DOCUMENT:Q184332  23-APR-1998  [vbwin]
TITLE   :BUG: UserControl in VB4 or VC++ 5.0 Container Causes IPF on Exit
PRODUCT :Microsoft Visual Basic for Windows
PROD/VER:WINDOWS:4.0,5.0; WINNT:5.0
OPER/SYS:WINDOWS winnt
KEYWORDS:

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

 - Microsoft Visual Basic Learning, Professional, and Enterprise Editions
   for Windows, version 5.0
 - Microsoft Visual Basic Standard, Professional, and Enterprise Editions,
   32-bit only, for Windows, version 4.0
 - Microsoft Visual C++, 32-bit Editions, version 5.0
---------------------------------------------------------------------

SYMPTOMS
========

An application compiled in Visual Basic 4.0 or Visual C++ 5.0 generates an
"Invalid Page Fault in MSVBVM50.DLL" error when the application is closed.

CAUSE
=====

This error can occur in the following situation:

- The Visual Basic 4.0 or Visual C++ 5.0 application contains a UserControl
  created in Visual Basic 5.0.

  -and-

- The UserControl contains a Class that creates a reference to the
  UserControl.

  -and-

- You are running Windows 95.

RESOLUTION
==========

To workaround this problem, use the ObjPtr function in combination with the
CopyMemory API call to create the UserControl reference in the Class
Module. The steps to complete the workaround are listed in the MORE
INFORMATION section of this article.

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 5.0.

2. Name the project "passcontrol".

3. Add a Class Module to the project.

4. Add the following code to Class1:

      Private mycontrol As passcontrol.UserControl1

      Friend Sub passSelf(obj As passcontrol.UserControl1)
        Set mycontrol = obj
        mycontrol.SayHello
      End Sub

      Private Sub Class_Terminate()
        Set mycontrol = Nothing
      End Sub

5. Add the following code to UserControl1:

      Private myclass As New passcontrol.Class1

      Private Sub UserControl_Initialize()
        myclass.passSelf Me
      End Sub

      Friend Sub SayHello()
        MsgBox "Hello!"
      End Sub

6.  On the File menu, click "Make Passcontrol.ocx" to build the control.

7.  Create a new project in Visual Basic 4.0 (32 bit) or Visual C++ 5.0.

8.  Add Passcontrol.ocx to your project's form.

9.  Compile (or build) the Visual Basic or Visual C++ application.

10. Run the application on a computer running Windows 95.

11. Exit the application. You should receive the error "<application>
   caused an Invalid Page Fault in Module MSVBVM50.DLL."

WARNING: If you reproduce this problem, it is advisable to restart the
computer you are testing on after you receive the error. Passcontrol.ocx is
locked by the system, and any attempt to manipulate this file causes your
computer to freeze.

Workaround
----------

In the Class Module, substitute the following code:

   Private Declare Sub CopyMemory _
         Lib "kernel32" Alias "RtlMoveMemory" _
            (pDest As Any, _
            pSrc As Any, _
            ByVal ByteLen As Long)

   Private mycontrol As passcontrol.UserControl1

   Friend Sub passSelf(ByVal ptrobj As Long)
      CopyMemory mycontrol, ptrobj, 4
      mycontrol.SayHello
   End Sub

   Private Sub Class_Terminate()
      Set mycontrol = Nothing
   End Sub

In the UserControl, substitute the following code:

   Private myclass As New passcontrol.Class1

   Private Sub UserControl_Initialize()
      myclass.passSelf ObjPtr(Me)
   End Sub

   Friend Sub SayHello()
      MsgBox "Hello!"
   End Sub

Additional query words: VC++ C++ user control activex ocx
======================================================================
Version           : WINDOWS:4.0,5.0; WINNT:5.0
Platform          : WINDOWS winnt
Issue type        : kbbug

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

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Copyright Microsoft Corporation 1998.