HOWTO: Access the Application Desktop from a Service

• Microsoft Win32 Application Programming Interface (API) included with: - Microsoft Windows NT versions 3.51, 4.0

SUMMARY

The configuration of a Windows NT service determines how it may access the application desktop. The application desktop is named "default" and belongs to the "WinSta0" Window station object. The application desktop is associated with the interactively logged-on user. A process that has access to the application desktop can display message boxes, windows, and dialog boxes that can be seen visually by the interactively logged-on user. In addition, a process with access to the application desktop can send messages to other processes running on the desktop. The application desktop is not destroyed when the interactively logged-on user logs off.

NOTE: Running interactive services under the LocalSystem account is a VERY dangerous practice. This is especially true of the command processor and batch files. A user who wants to control the system can just hit CTRL+C to get an interactive system command prompt.

MORE INFORMATION

A service that is configured in the LocalSystem account and is interacting with the desktop (the service type includes the SERVICE_INTERACTIVE_PROCESS flag) has access to the application desktop.

A service that is configured in the LocalSystem account and is not interacting with the desktop does not have access to the application desktop by default. If the service needs to display information through a message box, you can do this by specifying one of the following two flag types:

   MB_DEFAULT_DESKTOP_ONLY - The message box will appear on the application
                             desktop, for example, "winsta0\default".
   MB_SERVICE_NOTIFICATION - The message box will appear on the currently
                             active desktop.

Sample Code

   BOOL ThreadInteract(void)
   {
      HDESK   hdeskCurrent;
      HDESK   hdeskTest;
      HDESK   hdesk;
      HWINSTA hwinstaCurrent;
      HWINSTA hwinsta;

      //
      // Save the current Window station
      //
      hwinstaCurrent = GetProcessWindowStation();
      if (hwinstaCurrent == NULL)
         return FALSE;

      //
      // Save the current desktop
      //
      hdeskCurrent = GetThreadDesktop(GetCurrentThreadId());
      if (hdeskCurrent == NULL)
         return FALSE;

      //
      // Obtain a handle to WinSta0 - service must be running
      // in the LocalSystem account
      //
      hwinsta = OpenWindowStation("winsta0", FALSE,
                                  WINSTA_ACCESSCLIPBOARD   |
                                  WINSTA_ACCESSGLOBALATOMS |
                                  WINSTA_CREATEDESKTOP     |
                                  WINSTA_ENUMDESKTOPS      |
                                  WINSTA_ENUMERATE         |
                                  WINSTA_EXITWINDOWS       |
                                  WINSTA_READATTRIBUTES    |
                                  WINSTA_READSCREEN        |
                                  WINSTA_WRITEATTRIBUTES);
      if (hwinsta == NULL)
         return FALSE;

      //
      // Set the windowstation to be winsta0
      //
      if (!SetProcessWindowStation(hwinsta))
         return FALSE;

      //
      // Get the desktop
      //
      hdeskTest = GetThreadDesktop(GetCurrentThreadId());
      if (hdeskTest == NULL)
         return FALSE;

      //
      // Get the default desktop on winsta0
      //
      hdesk = OpenDesktop("default", 0, FALSE,
                            DESKTOP_CREATEMENU |
                  DESKTOP_CREATEWINDOW |
                            DESKTOP_ENUMERATE    |
                            DESKTOP_HOOKCONTROL  |
                            DESKTOP_JOURNALPLAYBACK |
                            DESKTOP_JOURNALRECORD |
                            DESKTOP_READOBJECTS |
                            DESKTOP_SWITCHDESKTOP |
                            DESKTOP_WRITEOBJECTS);
   if (hdesk == NULL)
           return FALSE;

   //
   // Set the desktop to be "default"
   //
   if (!SetThreadDesktop(hdesk))
           return FALSE;

   //
   // Do a message box
   //
   MessageBox(NULL, "MB_OK", "test_interact", MB_OK);

   //
   // Reset the Window station and desktop
   //
   if (!SetProcessWindowStation(hwinstaCurrent))
           return FALSE;

   if (!SetThreadDesktop(hdeskCurrent))
      return FALSE;

   //
   // Close the windowstation and desktop handles
   //
   if (!CloseWindowStation(hwinsta))
      return FALSE;

   if (!CloseDesktop(hdesk))
           return FALSE;

      return TRUE;
   }

Keywords          : BseService
Version           : WINDOWS NT:3.51,4.0
Platform          : NT WINDOWS
Issue type        : kbhowto