[V8i C++] The closed curve is recognized as curve or surface?

Hi

If I create a closed curve in Mirostation V8i, and see it in "Smooth" display, the area of the curve will be filled. The picture below shows the closed curve in the display style of "Wireframe" and "Smooth".

In the code, the element is recognized as curve, and will do not go to the code of handling surface.

     CurveVectorPtr pathCurve = ICurvePathQuery::ElementToCurveVector (eh);
     if (pathCurve.IsValid ())

So my question is that why the closed curve will be filled? For the closed curve, shall I considered it as a curve or a surface? If it is surface, how to get it from toolkit?