Q174289: FIX: Memory Leak Occurs After Form is Unloaded

Article: Q174289
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:4.0 5.0
Operating System(s): 
Keyword(s): kbVBp400 kbVBp500 kbGrpDSVB kb32bitOnly
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Control Creation Edition for Windows, version 5.0 
- Microsoft Visual Basic Learning Edition for Windows, version 5.0 
- Microsoft Visual Basic Professional Edition for Windows, version 5.0 
- Microsoft Visual Basic Enterprise Edition for Windows, version 5.0 
- Microsoft Visual Basic Standard Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0 
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SYMPTOMS
========

A memory leak occurs in a Visual Basic project that has forms that are loaded
and unloaded several times.

CAUSE
=====

The small icon of a form remains in memory after the form is unloaded, which
results in a memory leak.

RESOLUTION
==========

To work around this bug in Visual Basic 4.0, use the DestroyIcon function during
the Form_Unload event to remove the small icon from memory. Copy the following
code sample to the code window of each form in your project:

     Private Declare Function SendMessage Lib "user32" _
        Alias "SendMessageA" _
        (ByVal hwnd As Long, _
         ByVal wMsg As Long, _
         ByVal wParam As Long, _
         lParam As Long) As Long

     Private Declare Function DestroyIcon Lib "user32" _
        (ByVal hIcon As Long) As Long

     Private Const WM_SETICON = &H80
     Private Const MiniIcon = 0
     Private Const NullIcon = 0

     Private Sub Form_Unload(Cancel As Integer)
        Dim hIcon As Long
        hIcon = SendMessage(hwnd, WM_SETICON, MiniIcon, ByVal _
        NullIcon)
        If hIcon <> 0 Then
           DestroyIcon hIcon
        End If
     End Sub

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug has been fixed in Visual Basic Version 5.0.

MORE INFORMATION
================

Programs that load and unload forms frequently can cause a significant leak in
memory because the operating system will lose 1148 bytes every time a form is
unloaded. The workaround is to destroy the icon when the form is unloaded.

Use the SendMessage function with the WM_SETICON message to get the handle of
either the large or small icon and to then replace it with the icon specified by
the NullIcon parameter; in this case replacing it with Null. With the MiniIcon
parameter set to zero, the message applies to the small icon.

The return value is the handle to the old icon which is removed from memory by
the DestroyIcon function.

Steps to Reproduce the Behavior
-------------------------------

1. Start a new project. Form1 is created by default.

2. Add another Form to the project.

3. Add the following code to the Click event of Form1:

        Dim i as Integer
        For i = 1 To 1000
           Form2.Show
           Form2.Hide
        Next i

4. Press the F5 key to run the project. You will notice a performance drop where
  Form2 loads more slowly as the iterations progress. Press the CTRL+BREAK keys
  to end the application.

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Keywords          : kbVBp400 kbVBp500 kbGrpDSVB kb32bitOnly 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVBA500Search kbVBA500 kbVB500 kbVB400Search kbVB400 kbZNotKeyword3
Version           : WINDOWS:4.0 5.0
Issue type        : kbbug
Solution Type     : kbfix

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