WPF Window redraw issue after showing MessageBox by MJosephson

The standard MessageBox dialog uses the CS_SAVEBITS flag. This causes the window manager to save the contents of the screen behind the MessageBox dialog into a bitmap, and to restore them when the dialog is dismissed.

Of course, any changes to the contents behind this window should trump whatever the saved bitmap has. Windows detects this in a variety of ways: window moves, invalidate rect calls, GetDC calls, etc.

WPF complicates the story some because it renders to its windows from a separate rendering thread. There have been reported issues where it appears that WPF changes to the contents of the underlying window are not detected by the window manager code that is implementing CS_SAVEBITS, and so the window manager puts back the old content.

This does not happen often, but when it does it can be very annoying. If you encounter one of these scenarios, you can try to work around the problem is various ways.

1) Cause WPF to repaint after dismissing the dialog. This can be difficult, since WPF likes to optimize its rendering passes, and you would have to figure out how to trick it. One idea is to use put a large, transparent rectangle into the adorner layer temporarily.

2) Remove the CS_SAVEBITS class style. This is difficult for several reasons, the least of which is that it is hard to get the dialog HWND.

3) Force the window manager to discard the contents it saved. Raymond talks about some of the triggers that the window manager respects here: http://blogs.msdn.com/b/oldnewthing/archive/2006/04/28/586018.aspx. I think it would be easiest to just call InvalidateRect() on the WPF window. You could even post a Dispatcher work item to do this after the message box is displayed.

This is not a WPF product bug per-se, but rather the unfortunate consequence of some of the design choices (separate render thread, using DX, etc) interacting with other systems. I hope some of these work arounds work for you.

Good luck,
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