void ValidateRect( LPRECT lpRect );
lpRect
Points to a CRect or RECT structure that contains the rectangle (in client coordinates) to be removed from the update region.
Validates the client area within the given rectangle by removing the rectangle from the update region of the given window. If lpRect is NULL, the entire window is validated.
The BeginPaint member function automatically validates the entire client area. Neither the ValidateRect nor ValidateRgn member function should be called if a portion of the update region needs to be validated before WM_PAINT is next sent.
Windows continues to send WM_PAINT messages until the current update region is validated.
CWnd::BeginPaint, ::ValidateRect, CWnd::ValidateRgn