Bug: Unexpected behavior of CollectionViewSource with grouping

[DmitryProskurin]

Description

Hello.

I'm porting a UWP application to .NET 9 and I'm running into issues with CollectionViewSource when working with grouped collections.

I use a custom grouped observable collection. It has its own Grouping<TKey, TItem> type with a Key property and a Values property (which contains the items in that group).

The porting process is stuck because of problems with my custom collection.

I created a minimal reproducible example with two variants:

1. My custom grouped observable collection (shows groups by DateTime + view model class or DateTime values)
2. Manually created grouped collection (like in issue Bug: ListView doesn't render its items when using CollectionViewSource & NativeAot #2001, also using DateTime + view model or DateTime value)

When the app runs with both variants present — all groups and items are displayed correctly.

But when I comment out the creation of the manual grouped collection (from issue #2001) — only the group headers are shown in my custom collection; the items inside the groups disappear.

Right column display DateTime(group key and values). Left column display DateTime(header), view model in values.

Commented:

Why my custom collection stop working?
Archive with sources: TestNet9.zip

Steps To Reproduce

1. Launch the application using the archived build. → Expected: all groups and their items are displayed correctly.
2. Close the application.
3. Open MainPage.xaml.cs and comment out lines 36 through 57 (the manual grouped collection creation code).
4. Rebuild and run again. → Actual: only group headers appear; items within groups are not rendered.

Expected Behavior

Items from my custom collection display without issues. And i don't need another two collections.

Version Info

Microsoft.Windows.CsWinRT 2.3.0-prerelease.251115.2(add as PackageReference)
CommunityToolkit.Mvvm 8.4.0

Microsoft Visual Studio Community 2022
Version 17.14.27 (February 2026)

Additional Context

No response

[DmitryProskurin]

I ran some tests and found that my example was redundant. To make the lists display items from my custom grouped observable collection, I just need to create a couple more CollectionViewSources and assign them ObservableCollection containing one item each of the types I want to display in the grouped list.
If i comment initialization of new CVS, grouped lists display only group headers.

To see changes, please build project with/without commented lines(36-43) in MainPage.xaml.cs.

TestNet9-reduced2.zip

[manodasanW]

I was able to get a repro with your reduced sample. The issue you are seeing is from a limitation of our source generator with respect to analyzing generic classes. With a source generator, we aren't able to easily figure out the generic instantiations you do of a non-WinRT class like BaseGroupedObservableCollection<TKey, TElement> and then use that to determine which WinRT generic type instantiation you use within it such as the ObservableCollection<T> within Grouping<TKey, TElement>. So, for ObservableCollection<T>, we aren't automatically generating the vtable for it causing this issue.

From your code, we can tell you use both ObservableCollection<DateTime> and ObservableCollection<MyViewModel>. To work around the limitation of our automated analysis, you are able to use the opt-in attributes for vtable generation to specify the specific instantiations you use. With that I was able to get your repro working as expected.

[assembly: WinRT.GeneratedWinRTExposedExternalType(typeof(ObservableCollection<MyViewModel>))]
[assembly: WinRT.GeneratedWinRTExposedExternalType(typeof(ObservableCollection<DateTime>))]

[DmitryProskurin]

@manodasanW, thanks for clarification, now i can remove all new CVS with view models which i was add to another places in app.

I also noticed similar behavior in a collection(custom grouped) where the element must implement an interface. Everything worked almost immediately; I just added a partial modifier for the models(implemented interface) and GeneratedBindableCustomProperty for the TKeys classes. Unfortunately, I couldn't reproduce it in a separate project.