[CE U13 C#] Measure Volume Elements

I want to measure a simple volume (solid) element but with the code below, CurveVector is null. As it looks, CurveVector is not right for that element.

public static decimal getValueAsDecimal(Bentley.DgnPlatformNET.Elements.Element ElementIn, MSElementProperties elemPropIn)
      {
         decimal decValue = 0;
         CurveVector c = CurvePathQuery.ElementToCurveVector(ElementIn);


         if (c == null)
            return 0;

         switch (elemPropIn)
         {
            case MSElementProperties.Area:
               {
                  double dArea;
                  DPoint3d pt;
                  DVector3d normal;

                  c.CentroidNormalArea(out pt, out normal, out dArea);
                  decValue = (decimal)dArea;
                  break;
               }
               ...
                return decValue;
      }