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Problem with accessing form controls in response to handling COM object events

Posted on 2006-10-26
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Last Modified: 2010-04-30
Hi all,

I am a newcomer to VB 2005, and have a project working that handles events from a COM object. Strangely, simple things -like displaying a message box inside the event handling subroutine, or changing the text of an edit control- work, but others, like starting a timer or updating a progress bar inside a status strip won't. In the code below, the attempt to set the progess bar to 50 in response to eventcode 1 does nothing and generates "A first chance exception of type 'System.InvalidOperationException' occurred in System.Windows.Forms.dll". The DelayedAction timer in response to eventcode 0 will not execute, or -strangely- only the second time it is started??? I have checked that these will work if called from inside the form1_load sub or anywhere else as long as the event handler is not "involved".

Friend Class Form1
Public WithEvents Camera As MaxIm.CCDCamera

Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
Camera = CreateObject("Maxim.CCDCamera")
Camera.LinkEnabled = True
Camera.EventMask = (-1)
Exposure_ProgressBar.Value = 0
Exposure_ProgressBar.Maximum = 100
End Sub

Private Sub Process_CameraEvent(ByVal CameraEventCode As Short)
Select Case CameraEventCode
Case 0
DelayedAction.Start()
Case 1
Exposure_ProgressBar.Value = 50
Case 8
Case 14
MsgBox("Connected")
Sequence_StatusText.Text = "Connected"
Case 15
MsgBox("Disconnected")
End Select
End Sub

Private Sub Camera_Notify(ByVal EventCode As Short) Handles Camera.Notify
Process_CameraEvent(EventCode)
End Sub

Private Sub DelayedAction_Tick(ByVal sender As Object, ByVal e As System.EventArgs) Handles DelayedAction.Tick
MsgBox("Delayed Action")
DelayedAction.Stop()
End Sub

End Class


Frankly, I don't know what to do next. But something must be wrong in how I've set this up. Thanks in advance for any hints!

Robert
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Question by:schling
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Expert Comment

by:Mike Tomlinson
ID: 17815633
Do you know if the MaxIm.CCDCamera is using Threads?
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Author Comment

by:schling
ID: 17815686
No, unfortunately I don't. But could ask their user support.
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Author Comment

by:schling
ID: 18032517
I did find the answer. The solution was to make  "thread-safe" calls via invoke and a delegate function.
Since the question whether Maxim uses threads lead me to investigate invoke, I'd agree to split points.

Robert
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