Retrieves the update region into a region identified by pRgn.
BOOL bErase = FALSE
Identifies the update region.
Specifies whether the background will be erased and nonclient areas of child windows will be drawn. If the value is FALSE, no drawing is done.
Specifies a short-integer flag that indicates the type of resulting region. The value can take any one of the following:
SIMPLEREGION The region has no overlapping borders.
COMPLEXREGION The region has overlapping borders.
NULLREGION The region is empty.
ERROR No region was created.
The coordinates of this region are relative to the upper-left corner (client coordinates).
The BeginPaint member function automatically validates the update region, so any call to GetUpdateRgn made immediately after a call to BeginPaint retrieves an empty update region.