BUG: GDI Resource Leak Using Checkboxes in ListView Control

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

SYMPTOMS

While loading and unloading forms that contain ListView controls using Checkboxes, a large memory and GDI resource leak occurs.

CAUSE

The image list for the ListView control's Checkboxes is not being destroyed when the form is unloaded.

RESOLUTION

This problem can be worked around by setting the Checkboxes property at run-time rather than in the design environment. The Checkboxes property must be set to True in the form's Activate event and then to False in the form's QueryUnload event:

1. Open a new Standard EXE project. Form1 is generated by default.



2. From the Project menu, choose Components, select "Microsoft Windows Common Controls 6.0" and click OK.



3. Add a second form (Form2) to the project.



4. Add a ListView control (ListView1) to Form2. Leave the Checkboxes property for the control set to the default of False.



5. Add the following code to Form2's Activate Event:




Private Sub Form_Activate()
    ' Set the ListView's checkboxes property to True.
    ListView1.Checkboxes = True
    
    ' Do any initialization of the ListView here.
    With ListView1
        .ListItems.Add , , "Item 1"
        .ListItems.Add , , "Item 2"
    End With 
End Sub 

6. Add the following code to Form2's QueryUnload event:




Private Sub Form_QueryUnload(Cancel As Integer, UnloadMode As Integer)
    ' Set the checkboxes property
    ' of the listview to False.
    ListView1.Checkboxes = False
End Sub 

7. Add a CommandButton (Command1) to Form1. Add the following code to Form1's code window:




	Private Sub Command1_Click()
    		Dim f As Form2
  		Dim i As Long
  		For i = 0 To 100
    			Set f = New Form2
    			f.Show
    			Unload f
    			DoEvents
    			Set f = Nothing
  		Next i
	End Sub 

8. Start a program to monitor system resources as follows:



• Windows NT: Enter CTRL+ALT+DEL, and click Task Manager. Select the Performance tab.
• Windows 95/98: Start the System Monitor tool, which is located in the System Tools folder in the Accessories folder. If the System Monitor tool is not available, it can be installed from the Control Panel using the Add/Remove Programs option. On the Edit menu, click Add Item. Click the Memory Manager Category, click the Allocated Memory Item, and click OK.


9. Run the project and click on the Command1 button on Form1. Notice the resource usage.


Result: Excessive resource use doesn't occur.

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 Behavior

1. Open a new Standard EXE project. Form1 is generated by default.



2. From the Project menu, choose Components, check "Microsoft Windows Common Controls 6.0," and click OK.



3. Add a second form (Form2) to the project.



4. Add a ListView control (ListView1) to Form2 and set its Checkboxes property to True.



5. Add a CommandButton control (Command1) to Form1 and paste the following code into Form1's code window:




	Private Sub Command1_Click()
    		Dim f As Form2
  		Dim i As Long
  		For i = 0 To 100
    			Set f = New Form2
    			f.Show
    			Unload f
    			DoEvents
    			Set f = Nothing
  		Next i
	End Sub 

6. Start a program to monitor system resources as follows:



• Windows NT: Enter CTRL+ALT+DEL and click Task Manager. Select the Performance tab.
• Windows 95/98: Start the System Monitor tool, which is located in the System Tools folder in the Accessories folder. If the System Monitor tool is not available, it can be installed from the Control Panel using the Add/Remove Programs option. On the Edit menu, click Add Item. Click the Memory Manager Category, click the Allocated Memory Item, and click OK.


7. Run the project. Click on the Command1 button on Form1. On NT, you should watch as the memory used by the process climbs continuously while running the program, and is only cleaned up when exiting Visual Basic or the compiled program.


Result: Excessive resource use.

Additional query words:

Keywords          : kbsample kbCtrl kbListView kbResource kbVBp600bug kbGrpVB 
Version           : WINDOWS:6.0
Platform          : WINDOWS 
Issue type        : kbbug 

Last Reviewed: May 17, 1999