A class background brush prepares the client area of a window for subsequent drawing by the application. The system uses the brush to fill the client area with a solid color or pattern, thereby removing all previous images from that location whether they belong to the window or not. The system notifies a window that its background should be painted by sending the WM_ERASEBKGND message to the window. For more information, see Brushes.
To assign a background brush to a class, create a brush by using the appropriate GDI functions and assign the returned brush handle to the hbrBackground member of the WNDCLASSEX structure.
Instead of creating a brush, an application can set the hbrBackground member to one of the standard system color values. For a list of the standard system color values, see SetSysColors.
To use a standard system color, the application must increase the background-color value by one. For example, COLOR_BACKGROUND + 1 is the system background color. Alternatively, you can use the GetSysColorBrush function to retrieve the handle of a brush that corresponds to a standard system color, and then specify the handle in the hbrBackground member of the WNDCLASSEX structure.
The system does not require that a window class have a class background brush. If this parameter is set to NULL, the window must paint its own background whenever it receives the WM_ERASEBKGND message.