Question

I have a class that inherits from ObservableCollection and adds a few additional methods such as AddRange and RemoveRange

My base method call is this:

public void AddRange(IEnumerable<T> collection)
{
    foreach (var i in collection) Items.Add(i);

    OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
}

My problem with this is that I want to access e.NewItems or e.OldItems in the CollectionChanged event to perform an action on whatever item is in the collection, and the NotifyCollectionChangedAction.Reset action does not pass in these values

void Instances_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    if (e.NewItems != null) // e.NewItems is always null
    {
        foreach (var item in e.NewItems)
        {
            if (item is EventInstanceModel)
                ((EventInstanceModel)item).ParentEvent = this;
        }
    }
}

So I thought I could just use the NotifyCollectionChangedAction.Add instead of Reset, however that throws a Range actions are not supported exception

public void AddRange(IEnumerable<T> collection)
{
    var addedItems = collection.ToList();
    foreach (var i in addedItems) Items.Add(i);

    OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, addedItems));
}

So my question is, how can I raise a CollectionChanged event, and pass it the new or old item list?

Answer 1

I’ve been looking into it and apparently the CollectionChanged method cannot be raised with multiple items.

So I can call

OnCollectionChanged(new NotifyCollectionChangedEventArgs(
    NotifyCollectionChangedAction.Add, singleItem));

but I can’t call

OnCollectionChanged(new NotifyCollectionChangedEventArgs(
    NotifyCollectionChangedAction.Add, listOfItems));

For now what I have done is simply raise the Add event for every item added, but I am still rather unhappy at this since it means I raise the CollectionChanged event for every item in the AddRange method instead of only once.

public void AddRange(IEnumerable<T> collection)
{
    foreach (var i in collection) 
    {
        Items.Add(i);
        OnCollectionChanged(new NotifyCollectionChangedEventArgs(
            NotifyCollectionChangedAction.Add, i));
    }
}