Binding… and generic IList does not inherit from IList.

A note to myself, and to others who may find it useful:

Use System.Collections.ObjectModel.Collection<T> (or a subclass) as the return type for properties exposing collections of related items on your presentation model classes if you intend to bind to the relationships with Windows Forms BindingSources.

Collection<T> appears to be the lowest-level type in Framework 3.5 that provides both the non-generic IList implementation as required by the binding system and also provides strongly-typed programmatic access to the elements of the collection. It is used as the base for BindingList<T>, ObservableCollection<T> and many others and you can easily inherit from it yourself.

Using a return type of IList<T> or ICollection<T> instead, which may be preferred for being less concrete, is not sufficient as neither of these inherit from the non-generic IList and will fail to bind correctly (sometimes throwing exceptions) at run-time when the parent list of the bound relationship is empty.