The WM_DISPLAYCHANGE message is sent to all windows when the display resolution has changed.
WM_DISPLAYCHANGE
cBitsPerPixel = wParam;
cxScreen = LOWORD(lParam);
cyScreen = HIWORD(lParam);
Parameters
cBitsPerPixel
Specifies the new image depth of the display in bits per pixel.
cxScreen
Specifies the new horizontal resolution of the screen.
cyScreen
Specifies the new vertical resolution of the screen.