PRB: GetActiveWindow Behaves Differently Under Windows NT

• Standard, Professional, and Enterprise Editions of Microsoft Visual Basic, 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)

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.