Q129852: PRB: GetActiveWindow Behaves Differently Under Windows NT

Article: Q129852
Product(s): Microsoft Visual Basic for Windows
Version(s): 4.00
Operating System(s): 
Keyword(s): kbWndw
Last Modified: 11-JAN-2001

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The information in this article applies to:

- 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
========

If a Visual Basic version 4.0 application running under a 32-bit operating
system such as Windows NT calls the GetActiveWindow() API function and a window
of some other application currently has the focus, the handle of that window is
not returned; instead, a NULL is returned.

CAUSE
=====

In Windows NT, each thread of execution can set or get the focus for only those
windows created by the current thread. This prevents applications from
interfering with each other. One application's delay in responding cannot cause
other applications to suspend their response to user actions, as often happens
in Windows 3.x. Consequently, the following API functions also work differently
under Windows NT:

  GetCapture (VOID)
  GetFocus (VOID)
  ReleaseCapture (VOID)
  SetActiveWindow (HWND)
  SetCapture(HWND)
  SetFocus (HWND)

These functions now return NULL when the target window is owned by an application
other than the one that calls the function. Therefore, it is important to test
the return value of these functions before using it.

For example, if you call GetFocus and another thread's window has the input
focus, then instead of returning the handle of a window that belongs to another
thread, the function returns NULL. Similar considerations apply to GetCapture
and GetActiveWindow.

The Set functions can only specify a window created by the current thread. If you
attempt to pass a window handle created by another thread, the call to the Set
function fails.

RESOLUTION
==========

You can use the functions GetForegroundWindow and SetForegroundWindow instead of
GetActiveWindow and SetActiveWindow respectively, to manipulate windows created
by another thread. Note that these are "32-bit only" API functions.

STATUS
======

This behavior is by design.

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

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

1. Start a new project in Visual Basic. Form1 is created by default.

2. Add the following code to the General Declarations portion of Form1:

     Private Declare Function GetActiveWindow Lib "user32" () As Long
     Private Declare Function GetWindowTextA Lib "user32" (ByVal hwnd _
        As Long, ByVal lpString As String, ByVal aint As Long) As Long
     Private Declare Function GetForegroundWindow Lib "user32" () As Long

3. Add a Command Button (Command1) to Form1.

4. Add the following code to the Command1_Click event:

     Dim myhwnd As Long
     Dim s As String
     Dim iret As Long
     Dim nullhwnd As Long

     s = Space(256)

     myhwnd = GetActiveWindow()
     iret = GetWindowTextA(myhwnd, s, 256)
     Debug.Print myhwnd, s

     Shell "Calc.exe", 1

     ' Wait for CALC.EXE to launch under Windows NT:
     DoEvents

     nullhwnd = GetActiveWindow()
     myhwnd = GetForegroundWindow()

     s = Space(256)
     iret = GetWindowTextA(nullhwnd, s, 256)
     Debug.Print nullhwnd, s

     s = Space(256)
     iret = GetWindowTextA(myhwnd, s, 256)
     Debug.Print myhwnd, s

5. Press the F5 key to run the program. Click the Command1 button, and view the
  Debug Window. The first print statement will print Form1's hWnd and its
  Caption. The second print statement will merely print 0, because
  GetActiveWindow returns a null window handle. The third print statement will
  correctly print Calulator's hWnd and Caption.

Additional query words: 4.00 vb4win vb432

======================================================================
Keywords          : kbWndw 
Technology        : kbVBSearch kbAudDeveloper kbVB400Search kbVB400
Version           : 4.00
Issue type        : kbprb

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