NavigationView: Throw exception when a WUX NavigationViewItem is added to it as a menu item

[Felix-Dev]

Description

With PR #69, logic was added to throw an exception with a helpful error message when a developer accidentally adds a WUX NavigationViewItem to a MUX NavigationView. Unfortunately, with the move to ItemsRepeater in PR #1683, this logic did not survive and thus starting with WinUI 2.4, WinUI no longer guards against adding WUX NavigationViewItems.

This PR adds back this logic when a developer adds a WUX NavigationViewItem to a NavigationView (via the APIs MenuItems\MenuItemsSource or FooterMenuItems\FooterMenuItemsSource). The exception looks like this in a WinUI C# app:

Motivation and Context

Prevents developer confusion when they accidentally add a WUX NavigationViewItem to a NavigationView and see "weird" rendering behavior as a result. The image below shows the rendering difference between a MUX NavigationViewItem and a WUX NavigationViewItem when added to a MUX NavigationView:

How Has This Been Tested?

Tested manually. Unfortunately, while an API test for this scenario exists, it appears we cannot use a simple API test here because adding a WUX NavigationViewItem to the UI will lead to multiple calls of the NavigationViewItemsFactory::GetElementCore method during the UI update phase. Each of these calls will throw our added exception and I was unable to catch all of them and have the API test succeed. I tried adding an additional Application.Current.UnhandledException handler in the API test to catch all thrown exceptions in addition to the one we catch in

microsoft-ui-xaml/dev/NavigationView/NavigationView_ApiTests/NavigationViewTests.cs

Line 639 in 986d48a

Verify.Throws<Exception>(() => { navView.UpdateLayout(); });

but the VS test explorer still showed the test as failing because they apparently still managed to slip through my added unhandled exception handling (and I set UnhandledExceptionEventArgs.Handled to true).

Any other ideas here? If we cannot get this API test to work, we should add the reason why it is disabled as a comment (or alternatively just remove it, though keeping it will act as a documentation for this test case).

[StephenLPeters]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[StephenLPeters]

@Felix-Dev would you mind making an issue to track removing this code in winui 3 where this problem would no longer exist?

[Felix-Dev]

@StephenLPeters Created tracking issue #3236.