Question

I’ve got the following situation,

Private Sub MyButton_Click(sender as Object, args as EventArgs) Handles MyButton.Click
    Me.pleaseWaitFrm = New PleaseWaitForm()
    ' Fire up new thread to do some work in (AddressOf DoMyWork)
    Me.pleaseWaitFrm.ShowDialog()
End Sub

Private Sub DoMyWork()
   Dim log = Me.DoTheActualWork()

   Me.pleaseWaitFrm.Close()

   Using logFrm as New LogViewer(log)
       logFrm.ShowDialog()
   End Using
End Sub

If the DoTheActualWork() call exits fast enough, the Me.pleaseWaitFrm.Close() call is happening during the Me.pleaseWaitFrm.ShowDialog() call. The result, no surprise, is this exception:

An unhandled exception of type ‘System.InvalidOperationException’ occurred in System.Windows.Forms.dll

Additional information: Value Close() cannot be called while doing CreateHandle().

Obviously this is the old "you cannot .Close() a WinForm while it’s in the process of loading" problem. But what isn’t obvious to me is how best to prevent that from happening in this case? How does one safely and reliably delay the .Close() until it is safe to do so in this situation?

Answer 1

Me.pleaseWaitFrm.Close()

That’s an illegal call, you are not allowed to close a form from another thread. Not sure how you got away with it, that should raise an IllegalOperationException when you run with a debugger. Review your code and delete any assignment to the Control.CheckForIllegalCrossThreadCalls property.

This exception you’re getting is a clear side-effect of this. You must use Control.Invoke() to get the dialog closed. This automatically solves your problem, the delegate target cannot execute until the dialog is loaded.

Leverage the BackgroundWorker class, it makes this easy. You can close the dialog in a RunWorkerCompleted event handler.