样条曲线的长度！

(command "spline" pt1 pt2 pt3 pt4  "" pt1 pt4)                         
(setq ent1(entlast))
(setq lengthOfSpline(vlax-curve-getDistAtParam ent1        (vlax-curve-getEndParam ent1)))

//this doesn't give right value
        ptArray contains pt1, pt2, pt3, pt4
        for( int i = 0 ; i getEndParam(dummy1);
        retVal = spline->getDistAtParam(dummy1,dummy2);
        delete spline;
        return dummy2;
//i think this is not right as the outputs are different.
        //any other way to find it? is there a direct call like object.length() to find out?

  static void ExtTools_doit(void)
  {
    CPromptEntityResultPtr pRes = CEditor::GetEntity(_T("\nSelect Spline: "));
    if(pRes->Status != Acad::eNormal){
      return;
    }
    AcDbEntityPointer pEnt(pRes->ObjectId,AcDb::kForRead);
    if(pEnt.openStatus() != eOk){
      return;
    }
    AcDbSpline *pSpline = AcDbSpline::cast(pEnt);
    if(pSpline == NULL){
      return;
    }
    double endParam,length; 
    pSpline->getEndParam(endParam);
    pSpline->getDistAtParam(endParam,length);
    acutPrintf(_T("\nSpline Length is %g"),length);
  }

  static void ExtTools_doit(void)
  {
    ads_name ename = { 0L , 0L };
    ads_point pnt  = {0.0, 0.0, 0.0};
    AcDbObjectId eId;
    if(acedEntSel(_T("\nSelect Spline: "),ename,pnt) == RTNORM){
      acdbGetObjectId(eId,ename);
    }
    AcDbEntityPointer pEnt(eId, AcDb::kForRead);
    if(pEnt.openStatus() != eOk){
      return;
    }
    AcDbSpline *pSpline = AcDbSpline::cast(pEnt);
    if(pSpline == NULL){
      return;
    }
    double endParam,length; 
    pSpline->getEndParam(endParam);
    pSpline->getDistAtParam(endParam,length);
    acutPrintf(_T("\nSpline Length is %g"),length);
  }