How can I get a value of area which a polyline surrounds?

Hi, I'm now studying DraftSight API with C#, and I can't find a way to get area which a polyline surrounds. On DraftSight, I can get it from Geometry, Properties window although, it seems that the interface IPolyline doesn't have any property or method about getting area. Please let me know how I can get it with API.

Thank you,
Mas