App.axaml.cs

using Avalonia;
using Avalonia.Controls.ApplicationLifetimes;
using Avalonia.Data.Core.Plugins;
using Avalonia.Markup.Xaml;
using Tessera.Services;
using Microsoft.Extensions.DependencyInjection;
using Tessera.ViewModels;
using Tessera.Views;
using System;
using Avalonia.Controls.Shapes;
using static System.Runtime.InteropServices.JavaScript.JSType;

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

        public override void OnFrameworkInitializationCompleted()
        {
            if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
            {
                var services = new ServiceCollection();
                services.AddSingleton<IFilesService>(x => new FilesService(desktop.MainWindow));
                services.AddSingleton<IFileEntityRelationsService>(x => new FileEntityRelationsService());
                Services = services.BuildServiceProvider();

                // Line below is needed to remove Avalonia data validation.
                // Without this line you will get duplicate validations from both Avalonia and CT
                BindingPlugins.DataValidators.RemoveAt(0);
                
                desktop.MainWindow = new MainWindow
                {
                    DataContext = new MainWindowViewModel(),
                };
            }

            base.OnFrameworkInitializationCompleted();
        }

        public new static App? Current => Application.Current as App;

        /// <summary>
        /// Gets the <see cref="IServiceProvider"/> instance to resolve application services.
        /// </summary>
        public IServiceProvider? Services { get; private set; }
    }
}