How to get Curve from Polyline? Rhino API in C#

        double THeight = 2.0;
        Polyline _polygon = Polyline.CreateCircumscribedPolygon(iCircle, 6);
        Brep oExtrusion = Surface.CreateExtrusion(_polygon , new Vector3d(0, 0, THeight));

CreateExtrusion method takes in Curve, while I have a Polyline. How do I get this conversion?

You can create a PolylineCurve from a polyline