what is notifycollectionchangedaction reset value

Question

I have an observable collection...SelectableDataContext<T>..And in the generic class SelectableDataContext<T> is...having two private member variables

1. Private T item.
2. Private bool isSelected.

When the IsSelected property changes...My collection's changed property is not firing .

I think it should fire...because it's Reset in INotifyCollectionChangedAction.

Answer 1

This is an old question but for the benefit of anyone who may come across this through a search as I did:

NotifyCollectionChangedAction.Reset means "The content of the collection changed dramatically". One case where the Reset event is raised is when you call Clear() on the underlying observable collection.

With the Reset event, you don't get the NewItems and OldItems collections in the NotifyCollectionChangedEventArgs parameter.

This means you're better off using the "sender" of the event to get a reference to the modified collection and use that directly, i.e. assume it's a new list.

An example of this might be something like:

((INotifyCollectionChanged)stringCollection).CollectionChanged += new NotifyCollectionChangedEventHandler(StringCollection_CollectionChanged);
  ...

void StringCollection_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    switch (e.Action)
    {
        case NotifyCollectionChangedAction.Add:
            foreach (string s in e.NewItems)
            {
                InternalAdd(s);
            }
            break;

        case NotifyCollectionChangedAction.Remove:
            foreach (string s in e.OldItems)
            {
                InternalRemove(s);
            }
            break;

        case NotifyCollectionChangedAction.Reset:
            ReadOnlyObservableCollection<string> col = sender as ReadOnlyObservableCollection<string>;
            InternalClearAll();
            if (col != null)
            {
                foreach (string s in col)
                {
                    InternalAdd(s);
                }
            }
            break;
    }
}

Lots of discussions on this Reset event here: When Clearing an ObservableCollection, There are No Items in e.OldItems.

Answer 2

There is a difference between INotifyCollectionChanged and INotifyPropertyChanged.

When a value of a propery in an object changes, it should notify others using INotifyPropertyChanged interface implementation.

On the other hand, when number of items or items themselves change in a collection, it should let others know using INotifyCollectionChanged implementation.

Now, in your case, value of a property of an object in your collection changes. That is supposed to raise PropertyChanged event, not CollectionChanged event.