What is the difference between ObservableCollection and INotifyPropertyChanged?

mvvm, wpf

Solution

`ObservableCollection` is a specialized collection that can notify subscribers when its contents change, while `INotifyPropertyChanged` is an interface that allows implementors to notify subscribers when one of their properties changes value.

You are probably wondering how the two are related (because both are "involved" in the setter in your example).

Consider this code:

var model = new MyViewModel(); // assume it's the class with Payments inside
model.Payments.Add(new PaymentViewModel());

Subscribers to the `INotifyCollectionChanged.CollectionChanged` event would now know that things have changed and they should update accordingly.

But now look at this:

var model = new MyViewModel(); // assume it's the class with Payments inside
model.Payments.Add(new PaymentViewModel()); // OK, we know what this does

model.Payments = new ObservableCollection<PaymentViewModel>();

After adding an item to the collection we then swap the entire collection for another one. If an `ItemsControl` is bound to this collection we expect it to update itself and reflect the fact that `model.Payments` ends up being empty. But how can it do that?

`CollectionChanged` will not help because the original collection (after receiving its first item) was not modified; we just discarded it and installed another in its place. The only one who knows that the switch happened is the `Payments` property setter. So the setter utilizes `INotifyPropertyChanged` to tell subscribers that the collection has been replaced with another, and they should of course update their status.

Conclusion: Data binding works automagically in WPF because all the databound controls listen to the `INotifyPropertyChanged` of their `DataContext`, and if the binding target implements `INotifyCollectionChanged` they subscribe to that as well. If the binding target changes they are notified through `INotifyPropertyChanged`, unsubscribe from `INotifyCollectionChanged` on the old target and subscribe to it on the new one.

Problem

Original source