DOCUMENT:Q190519  07-AUG-1998  [vbwin]
TITLE   :BUG: UserControl Causes VB IDE to Crash on Shutdown
PRODUCT :Microsoft Visual Basic for Windows
PROD/VER:
OPER/SYS:WINDOWS
KEYWORDS:

======================================================================
---------------------------------------------------------------------
The information in this article applies to:

 - Microsoft Visual Basic Learning, Professional, and Enterprise Editions
   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:

   ARTICLE-ID: Q189156
   TITLE     : BUG: Crash When Closing Application That Uses ActiveX DLL

   ARTICLE-ID: Q184332
   TITLE     : BUG: UserControl in VB4 or VC++ 5.0 Container Causes IPF
               on Exit

Additional query words: kbDSupport kbDSD kbVBp kbVBp600bug kbVBp500bug
kbCtrlCreate kbIDE
======================================================================
Platform          : WINDOWS
Issue type        : kbbug

=============================================================================

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