Cooperative levels describe how DirectDraw interacts with the display and how it reacts to events that might affect the display. Use the IDirectDraw4::SetCooperativeLevel method to set cooperative level of DirectDraw. For the most part, you use DirectDraw cooperative levels to determine whether your application runs as a full-screen program with exclusive access to the display or as a windowed application. However, DirectDraw cooperative levels can also have the following effects:
The normal cooperative level indicates that your DirectDraw application will operate as a windowed application. At this cooperative level you won't be able to change the primary surface's palette or perform page flipping.
Because applications can use DirectDraw with multiple windows, IDirectDraw4::SetCooperativeLevel does not require a window handle to be specified if the application is requesting the DDSCL_NORMAL mode. By passing a NULL to the window handle, all of the windows can be used simultaneously in normal Windows mode.
At the full-screen and exclusive cooperative level, you can use the hardware to its fullest. In this mode, you can set custom and dynamic palettes, change display resolutions, and implement page flipping. The exclusive (full-screen) mode does not prevent other applications from allocating surfaces, nor does it exclude them from using DirectDraw or GDI. However, it does prevent applications other than the one currently with exclusive access from changing the display mode or palette.
DirectDraw takes control of window activation events for full-screen, exclusive mode applications, sending WM_ACTIVATEAPP messages to the window handle registered through the SetCooperativeLevel method as needed. DirectDraw only sends activation events to the top-level window. If your application creates child windows that require activation event messages, it is your responsibility to subclass the child windows.
SetCooperativeLevel maintains a binding between a process and a window handle. If SetCooperativeLevel is called once in a process, a binding is established between the process and the window. If it is called again in the same process with a different non-null window handle, it returns the DDERR_HWNDALREADYSET error value. Some applications may receive this error value when DirectSound® specifies a different window handle than DirectDraw—they should specify the same, top-level application window handle.
Note Developers using Microsoft Foundation Classes (MFC) should keep in mind that the window handle given to the SetCooperativeLevel method should identify the application's top-level window, not a derived child window. To retrieve your MFC application's top level window handle, you could use the following code:
HWND hwndTop = AfxGetMainWnd()->GetSafeHwnd();
See also, Multiple Monitor Systems.