mvvm-community-demo/MvvmCommunityApp/App.xaml.cs

using CommunityToolkit.Mvvm.DependencyInjection;
using Microsoft.Extensions.DependencyInjection;
using MvvmAppCore.ViewModels;
using MvvmCommunityApp.services;
using MvvmAppCore.Interfaces;
using System;
using System.Windows;

namespace MvvmCommunityApp
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application
    {
        public new static App Current => (App)Application.Current;

        public App()
        {
            // !!! Add constructor initialization HERE !!!!
            // configure mvvm ioc
            ConfigureServices();
        }

        private void ConfigureServices() =>
            // setup mvvm ioc
            Ioc.Default.ConfigureServices(
                new ServiceCollection()
                    .AddTransient<IocPageViewModel>()
                    .AddSingleton<IDemoService, DemoService>()
                    .AddSingleton<MainWindowViewModel>()
                    .BuildServiceProvider());

        protected override void OnStartup(StartupEventArgs e)
        {
            base.OnStartup(e);

            // setup mvvm ioc
            //Ioc.Default.ConfigureServices(
            //    new ServiceCollection()
            //        .AddTransient<IocPageViewModel>()
            //        .AddSingleton<IDemoService, DemoService>()
            //        .BuildServiceProvider());

            // load the main / entry page
            MainWindow wnd = new MainWindow();

            wnd.Title = "Something else";


            wnd.Show();
        }
    }
}