How can I use Server.MapPath() from global.asax?

            //引用：using System.Web.Hosting;
            string filePathcsv = "";
            string file = "";
            string strFile = "LukfookGeovindu";
            strFile = strFile + DateTime.Now.ToString("yyyyMMddhhmmss");
            strFile = strFile + ".csv";
            //file = strFile; 
            //file = HttpContext.Current.Server.MapPath("/ReportExcel/" + strFile); //此用法存在問題
            // Server.MapPath("/ReportExcel/" + strFile);//存在問題
            file = HostingEnvironment.MapPath("/ReportExcel/" + strFile);