BOOL CreateRectRgnIndirect( LPRECT lpRect );
lpRect
Points to a RECT structure or CRect object that contains the coordinates of the upper-left and lower-right corners of the region.
Creates a rectangular region that is stored in the CRgn object.
TRUE if the operation succeeded; otherwise FALSE.
CRgn::CreateRectRgn, CRgn::CreateRoundRectRgn, ::CreateRectRgnIndirect