FIX: Memory Leak Occurs After Form is Unloaded
ID: Q174289
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The information in this article applies to:
-
Microsoft Visual Basic Control Creation, Learning, Professional, and Enterprise Editions for Windows, version 5.0
-
Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 32-bit only, for Windows, version 4.0
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
- Start a new project. Form1 is created by default.
- Add another Form to the project.
- 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
- 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.
Keywords : kbVBp400 kbVBp500 kbGrpVB VB4WIN VBKBObj VBKBVB kb32bitOnly
Version : WINDOWS:4.0 5.0
Platform : WINDOWS
Issue type : kbbug