public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureAppConfiguration(configurationBuilder =>
                {
                    // nothing to do here
                })
                .ConfigureHostConfiguration(configurationBuilder =>
                {
                    // nothing to do here
                })
                .ConfigureServices((hostBuilderContext, serviceCollection) =>
                {
                    IConfigurationSection myConfigurationSection = hostBuilderContext.Configuration.GetSection("app");

                    serviceCollection.AddSingleton<IValidateOptions<AppOptions>, AppOptionsValidator>();
                    serviceCollection.Configure<AppOptions>(myConfigurationSection);

                    // ! Just for testing purposes !

                    IServiceProvider serviceProvider = serviceCollection.BuildServiceProvider();
                    IOptions<AppOptions> appOptions = serviceProvider.GetService<IOptions<AppOptions>>(); // appOptions threw a Microsoft.Extensions.Options.OptionsValidationException
                    AppOptions x = appOptions.Value;
                    string y = x.Foo;
                });