What is the point of INotifyCollectionChanged in terms of databinding (C# WinRt)

Question

Background: I was trying to roll my own observable collection, by implementing IEnumerable, INotifyPropertyChanged and INotifyCollectionChanged. It works fine, but when I databind the CollectionChanged event is always null. The databound property does however updated, since I am sending a Items[] property changed event. So this got me wondering what the point of INotifyCollectionChanged is in terms of databinding, since in my class it never gets triggered, but the databinding still works (it updates all the bindings to the collection).

I then decided to do some more digging, and decompiled ObservableCollection. When I databind to an ObservableCollection the CollectionChanged event isn't null like in my implementation.

So really I am wondering why ObservableCollection gets 'special' treatment, and what role INotifyCollectionChange plays in databinding (if any)

Answer 1

INotifyCollectionChanged can be implemented by collections so that when elements are added or removed from the collection, interested parties can be notified of those events. This is useful, for example, when you want a ListView or GridView or some other display control that displays the contents of collections to update its display when the contents of the collection have changed (through adding or removing elements). More generally, any object could data bind to the event to be notified when items are added/removed from the collection to do whatever the data bound component needs to do—it doesn't just have to be a GUI control. Any other operations on the collection, however, will result in no notifications being made to data bound controls/objects. In order for that to occur, you would also need to implement INotifyPropertyChanged on the collection, creating the other PropertyChanged events you also want to publish to notify data bound objects, and raise the event when the operation in question occurs.

Additionally, if you want each item within the collection to update its presentation in the UI when something about the item itself has changed, then the type representing the item should implement INotifyPropertyChanged.

It seems to me that you need to implement your own CollectionChanged event. The built-in System.Array and/or System.Collections.ArrayList classes do not have any events associated with them. So if you're using one of these classes as your backing store, then on each addition/removal of an item, you would need to be sure to raise the CollectionChanged event for your custom collection implementation.

However, I need to ask, why roll your own observable collection when Microsoft already provides the ObservableCollection<T> object, which you could possibly subclass and receive the functionality you're looking for for free?