It looks like we should make the feature flag around DisplayMemberName and the default template can still work.

TODO: how many DisplayMemberName-like features exist out there? Telerik, third party, etc.?

I updated the feature switch. It will now always just call ToString() on the item (the default behavior of bindings that set string properties) + added a warning if somebody uses DisplayMemberName.

Should this throw? and then we put [RequiresUnreferencedCode] on the setter of the DisplayMemberName property?

I wasn't sure if this should throw or not. When I'm running the app from the terminal, I don't see any stack trace or the exception message when the app crashes. Maybe do both?

The problem with [RequiresUnreferencedCode] on DisplayMemberName is that it causes ~50 trimming warnings similar to this one:

/.../maui/src/Controls/src/Core/Shell/SearchHandler.cs(398): Trim analysis warning IL2026: Microsoft.Maui.Controls.SearchHandler..cctor(): Using member 'Microsoft.Maui.Controls.SearchHandler.DisplayMemberName.set' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. DisplayMemberName is not supported. Consider implementing custom ItemTemplate instead. Alternatively, enable DisplayMemberName by setting the $(MauiShellSearchResultsRendererDisplayMemberNameSupported) MSBuild property to true. Note: DisplayMemberName is not trimming-safe and it might not work as expected in NativeAOT or fully trimmed apps. [/.../MyMauiApp/MyMauiApp.csproj::TargetFramework=net9.0-maccatalyst]

I am not sure why the static constructor would cause such a warning, but unfortunately, I don't know what to do about it.

So if we have:

public static readonly BindableProperty DisplayMemberNameProperty =
	BindableProperty.Create(nameof(DisplayMemberName), typeof(string), typeof(SearchHandler), null, BindingMode.OneTime);

//...

public string DisplayMemberName
{
	get { return (string)GetValue(DisplayMemberNameProperty); }
	[RequiresUnreferencedCode]
	set { SetValue(DisplayMemberNameProperty, value); }
}

Was it warning about nameof(DisplayMemberName)? I think we can probably leave the BindableProperty alone and only add the attribute to the setter of DisplayMemberName , if that is possible?

This is exactly the code that I had locally, and it produced the flood of trimming warnings (we can't apply the attribute to fields BTW). Maybe the warnings are a bug in ILC? @vitek-karas might know what's going on when he's back from vacation.

Ok, it's probably not a bug, but it's still strange. We get 2 exactly same warnings on each line where we use typeof(SearchHandler), for example:

// /.../maui/src/Controls/src/Core/Shell/SearchHandler.cs(145): Trim analysis warning IL2026: Microsoft.Maui.Controls.SearchHandler..cctor(): Using member 'Microsoft.Maui.Controls.SearchHandler.DisplayMemberName.set' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Use ItemTemplate instead. [/.../MyMauiApp/MyMauiApp.csproj::TargetFramework=net9.0-maccatalyst]
// /.../maui/src/Controls/src/Core/Shell/SearchHandler.cs(145): Trim analysis warning IL2026: Microsoft.Maui.Controls.SearchHandler..cctor(): Using member 'Microsoft.Maui.Controls.SearchHandler.DisplayMemberName.set' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. Use ItemTemplate instead. [/.../MyMauiApp/MyMauiApp.csproj::TargetFramework=net9.0-maccatalyst]
145: public static readonly BindableProperty CancelButtonColorProperty = BindableProperty.Create(nameof(CancelButtonColor), typeof(Color), typeof(SearchHandler), default(Color));

At the same time, the compiled XAML doesn't produce any warnings. That is because XamlC doesn't use the setter method, but it sets the value using BindableObject.SetValue:

<Shell.SearchHandler>
    <local:AnimalSearchHandler Placeholder="Enter search term"
                               ShowsResults="true"
                               DisplayMemberName="Name" />
</Shell.SearchHandler>

// decompiled IL
animalSearchHandler.SetValue(SearchHandler.DisplayMemberNameProperty, "Name");

This is very likely a problem in the trimmer - and possibly even in NativeAOT, but that I'm not sure of. The root cause is that the parameter to which we're passing typeof(SearchHandler) has DAM(PublicProperties | PublicMethods).

The PublicProperties will mark the DisplayMemberName property as reflection accessible, which in turn marks the getter of it as reflection accessible -> warning.
The PublicMethods will mark the getter direcly as reflection accessible -> warning.

@sbomer as FYI.