BUG: UserControls with Menus Cause Resource Leak

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

SYMPTOMS

When a usercontrol has any top level menus where the NegotiateMenus property is non-zero, resources drain slowly when the focus is set to and from the usercontrol. The problem occurs in both the Visual Basic Design Environment (IDE) and as an EXE.

CAUSE

Visual Basic destroys the UserControl's main menu, but does not destroy all of the submenus under the main menu when the UserControl loses input focus.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article.

MORE INFORMATION

Steps to Reproduce and Work Around the Problem

1. Start a new Visual Basic Standard EXE project. Form1 is created by default. The NegotiateMenus property of Form1 is set to True by default.



2. On the Project menu, click Add User Control to add UserControl1 to the project.



3. Add a CommandButton and a CheckBox control to UserControl1.



4. Create a menu with several items and sub items on UserControl1. Set the NegotiatePosition property of each top level menu item to 1-Left.



5. Add the following code to the General Declarations section of UserControl1:




Option Explicit
'
'
' Demonstrates how to free menu resources orphaned by a UserControl.

' Zero-based array and one-based counter
' of cached UserControl submenus

Private m_ahMenus() As Long
Private m_nMenus As Integer

Private Declare Function GetMenu Lib "user32" (ByVal hwnd As Long) As Long
Private Declare Function GetSubMenu Lib "user32" (ByVal hMenu As Long, _
   ByVal nPos As Long) As Long
Private Declare Function GetMenuItemCount Lib "user32" _
   (ByVal hMenu As Long) As Long
Private Declare Function IsMenu Lib "user32" (ByVal hMenu As Long) As Long
Private Declare Function DestroyMenu Lib "user32" (ByVal hMenu As Long) _
   As Long

Private Declare Function GetParent Lib "user32" (ByVal hwnd As Long) As Long

Private Sub UserControl_EnterFocus()
   ' the UserControl's main menu has been created and has
   ' replaced the form's main menu before any control on the
   ' UserControl gains input focus.

   If Check1.Value And (m_nMenus = 0) Then
      Call LoadMenus(GetMenu(GetParent(UserControl.hwnd))) ' Form1.hwnd))
      Debug.Print "UserControl submenus loaded: " & m_nMenus
   End If
End Sub

Private Sub UserControl_ExitFocus()
   ' the UserControl's main menu has been destroyed, and has
   ' replaced been by the form's main menu. *But* all submenus
   ' under UserControl's main menu have not been destroyed.

   If m_nMenus Then
      Debug.Print "UserControl submenus destroyed: " & DestroyMenus
   End If
End Sub

Private Sub UserControl_Initialize()
   Check1.Caption = "Workaround"
   BorderStyle = 1
End Sub

Private Sub Check1_Click()
   If (Check1.Value = vbChecked) And (m_nMenus = 0) Then
      Call LoadMenus(GetMenu(GetParent(UserControl.hwnd)))
      Debug.Print "UserControl submenus loaded: " & m_nMenus
   ElseIf (Check1.Value = vbUnchecked) And m_nMenus Then
      Erase m_ahMenus ' Clear the module level variables
      m_nMenus = 0
      Debug.Print "No UserControl submenus will be destroyed"
   End If
End Sub

' Pass the UserControl's top level menu on first call

Private Sub LoadMenus(hMenu As Long)
   Dim nItems As Integer
   Dim i As Integer
   Dim hSubmenu As Long
   
   nItems = GetMenuItemCount(hMenu)

   For i = 0 To nItems - 1
      hSubmenu = GetSubMenu(hMenu, i)

      If IsMenu(hSubmenu) Then
         ReDim Preserve m_ahMenus(m_nMenus)
         m_ahMenus(m_nMenus) = hSubmenu
         m_nMenus = m_nMenus + 1
      End If

      Call LoadMenus(hSubmenu) ' recurse through sub menu
   Next
End Sub

Private Function DestroyMenus() As Integer
   Dim i As Integer
   Dim n As Integer

   For i = m_nMenus - 1 To 0 Step -1
      If IsMenu(m_ahMenus(i)) Then
         ' Will destroy all submenus under the current submenu, toggle the
         ' For expression comments above to destroy individual submenus.
         Call DestroyMenu(m_ahMenus(i))
         n = n + 1
      End If
   Next

  ' Clear the mod level variables
   Erase m_ahMenus
   m_nMenus = 0

   DestroyMenus = n

End Function 

6. Close all of UserControl1's open windows.



7. Add a CommandButton and a UserControl1 to Form1. UserControl11 is created by default.



8. Add the following code to the General Declarations section of Form1:




Private Sub Form_Click()
   Dim i As Integer
   For i = 1 To 2000
      UserControl11.SetFocus
      DoEvents ' IMPORTANT:  Allows SetFocus to complete before proceeding
      Command1.SetFocus
      DoEvents
      Me.Caption = i
   Next i
End Sub 

9. Save your project.



10. On Windows 95 or Windows 98, start the Resource Meter. The Windows NT Task Manager does not provide the same level of functionality.



11. Run the project, and click on the form. Under Win9x, the Resource Meter shows a steady decline in User and System resources.



12. Click the form again. On most systems, you observe problems with menu-related functionality such as Copy, Paste, and the Start menu. You may also see changes to fonts and icons used by the system.



13. Click the Stop button. You may receive an Out of Memory dialog, which you can dismiss. Terminate the Visual Basic IDE and observe that the resources are released.



14. Restart the Visual Basic IDE. Reload and run your project.



15. Select the checkbox labeled Workaround and click the form. The Resource Meter shows no decline in resources.

Additional query words:

Keywords : kbActiveX kbAPI kbCtrl kbMenu kbSDKWin32 kbVBp kbVBp500bug kbVBp600bug kbGrpVB kbDSupport
Version : WINDOWS:5.0,6.0
Platform : WINDOWS
Issue type : kbbug