Question

The MSDN reference page for ObservableCollection<T> notes:

“The objects in your collection must satisfy the requirements described in the Binding Sources Overview. In particular, if you are using OneWay or TwoWay (for example, you want your UI to update when the source properties change dynamically), you must implement a suitable property changed notification mechanism such as the INotifyPropertyChanged interface.”

Since ObservableCollection<T> already implements INotifyPropertyChanged, why do I need to again implement INotifyPropertyChanged on T also?

Answer 1

Consider your observable collection as a data source for a table. Each object from the collection occupies one row, and is displayed in the table across multiple columns.

The view (i.e. your table) needs to know when to modify each cell in response to changing properties of objects, but also in response to adding and removing objects to and from the collection.

Your observable collection takes care of dealing with table rows: it notifies its observers when an object gets inserted, removed, moved, and so on. However, it lacks knowledge of what’s going on with individual objects, so it is of no help in dealing with table columns.

This is where your objects come in: by implementing INotifyPropertyChanged they let your table manage the data in the columns.