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Q192682: BUG: Dynamically-Added OCX Event Parameter Is Not Passed ByRef

Article: Q192682
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:6.0
Operating System(s): 
Keyword(s): kbGrpDSVB
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Learning Edition for Windows, version 6.0 
- Microsoft Visual Basic Professional Edition for Windows, version 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, version 6.0 
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SYMPTOMS
========

When working with an ActiveX control that is added dynamically at run-time, an
event procedure passes a Variant parameter by reference (ByRef) to the client,
but any changes made by the client are not returned to the control.

CAUSE
=====

This occurs when the event procedure is written to pass a Variant, but the
variable being passed by the control is not a Variant. This causes Visual Basic
to create a temporary Variant to use as the event parameter. When the client
code makes changes to the parameter, the changes are made to this temporary copy
and not to the original variable.

Under normal circumstances, Visual Basic updates the original variable with any
changes made to the Variant copy. However, this does not occur for event
procedures in a dynamically-loaded UserControl.

RESOLUTION
==========

To avoid the problem, make sure the data types of your UserControl event
procedures are as specific as possible. Also, see that the parameters used
during a RaiseEvent call match the expected parameter types.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

MORE INFORMATION
================

Steps to Reproduce Behavior
---------------------------

1. Create a Standard EXE project. Form1 is created by default.

2. Add a UserControl project to the group. This should be Project2.

3. Add a CommandButton to the UserControl and place the following in the code
  window for UserControl1:

        Event Click(X As Variant)

        Private Sub Command1_Click()
           Dim MyValue As Long  ' Notice MyValue is declared as Long
           MyValue = 1
           MsgBox "Value before event is raised: " & MyValue
           RaiseEvent Click(MyValue)
           MsgBox "Value after then event returns: " & MyValue
        End Sub

4. Close the UserControl designer.

5. In the code window for Form1, add the following:

        Dim WithEvents DynCtl As VBControlExtender

        Private Sub Form_Load()
           Set DynCtl = Controls.Add("Project2.UserControl1", "Tester")
           DynCtl.Visible = True
        End Sub

        Private Sub DynCtl_ObjectEvent(Info As EventInfo)
           ' Change the ByRef value to 1000000
           Info.EventParameters.Item(0).Value = 1000000
           MsgBox "Value after change in ObjectEvent: " & _
              Info.EventParameters.Item(0).Value
        End Sub

6. Press the F5 key to run Project1 and click on the CommandButton. You should
  see a message box showing the value of the original variable before raising
  the "Click" event. This is followed by another message box showing the value
  of the parameter after it has been changed by the client. Finally, the last
  message box shows the value of the original variable after the event has been
  raised. Note that the changes made by the client are not reflected in the
  last message box.

7. To correct the problem, change the declaration of the MyValue variable (in
  Project2.UserControl1's CommandButton code) from a Long to a Variant. Re-run
  Project1. The code should now work correctly because Visual Basic no longer
  needs to create a temporary variable for the event parameter.

REFERENCES
==========

For additional information, please see the following article in the Microsoft
Knowledge Base:

  Q190670 : HOWTO: Dynamically Add Controls to a Form with Visual Basic 6.0

Additional query words: kbDSupport kbDSD kbVBp kbCtrl kbVBp600bug

======================================================================
Keywords          : kbGrpDSVB 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB600Search kbVBA600 kbVB600
Version           : WINDOWS:6.0
Issue type        : kbbug

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