Question

Edited to address F Ruffell’s answer

I have the following xaml

<StackPanel>
    <ListBox x:Name="_list1"/>
    <ListBox x:Name="_list2"/>
</StackPanel>

and this code-behind:

var ints = new[] { 1, 2, 3 };
_list1.ItemsSource = ints;
_list2.ItemsSource = ints;

_list1.Items.Filter = i => ((int)i) < 2;

For some reason after setting filter only for the first ListBox both lists are filtered. I expect the lists to have completely different CollectionViews and indeed _list1.Items != _list2.Items. Meanwhile setting filter to one of them also somehow sets that very filter to the other.
The question is why and how are CollectionViews synchronized?

Answer 1

The question is why and how are CollectionViews synchronized?

They are synchronized because even though both ListBoxes have different Items, they share the same CollectionView, which is the default view for the source collection.

The Items property of ItemsControl is of type ItemCollection and the CollectionView property of ItemCollection is internal so we can’t access it directly to verify that this is true. However, we can just enter these three values in the debugger to verify this, they all come out as true

_list1.Items.CollectionView == _list2.Items.CollectionView // true
_list1.Items.CollectionView == CollectionViewSource.GetDefaultView(ints) // true
_list2.Items.CollectionView == CollectionViewSource.GetDefaultView(ints) // true

Alternatively, we can use reflection to do the comparison in code

PropertyInfo collectionViewProperty =
    typeof(ItemCollection).GetProperty("CollectionView", BindingFlags.NonPublic | BindingFlags.Instance);
ListCollectionView list1CollectionView = collectionViewProperty.GetValue(_list1.Items, null) as ListCollectionView;
ListCollectionView list2CollectionView = collectionViewProperty.GetValue(_list2.Items, null) as ListCollectionView;
ListCollectionView defaultCollectionView = CollectionViewSource.GetDefaultView(ints) as ListCollectionView;

Debug.WriteLine(list1CollectionView == list2CollectionView);
Debug.WriteLine(list1CollectionView == defaultCollectionView);
Debug.WriteLine(list2CollectionView == defaultCollectionView);

The way to work around this has already been posted by F Ruffell, create a new ListCollectionView as ItemsSource for each ListBox.

_list1.ItemsSource = new ListCollectionView(ints);
_list2.ItemsSource = new ListCollectionView(ints);

Also note that after this, the 3 comparisons above comes out as false