M....
I am wondering whether:
1. InvalidateRect(NULL) is having the same effect as Invalidate().

2. In MSDN, it says: " If bErase is TRUE for any part of the update region, the background in the entire region, not just in the given part, is erased. ".

So is this mean if we use Invalidate(), then no matter how small the clip regions are, all client area will be set invalid? (Repaint the complete client area?)

Thanks for you guys' help.