I have an observabelCollection of a data object:

public ObservableCollection<GistViewModel> Gists { get => gists; set => SetProperty(ref gists, value); }

Each GistViewModel has the following:

public BindingList<GistFileViewModel> GistFiles { get => gistFiles; }

In terms of what this represents, each Gist has a collection of GistFiles. I am mapping this onto a TreeViewControl (root being Gists, Child being GistFiles) via the following xaml (abridged):

<!-- GistLevel -->
<HierarchicalDataTemplate DataType="{x:Type viewmodels:GistViewModel}" ItemsSource="{Binding Path=GistsFiles}">

    <StackPanel Orientation="Horizontal">
        <TextBlock Text="{Binding GistFiles, Converter={StaticResource GistFilesToFirstFilenameConverter}}"></TextBlock>
    </StackPanel>
    
</HierarchicalDataTemplate>

<!-- GIST FILE LEVEL -->

<DataTemplate DataType="{x:Type viewmodels:GistFileViewModel}">
    <StackPanel Orientation="Horizontal">
        <TextBlock  VerticalAlignment="Center" Margin="2,0,2,0" Text="{Binding Filename}"></TextBlock>
    </StackPanel>
</DataTemplate>

<TreeView x:Name="GistsTV" ItemsSource="{Binding Gists}"/>

This works successfully. Now I am trying to introduce a filter feature. I have followed a guide online somewhere which advises using ICollectionView. I implement this thus:

public ICollectionView GistsView { get => CollectionViewSource.GetDefaultView(Gists); }

private async Task GetAllGistsAsync()
{    
    Gists = await gistManager.LoadGistsAsync();
    GistsView.Refresh();
}

And changing my xaml to:

<TreeView x:Name="GistsTV" ItemsSource="{Binding GistsView}"/>

I’m just trying to get the default list presented first. However, this does not work. No list items are displayed.

GistFilesToFirstFilenameConverter doesn’t get called when Binding = GistsView

The full code is here: https://github.com/stigzler/VisGist/blob/master/VisGist/ToolWindows/MainWindow.xaml

What am I missing?