/// <summary>
 /// Call this function to see if the point was on an object. If the point was
 /// on an object an ObjRef is returned; otherwise null is returned.
 /// </summary>
 /// <returns>A point object reference.</returns>
 public Rhino.DocObjects.ObjRef PointOnObject()
 {
   Rhino.DocObjects.ObjRef rc = new Rhino.DocObjects.ObjRef();
   IntPtr pThis = ConstPointer();
   IntPtr pObjRef = rc.NonConstPointer();
   if (!UnsafeNativeMethods.CRhinoGetPoint_PointOnObject(pThis, pObjRef))
   {
     rc.Dispose();
     rc = null;
   }
   return rc;
 }