AvaloniaStack/Avalonia-PC/App.axaml.cs

using Avalonia;
using Avalonia.Controls.ApplicationLifetimes;
using Avalonia.Markup.Xaml;
using Avalonia_PC.ViewModels;
using Avalonia_PC.Views;
using Microsoft.Extensions.DependencyInjection;

namespace Avalonia_PC
{
    public partial class App : Application
    {
        public override void Initialize()
        {
            AvaloniaXamlLoader.Load(this);
        }

        public override void OnFrameworkInitializationCompleted()
        {
            if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
            {
                desktop.MainWindow = Program.Services.GetRequiredService<MainWindow>();
                desktop.MainWindow.DataContext = new MainWindowViewModel();
            }

            base.OnFrameworkInitializationCompleted();
        }
    }
}