The WM_ICONERASEBKGND message is sent to a minimized window when the background of the icon must be filled before painting the icon. A window receives this message only if a class icon is defined for the window; otherwise, WM_ERASEBKGND is sent.
WM_ICONERASEBKGND hdc = (HDC) wParam; // handle of device context
An application should return nonzero if it processes this message.
The DefWindowProc function fills the icon background with the class background brush of the parent window.