只有在AfterDraw 中不需要StartDrawing and FinishDrawing

DrawPolyline draws a Polyline object with the symbol that must be specified before hand with the SetSymbol method. All draw methods must be enclosed between the calls to StartDrawing and FinishDrawing unless you are drawing in response to the IActiveViewEvents::AfterDraw event.
 

只有在AfterDraw event.中不需要StartDrawing and FinishDrawing

转载自：https://blog.csdn.net/mydriverc/article/details/1711413