MapChanged Event Raises Null args in WinUI 3 with ApplicationData.LocalSettings.Values

[cdaylin]

Summary:
In a WinUI 3 app using ApplicationData.Current.LocalSettings.Values.MapChanged, the MapChanged event occasionally fires with a null args parameter, causing a NullReferenceException when accessing args.CollectionChange.

Expected behavior:
The IMapChangedEventArgs parameter (args) should never be null. All values should be non-null per WinRT metadata contracts.

Environment:

WinUI version: Microsoft.WindowsAppSDK 1.5 (or applicable version)

SDK / runtime: Microsoft.Windows.SDK.NET, WinRT.Runtime

.NET version: .NET 8

Platform: Windows 11

Architecture: x64

Repro Steps:

Subscribe to MapChanged:

ApplicationData.Current.LocalSettings.Values.MapChanged += HandleBackingPropertySetChanged;

Assign a value:

ApplicationData.Current.LocalSettings.Values["Temperature"] = 0.75;

In your handler:

private void HandleBackingPropertySetChanged(
    IObservableMap<string, object> sender,
    IMapChangedEventArgs<string> args)
{
    // This line throws unexpectedly
    switch (args.CollectionChange) { ... }
}

Observe args == null and resulting NullReferenceException.

Notes:

args is normally a WinRT-projected value and should never be null.

This seems to originate from Do_Abi_Invoke(...) in the auto-generated interop.

Bug occurs only occasionally, likely due to ABI event wiring edge case.