Question

What is the purpose of ObservableCollection raising a PropertyChange of ‘Item[]’?

Is this something I should be doing if I have a class that implements INotifyCollectionChanged?

Do WPF controls use this PropertyChange of ‘Item[]’ somehow?

Answer 1

ObservableCollection implements both INotifyCollectionChanged and INotifyPropertyChanged.

INotifyPropertyChanged is used to indicate that a property of the ObservableCollection has changed, like the number of its elements ('Count') or an element accessible through the collection’s indexer ('Item[]'). Additionally, ObservableCollection implements INotifyCollectionChanged to indicate which element has changed exactly and how.

Have a look at the Mono implementation of ObservableCollection to see what the ObservableCollection does exactly. For example, here is the InsertItem method:

protected override void InsertItem (int index, T item) {     CheckReentrancy ();      base.InsertItem (index, item);      OnCollectionChanged (new NotifyCollectionChangedEventArgs (         NotifyCollectionChangedAction.Add, item, index));     OnPropertyChanged (new PropertyChangedEventArgs ('Count'));     OnPropertyChanged (new PropertyChangedEventArgs ('Item[]')); } 

If you want to implement your own ObservableCollection-like collection class, it seems the proper way to implement both INotifyCollectionChanged and INotifyPropertyChanged.