MarkupExtension with DI

App.xaml.cs

public partial class App : Application
{
    [NotNull]
    public static IServiceProvider? ServiceProvider { get; private set; }

    [STAThread]
    public static void Main(string[] args)
    {
        using IHost host = CreateHostBuilder(args).Build();
        host.Start();
        ServiceProvider = host.Services;

        App app = new();
        app.InitializeComponent();
        app.MainWindow = host.Services.GetRequiredService<MainWindow>();
        app.MainWindow.Visibility = Visibility.Visible;
        app.Run();
    }

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureAppConfiguration((hostBuilderContext, configurationBuilder) =>
            {
                //TODO: Any config setup here
            })
            .ConfigureServices((hostContext, services) =>
            {
                services.AddSingleton<MainWindow>();
                services.AddSingleton<MainWindowViewModel>();
                services.AddSingleton<IService, Service>();
                //TODO: Register services

            });
}

Service.cs

public interface IService { }

public class Service : IService { }

TestExtension.cs

public class TestExtension : MarkupExtension
{
    // Default ctor needed for XAML instantitation
    public TestExtension()
        : this(App.ServiceProvider.GetService<IService>()!)
    { }

    //Overloaded constructor to make testing easier
    public TestExtension(IService service)
    {
        Service = service ?? throw new ArgumentNullException(nameof(service));
    }

    private IService Service { get; }

    public override object? ProvideValue(IServiceProvider serviceProvider)
    {
        //TODO Use dependencies, produce value
    }
}