linq to sql 修改 web.config的值

string path = Server.MapPath("~/Web.config");
                //"Web.config";
                var xml = XElement.Load(path);

                var temp = from k in xml.Descendants("appSettings").Elements("add")
                           where k.Attribute("key").Value == "pas"
                           select k;
                var query = temp.SingleOrDefault();
               

                if (query != null)
                {
                    query.Attribute("value").Value = newpassword;
                    xml.Save(path);

                }