How to find out which surface the surface is under?

Question

I need to which surface the surface is under using eyeshot library. Below code is not working correctly.

public static Point3D[] GetPolygon(List<ICurve> trimmingContourList, Point3D[] vertices)
{
    ArrayList polygonResult = new ArrayList();
    CompositeCurve compCurve;
    int index = 0;
    for (int i = 0; i < trimmingContourList.Count; i++)
    {
        compCurve = trimmingContourList[i] as CompositeCurve;
        if (compCurve != null)
        {
            int j;
            for (j = 0; j < compCurve.CurveList.Count; j++)
            {
                polygonResult.Add(vertices[index]);
                index++;
            }
            if (j > 0)
            {
                polygonResult.Add(vertices[index - j]);
            }
        }
    }
    return polygonResult.ToArray(typeof(Point3D)) as Point3D[];
}