youre not setting the text somewhere in your "lots of procedures" and you are setting the text by means of Ctrl+C or right-click Copy, then you should be sending the text in as a parameter and getting it back out as such, as jmc has pointed out.

 Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
     BGW.RunWorkerAsync(Clipboard.GetText)
 End Sub

 Private Sub BGW_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles BGW.DoWork
     Dim ClipObj As Object = e.Argument
     Dim ClipTxt As String = String.Empty
     If ClipObj IsNot Nothing Then
         ClipTxt = ClipObj.ToString

         'Do Stuff

         'Send clip text back to UI thread if needed
         e.Result = ClipTxt
     End If
 End Sub

 Private Sub BGW_RunWorkerCompleted(sender As Object, e As RunWorkerCompletedEventArgs) Handles BGW.RunWorkerCompleted
     If e.Error Is Nothing Then
         Dim ClipText As String = e.Result.ToString
     End If
 End Sub