XmlLoading（Linq）

        void Page_Loaded(object sender, RoutedEventArgs e)
        {
            WebClient client=new WebClient ();
            client .OpenReadCompleted+=new OpenReadCompletedEventHandler(client_OpenReadCompleted);
            client .OpenReadAsync(new Uri ("Data.xml" ,UriKind.Relative));
        }

        void client_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
        {
            XElement xml=XElement .Load (XmlReader.Create (e.Result ));
            var query=
                from i in xml.DescendantsAndSelf("item")
                let cid=(string)i.Ancestors ("customer").First().Attribute ("id")
                group (decimal) i.Attribute ("price")*(int)i.Attribute ("quantity")
                by cid
                into groupedSales
                let totalSales=groupedSales .Sum()
                orderby totalSales descending 
                select new SalesData {
                    Customer =groupedSales.Key ,
                    TotalSales =totalSales 
                };
            LayoutRoot .DataContext =query .ToList ();
        }