ObservableCollection event for items being added, but not removed

Question

Is there a way to fire an event when an item is added to an ObservableCollection, but not when one is removed?

I believe there isn't an actual event, but perhaps a way to filter the CollectionChanged event?

Why not use a List and manually handle NotifyPropertyChange instead? — David L, Feb 24 '13 at 19:35
Could I just create a custom class, inherit List, and add the event to Add() and AddRange()? — Wilson, Feb 24 '13 at 19:37

score 15 · Answer 1 · answered Feb 24 '13 at 20:59

The CollectionChanged event includes information such as what action was performed on the collection (e.g., add or remove) and what items were affected.

Just add a check in your handler to only perform the desired action if an Add was performed.

ObservableCollection<T> myObservable = ...;
myObservable.CollectionChanged += (sender, e) =>
{
    if (e.Action == NotifyCollectionChangedAction.Add)
    {
        // do stuff
    }
};

score 1 · Answer 2 · answered Feb 24 '13 at 19:50

Subclassing the ObservableCollection class and create your own ItemAdded event should work I would think.

public class MyObservableCollection<T> : ObservableCollection<T>
{
    public event EventHandler<NotifyCollectionChangedEventArgs> ItemAdded;

    public MyObservableCollection()
    {
        CollectionChanged += MyObservableCollection_CollectionChanged;
    }

    void MyObservableCollection_CollectionChanged(object sender,            NotifyCollectionChangedEventArgs e)
    {
        if (e.Action == NotifyCollectionChangedAction.Add)
            ItemAdded(sender, e);
    }
}

If subclassing, it would be better to override the `OnCollectionChanged()` method to add this logic. This would prevent client code from removing the handler. — Jeff Mercado, Feb 24 '13 at 20:50

ObservableCollection event for items being added, but not removed

2 Answers2