Question

From the MSDN about OnCollectionChanged: “Occurs when an item is added, removed, changed, moved, or the entire list is refreshed.”

I’m changing a property attached to an obj that resides in my collection, but OnCollectionChanged isn’t fired. I am implementing iNotifyPropertyChanged on the obj class.

public class ObservableBatchCollection : ObservableCollection<BatchData>
    {
        protected override void OnCollectionChanged(System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
        {
            if(e.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Add)
            {
                foreach (BatchData item in e.NewItems)
                {

                }
            }
            base.OnCollectionChanged(e);
        }

        public ObservableBatchCollection(IEnumerable<BatchData> items)
            : base(items)
        {

        }
}

To me, that reads that when an item in the collection is changed, such as a property of the object, that this event should fire. It’s not, however. I want to be able to know when an item in my custom collection changes so I can perform a calculation on it, if needed.

Any thoughts?

Answer 1

ObservableCollection<T> raises events only when the collection itself changes. An item contained in the collection that has its internal state mutated has not altered the structure of the collection, and ObservableCollection<T> will not report it.

One option is to subclass ObservableCollection<T> and subscribe to each item’s OnPropertyChanged event when it is added. In that handler, you can raise either a custom event, or fall back to the collection’s own PropertyChanged event. Note that if you do go this route, you should add a generic constraint so that T : INotifyPropertyChanged.