public CanvasDrawingSession CreateDrawingSession( Rect updateRectangle )
The specified region of the image is cleared to the ClearColor before this method returns. The region is in device independent pixels (DIPs). Regions that need to be updated are provided by the InvalidatedRegions property passed to the RegionsInvalidated event handler.
CreateDrawingSession may be called on any thread, but the returned drawing session must be disposed on the UI thread. If you want to move your drawing work to a different thread, either: