Avalonia/samples/SingleProjectSandbox/App.axaml.cs

using Avalonia;
using Avalonia.Controls.ApplicationLifetimes;
using Avalonia.Markup.Xaml;

namespace SingleProjectSandbox;

public class App : Application
{
    public static AppBuilder BuildAvaloniaApp()
        => AppBuilder.Configure<App>()
            .LogToTrace();

    public override void Initialize()
    {
        AvaloniaXamlLoader.Load(this);
    }

    public override void OnFrameworkInitializationCompleted()
    {
        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktopLifetime)
        {
            desktopLifetime.MainWindow = new MainWindow();
        }
        else if (ApplicationLifetime is ISingleViewApplicationLifetime singleViewLifetime)
        {
            singleViewLifetime.MainView = new MainView();
        }

        base.OnFrameworkInitializationCompleted();
    }
}