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

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

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, Windows 98, or Windows Me.
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