ObservableCollection<T>.CollectionChanged Caveat

DO NOT return null values from collection properties or from methods returning collections. Return an empty collection or an empty array instead.

Users of collection properties often assume that the following code will always work:

IEnumerable<string> list = GetList();
foreach (string name in list)
{
...
}

The general rule is that null and empty (0 item) collections or arrays should be treated the same.

– Framework Design Guidelines, p. 217.

System.Collections.ObjectModel.ObservableCollection<T>‘s event CollectionChanged uses the NotifyCollectionChangedEventArgs event arguments class, which holds two lists: OldValue and NewValue. When either is empty, it will be raised with a null as the list’s reference.

I hope they fix this for the next release.

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