c# Arc engine 二次开发，面积计算问题

计算多边形的面积，直接调用Iarea接口的area属性得到的面积和在arcgis里计算的面积不一样，请问怎么回事，是不是代码中需要设置坐标系统，请高人指点，谢谢！
部分代码
IFeatureClass pafeature = pFeatCl as IFeatureClass;
for (int i = 0; i < pafeature.FeatureCount(null); i++)
{
IFeature pFeature = pFeatCl.GetFeature(i);
IArea parea = pFeature.Shape as IArea;
pFeature.set_Value(pFeature.Fields.FindField("area"), parea.Area);
pFeature.Store();
}

坐标单位需要设置，可查看以下链接“https://blog.csdn.net/xiepeng0305/article/details/78911629”