Problem
1: The DriveListBox control crashes Visual Basic when used with ReSize.
Problem 2: The DriveListBox fails to drop
down the list of drives when used with ReSize.
Last updated:
January 21, 2007
Applies to: The 16 and 32 bit
versions of VB 4.0, VB 5.0, (VB 6.0? see footnote).
Cause: This is due to a bug in the
DriveListBox control.
Background:
If the font of a DriveListBox is reduced in size past a certain
point it will cause the DriveListBox to fail to drop down its list of
drives. This happens in either VB 4.0 or VB 5.0. The DriveListBox
appears to work correctly in VB 3.0. In VB 4.0 the DriveListBox also
severely corrupts internal memory structures when this happens. With
just the right combination of controls and events it is possible to
produce a situation where controls disapear from the form. It is also
possible to crash other programs and corrupt the system registry under
Windows 95 when the DriveListBox currupts memory. Basically the
operating system becomes unstable when the memory is corrupted and that
can lead to just about anything. We have contacted Microsoft about this
problem and they have now issued a problem report which can be found
under Article ID: Q191104
in the Microsoft knowledge base.
Unfortunately, any resizing technique that
is used in combination with the DriveListBox will suffer from this
problem. Even native VB code can not overcome the problem.
Remedy:
Don't use the DriveListBox control. If possible, use the
CommonDialog control. Another alternative would be to use VB 5.0 to
write your own DriveListBox control. Yet another alternative would be to
use the DriveListBox, but don't resize the font. As long as the font is
not made smaller at runtime you will not experience the problem.
Steps to recreate problem:
To demonstrate this problem create a VB program with one form
containing a CommandButton and a DriveListBox. Place the following line
of code in the click event of a command button:
Drive1.FontSize = Drive1.FontSize * 0.8
Run the program and verify that the
DriveListBox will initially drop down. Now press the CommandButton and
then try to drop down the list. You may have to press the CommandButton
several times before the font size of the DriveListBox crosses the magic
point that causes it to fail. At some point the DriveListBox will fail
to drop down the list of available drives.
To demonstrate more serious memory
corruption problems under VB 4.0, try the following (at your own risk).
Create one form with two CommandButtons create and a DriveListBox and a
second form with nothing on it at all. In the click event of one of the
CommandButtons on the first form, place this line of code:
Form2.Show 1 'Show form two as modal
In the click event of the other
CommandButton place:
Drive1.FontSize = Drive1.FontSize * 0.8
Run the program. Press the first button
that shows Form2. Close Form2. Press the second button that changes the
font size of the DriveListBox. Try to drop down the list of the
DriveListBox. The program may hang, generate a fatal error, a control
may vanish from the form or all of the above.
Footnote:
We did limited testing and were not able to recreate this problem in
VB 6.0, however, the Microsoft knowledge base article (Q191104)
indicates that VB 6.0 is still a problem.
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