Update list when ObservableCollection changes having binding with in-between layer
===

A ObservableCollection 

    private ObservableCollection<string> _items = new ObservableCollection<string>();
    
    public ObservableCollection<string> Items { get { return _items; } }

is updated on user interactions (a TextBox event).

In a ListBox I'll show the current values

    Binding listBinding = new Binding {Source = Items};
    listbox.SetBinding(ListBox.ItemsSourceProperty, listBinding);

That works so far: When adding a new value the list is immediately updated.

But now I have to requirements:

 * Sort values
 * Add one item at the beginning of the list

I solved as followed:

    public IEnumerable<string> ItemsExtended
    {
      get
      {
        return new[] { "first value" }.Concat(Items.OrderBy(x => x));
      }
    }

I changed the binding to that IEnumerable and the list contains a sorted list with "first value" at position one.

Unfortunately when the list should be updated on user interaction it does not work any more. Even changing IEnumerable to ObservableCollection again and directly referring to the private ObservableCollection does not solve the issue:

    return new ObservableCollection<string> (new[] { "bla" }.Concat(_items.OrderBy(x => x)));

How to update the list when _items changes?