BUG: UserControl Causes VB IDE to Crash on Shutdown

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0

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

Keywords : kbGrpVB
Version :
Platform : WINDOWS
Issue type : kbbug