Viv,

Thats correct, however you can paint to a context without invalidating a region. Consider this, you have a hot tracking effect in your client area - you want the fastest mechanism for updating a tiny portion of the display avoiding overdraw, flickering etc. Do you invalidate a region and give windows its dirty rectangle (slow) or do you draw the effect directly to the controls display context (quick) and pay the price of not supporting standard behaviour by re-creating the tooltip?

Its a design tradeoff, if your control is fairly light then you don't have to worry about optimising your drawing pipeline. Off course if you do a lot of control authoring then its worth while investing time in things like span lists to protect against overdraw.

Regards
Neil