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

ID: Q184332

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 or Windows 98.

RESOLUTION

To work around 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.

MORE INFORMATION

WARNING: One or more of the following functions are discussed in this article; VarPtr, VarPtrArray, VarPtrStringArray, StrPtr, ObjPtr. These functions are not supported by Microsoft Technical Support. They are not documented in the Visual Basic documentation and are provided in this Knowledge Base article "as is." Microsoft does not guarantee that they will be available in future releases of Visual Basic.

Steps to Reproduce Behavior

Start a new ActiveX Control project in Visual Basic 5.0.

Name the project "passcontrol."

Add a Class Module to the project.

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

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

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

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

Add Passcontrol.ocx to your project's form.

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

Run the application on a computer running Windows 95 or Windows 98.

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


Keywords          : kbCRT kbCtrlCreate kbVBp kbVBp400bug kbVBp500bug kbVC500bug 
Version           : WINDOWS:4.0,5.0; winnt:5.0
Platform          : WINDOWS winnt 
Issue type        : kbbug

Last Reviewed: June 10, 1999