Why does MarkupExtension return itself instead of ProvideValue?

[CodingOctocat]

I'm using DevExpress Early Access Preview (v24.2) for MAUI, which provides the dx:Ioc MarkupExtension, so that's how I can use it:

<ContentPage ...
             xmlns:dx="http://schemas.devexpress.com/maui"
             BindingContext="{dx:Ioc Type={x:Type vm:MainViewModel}}">

However, in Code-Behind, I can't put BindingContext as MainViewModel, throw runtime error: InvalidCastException: BindingContext is IocExtension, can not cast to MainViewModel. What is the reason for this?

---

public class IocExtension : IMarkupExtension
{
    public Type Type { get; set; }

    public object ProvideValue(IServiceProvider serviceProvider)
    {
        return ApplicationHelper.ServiceProvider.GetRequiredService(Type);
    }

    //     Initializes a new instance of the DevExpress.Maui.Core.IocExtension class.
    public IocExtension()
    {
    }
}

internal class ApplicationHelper
{
    public interface IApplicationService
    {
        AppTheme RequestedTheme { get; }

        event EventHandler<AppThemeChangedEventArgs> RequestedThemeChanged;
    }

    private class DefaultApplicationService : IApplicationService
    {
        public AppTheme RequestedTheme => Application.Current.RequestedTheme;

        public event EventHandler<AppThemeChangedEventArgs> RequestedThemeChanged
        {
            add
            {
                Application.Current.RequestedThemeChanged += value;
            }
            remove
            {
                Application.Current.RequestedThemeChanged -= value;
            }
        }
    }

    private static IApplicationService applicationService;

    public static IApplicationService ApplicationService
    {
        get
        {
            return applicationService ?? (applicationService = new DefaultApplicationService());
        }
        internal set
        {
            applicationService = value;
        }
    }

    public static IServiceProvider ServiceProvider => IPlatformApplication.Current.Services;
}

[stephenquan]

I am not sure of the DevExpress, but, if I were to build the IocExtension myself, I'd do it with BindableObject, IMarkupExtension and IValueConverter like this:

// IocExtension.cs
public class IocExtension : BindableObject, IMarkupExtension<BindingBase>, IValueConverter
{
    public static readonly BindableProperty TypeProperty = BindableProperty.Create(nameof(Type), typeof(Type), typeof(IocExtension), null);
    public Type Type
    {
        get => (Type)GetValue(TypeProperty);
        set => SetValue(TypeProperty, value);
    }
    public object ProvideValue(IServiceProvider serviceProvider)
        => (this as IMarkupExtension<BindingBase>).ProvideValue(serviceProvider);
    BindingBase IMarkupExtension<BindingBase>.ProvideValue(IServiceProvider serviceProvider)
        => new Binding(nameof(Type), BindingMode.OneWay, this, null, null, this);
    public object? Convert(object value, Type targetType, object parameter, CultureInfo culture)
        => (value is Type type) ? IPlatformApplication.Current?.Services.GetService(type) : null;
    public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
        => throw new NotImplementedException();
}

Then, I can use it with MainViewModel.cs and MainPage.xaml like this:

// MainViewModel.cs
public class MainViewModel
{
    public string Title { get; } = "Hello, .NET MAUI!";
    public string Description { get; } = "Welcome to .NET Multi-platform App UI!";
}

<!-- MainPage.xaml snippet -->
<Frame>
    <VerticalStackLayout x:DataType="local:MainViewModel" BindingContext="{local:Ioc Type={x:Type local:MainViewModel}}">
         <Label Text="{Binding Title}" />
         <Label Text="{Binding Description}" />
    </VerticalStackLayout>
</Frame>

// MauiProgram.cs snippet
builder.Services.AddTransient<MainViewModel>();