Use a ReadOnlyObservableCollection when you allow people to subscribe to changes to a collection, without allowing them to make changes to the collection. ReadOnlyObservableCollection is a object from System.Collections.ObjectMo... ReadOnly: Because there's no Add/Remove method Observable: When an item is added or removed, it will fire an event (INotifyCollectionChanged). But, if it's ReadOnly, how can it be Observable? Because the object is actually a wrapper to a "real" collection. When you create ......