PRB: GetActiveWindow Behaves Differently Under Windows NT

ID: Q129852

The information in this article applies to:

Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 32-bit only, for Windows, version 4.0

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

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

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

Add a Command Button (Command1) to Form1.

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

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
Version : 4.00
Platform : NT WINDOWS
Issue type :