How Can I Create an ObservableCollection That Monitors Changes in its Element Properties?

ObservableCollection to monitor element changes

Problem Statement

Imagine an ObservableCollection where the elements implement INotifyPropertyChanged and the collection itself monitors those elements for any changes. Although ObservableCollection exists, it's not clear if there is a pre-existing collection that satisfies this exact requirement.

Solution

ObservableCollectionEx class

The proposed solution introduces ObservableCollectionEx, which is an extension of ObservableCollection. In this implementation:

• OnCollectionChanged: Updates event subscriptions for items that have been added or removed.
• ClearItems: Unsubscribe from an element's property changes before clearing.
• Subscribe: Subscribe to property changes of newly added items.
• Unsubscribe: Unsubscribe from property changes of deleted items.
• ContainedElementChanged: Propagates the element's property changes to the collection.

Usage

ObservableCollectionEx is used as follows:

ObservableCollectionEx<Element> collection = new ObservableCollectionEx<Element>();
((INotifyPropertyChanged)collection).PropertyChanged += (x, y) => ReactToChange();

Notes

While this implementation fires the collection's PropertyChanged event when an element's property changes, this can be confusing. Therefore, an additional event can be introduced specifically for such changes.

Furthermore, the ObservableCollection implementation in BCL only explicitly exposes the INotifyPropertyChanged interface. Subscribing to events requires casting as follows:

((INotifyPropertyChanged)collection).PropertyChanged += (x, y) => ReactToChange();

Note that unsubscribing from PropertyChanged requires setting the event handler to null:

collection.PropertyChanged -= (s, e) => { Trace.WriteLine("Changed " + e.PropertyName); };

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