ObservableCollection PropertyChanged event

Question

OK so I want to subclass ObservableCollection to add a property to it. Unfortunately the PropertyChanged event is protected. Basically I want to subclass it to have a SelectedItem that I can bind to for lists in my MVVM WPF app.

Here's the skeleton of my class:

public class SelectableList<T> : ObservableCollection<T>
{
    public T SelectedItem {get;set;}
}

But I cannot do the following:

SelectableList<int> intList = new SelectableList<int>();
intList.PropertyChanged += new PropertyChangedEventHandler(intList_Changed);

because of access restrictions. This causes me to ask a deeper question. How come the UI can get notified of PropertyChanged events(e.g. Count property) and I can't do it in code-behind?

My head is spinning, can someone please enlighten me?

Answer 1

SelectableList<int> intList = new SelectableList<int>();
((INotifyPropertyChanged)intList).PropertyChanged += 
    new PropertyChangedEventHandler(intList_Changed);

ObservableCollection implements INotifyPropertyChanged explicitly, which means you have to cast the instance to the interface before you can access the interface's methods, properties and events. As to why this is done, I don't know. The Binding markup extension doesn't "know" ObservableCollections or any other type. It checks types to see if they implement or extend specific interfaces/base classes (INPC, INCC, DependencyObject, etc) and so doesn't care if the interface is implemented explicitly.