protected override void SetItem(int index, SelectableItem <T> item)
 {
     this[index].PropertyChanged -= this.ItemOnPropertyChanged;
     base.SetItem(index, item);
     item.PropertyChanged += this.ItemOnPropertyChanged;
 }
        protected override void InsertItem(int index, SelectableItem <T> item)
        {
            base.InsertItem(index, item);

            item.PropertyChanged += this.ItemOnPropertyChanged;
        }