This function retrieves the device context (DC) for the entire window, including title bar, menus, and scroll bars.
The handle of a device context for the specified window indicates success.
NULL indicates an error or an invalid hWnd parameter.
To get extended error information, call GetLastError.
GetWindowDC is intended for special painting effects within a window's nonclient area. Painting in nonclient areas of any window is not recommended.
The GetSystemMetrics function can be used to retrieve the dimensions of various parts of the nonclient area, such as the title bar, menu, and scroll bars.
The GetDC function can be used to retrieve a device context for the entire screen.
After painting is complete, the ReleaseDC function must be called to release the device context. Not releasing the window device context has serious effects on painting requested by applications.