WP7 ListPicker设置SelectedItem问题

        XmlReader xml = XmlReader.Create(\"file.xml\");
        XDocument _doc = XDocument.Load(xml);

        var stringNames = from query in _doc.Descendants(\"string\")
                     select new CustomValue
                     {
                         StringName = (string)query.Attribute(\"name\"),
                         StringValue = (string)query.Attribute(\"value\"),
                     };

        Listpicker.ItemsSource = stringNames;

    public class CustomValue
    {
        public string StringName             
        {
            get;
            set;
        }

        public string StringValue
        {
            get;
            set;
        }
    }

((appname.pagename.CustomValue)(this.Listpicker.SelectedItem)).StringValue