WindowsPhone8中LongListSelector的扩展解决其不能绑定SelectdeItem的问题

  public enum PositionOnAdd
    {
        Top,
        Default,
        NewItem
    }
    public class ExtendedSelector : LongListSelector
    {
        public static readonly DependencyProperty SelectedItemProperty =
            DependencyProperty.Register("SelectedItem", typeof(object), typeof(ExtendedSelector), new PropertyMetadata(default(object)));

        public static readonly DependencyProperty SelectionModeProperty =
            DependencyProperty.Register("SelectionMode", typeof(SelectionMode), typeof(ExtendedSelector), new PropertyMetadata(SelectionMode.Single));

        public static readonly DependencyProperty RepositionOnAddStyleProperty =
            DependencyProperty.Register("RepositionOnAddStyle", typeof(PositionOnAdd), typeof(ExtendedSelector), new PropertyMetadata(PositionOnAdd.Default));

        public PositionOnAdd RepositionOnAddStyle
        {
            get { return (PositionOnAdd)GetValue(RepositionOnAddStyleProperty); }
            set { SetValue(RepositionOnAddStyleProperty, value); }
        }

        public SelectionMode SelectionMode
        {
            get { return (SelectionMode)GetValue(SelectionModeProperty); }
            set { SetValue(SelectionModeProperty, value); }
        }

        public new object SelectedItem
        {
            get { return GetValue(SelectedItemProperty); }
            set { SetValue(SelectedItemProperty, value); }
        }

        public ExtendedSelector()
        {
            SelectionChanged += (sender, args) =>
            {
                if (SelectionMode == SelectionMode.Single)
                    SelectedItem = args.AddedItems[0];
                else if (SelectionMode == SelectionMode.Multiple)
                {
                    if (SelectedItem == null)
                    {
                        SelectedItem = new List