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Q190519: BUG: UserControl Causes VB IDE to Crash on Shutdown

Article: Q190519
Product(s): Microsoft Visual Basic for Windows
Version(s): 
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, versions 5.0, 6.0 
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 
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SYMPTOMS
========

A UserControl is added to a Visual Basic project. When you try to exit the IDE,
Visual Basic causes an Invalid Page Fault (IPF) and terminates. This is preceded
by the error message:

  The memory could not be "written".

CAUSE
=====

The problem is that the UserControl has two module level variables, one that
holds onto a reference to the UserControl itself (Set m_oMe = Me) and another
that holds a reference to control's parent (Set m_oParent = Extender.Parent).
This causes a circular reference count on the control and the container. When
you try to exit the IDE, Visual Basic destroys the design-time form container
and attempts to release the UserControl instance. Because the control has a
reference to itself and the form, Visual Basic cannot properly destroy the
objects; consequently an Invalid Page Fault occurs.

RESOLUTION
==========

Avoid setting module level variables that reference both a control and its
container. Setting the variables to Nothing in the UserControl_Terminate event
will not avoid the error because the Terminate event will never fire if the
control has a circular reference.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. We are researching this bug and will post new
information here in the Microsoft Knowledge Base as it becomes available.

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

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

1. Start a new ActiveX Control project in Visual Basic. UserControl1 is created
  by default.

2. Copy the following code to the Code window of UserControl1:

        Option Explicit
        Private m_oMe As Object
        Private m_oParent As Object

        Private Sub UserControl_InitProperties()
           Set m_oMe = Me
           Set m_oParent = UserControl.Extender.Container
           Let UserControl.BackColor = &HFF0000
        End Sub

        Private Sub UserControl_Terminate()
           Set m_oMe = Nothing
           Set m_oParent = Nothing
        End Sub

3. [Optional] Save and compile Project1.

4. Close UserControl1's design window.

5. Choose Add Project from the File menu and add a Standard EXE project to the
  project group. Form1 is created by default.

6. Add the UserControl you created to Form1.

7. Now exit the Visual Basic IDE, by pressing ALT+Q or the close button on the
  title bar. When prompted to save changes, select "No". The IDE will crash on
  exit.

REFERENCES
==========

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

  Q189156 : BUG: Crash When Closing Application That Uses ActiveX DLL

  Q184332 : BUG: UserControl in VB4 or VC++ 5.0 Container Causes IPF on Exit

Additional query words: kbDSupport kbDSD kbVBp kbVBp600bug kbVBp500bug kbCtrlCreate kbIDE

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Keywords          : kbGrpDSVB 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500 kbVBA600 kbVB500 kbVB600
Issue type        : kbbug

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