[BUG] XamlTypeInfo.g.cs generates invalid code for types with required or init properties

[mikernet]

Describe the bug

If a type with required properties is used as a public property of a user control / page / dialog / etc (or an internal property that is actually used in XAML), the build fails with an error in XamlTypeInfo.g.cs because the generated code attempts to naively new it up for some reason.

Steps to reproduce the bug

public class User
{
    required public string UserName { get; set; }
}

public class UserInfoControl : UserControl
{
    public User? User { get; set; }
}

This is enough for the build to fail when the property is public, like above. If the property is internal then the control builds, but then compilation fails when the property is actually used from XAML, i.e:

<controls:UserInfoControl User="{x:Bind whatever}" />

In either case, you get:

ERROR: CS9035
Required member 'User.UserName' must be set in the object initializer or attribute constructor.
C:\[project path]\obj\x86\Debug\net7.0-windows10.0.19041.0\win10-x86\XamlTypeInfo.g.cs

Occurs on this line:

        private object Activate_8_User() { return new global::TestWinUIApp.User(); }

I am also having a similar problem with { get; init; } properties where XamlTypeInfo.g.cs sometimes tries to generate a method that sets init props on view models.

Expected behavior

Should just work the same as types that don't have parameterless constructors and not attempt to new it up in XamlTypeInfo.g.cs.

Screenshots

No response

NuGet package version

WinUI 3 - Windows App SDK 1.4 Preview 1: 1.4.230628000-preview1

Windows version

No response

Additional context

No response

[codenamelab]

Same problem. So we can't use types with the required keyword in WinUI?

[mikernet]

FWIW I haven't found any workarounds that let me use it so I've had to jump some hoops to avoid using both required and init properties anywhere WinUI sees.

[karmeye]

Is there some attribute one can use on the class or property so that it is not included in XamlTypeInfo.g.cs similar to the WinForms [Browsable (false)], [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]?

Trying to forward the property so it's not used in the XAML didn't solve the problem, i.e.:

private string? MyClassTitle => MyClass.Title;

Ended up declaring all properties that causes the issue like this:

/// <remarks>
/// #415: XamlTypeInfo.g.cs generates invalid code for types with required or init properties.
/// <para>
/// Want this:	
/// <code>
/// public required MyClass ViewModel { get; init; }
/// </code>
/// </para>
/// </remarks>
public MyClass ViewModel { get; set; } = null!;

Because if I let them be nullable then my whole code changes with null checks so it will be cumbersome to go back if issue is ever resolved.

[mikernet]

@karmeye What I've done in these situations is usually something like:

private MyClass? _viewModel = null;

public MyClass ViewModel
{
    get => ViewModelHelper.Get(_viewModel);
    set => ViewModelHelper.Set(ref _viewModel, value);
}

internal static class ViewModelHelper
{
    public static T Get<T>(T? field) where T : class
    {
        return field ?? throw new InvalidOperationException("View model was not initialized.");
    }
    
    public static void Set<T>([NotNull] ref T? field, T value) where T : class
    {
        ArgumentNullException.ThrowIfNull(value);
        
        if (field is not null)
            throw new InvalidOperationException("View model has already been set.");

        field = value;
    }
}

[mikernet]

Is there any update on this issue? This is becoming a large problem for us. It affects our ability to use modern C# features (if you can even call init a modern feature...it's been around for a very long time) on any shared code that is involved in bindings.