The array identified by paBlock contains a series of structures, each of which identifies a DC and supplies the pointer (pInstance) to its instance data. The display driver responds by setting a flag (HDC_IS_DIRTY) in the instance data of each DC identified in the array. The handling routines for all drawing functions should check the HDC_IS_DIRTY flag before drawing. If the flag is set, VisRegionNotify must be called to revalidate the DC's visible region.

This function allows the system to defer the calculations caused by visible region changes. This enables menus and dialogs to perform more efficiently.