C Sharp - Refresh von Panel bzw. Grafik

There's an additional performance-related reason. Suppose at about the same time a couple of different requests to draw part of the screen come in. Perhaps, your code has just done something to modify the document and so wants to make sure the updated document is displayed, just as in our example, while at the same time the user has just restored the window or moved another window that was covering part of the client area out of the way. By calling Invalidate(), you are giving windows a chance to notice that this has occurred. Windows can then merge the Paint events if appropriate, combining the invalidated areas, so that the painting is only done once. (Calling a method to do the painting directly from your code might needlessly result in the same area of screen being repainted more than once.)