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

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, version 6.0

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 : kbGrpVB
Version : WINDOWS:6.0
Platform : WINDOWS
Issue type : kbbug