static void Main()
{
    // Create the service collection and add services.
    ServiceCollection services = new();
    services.AddTransient<Application>();
    services.AddTransient<IFoo, Foo>();
    // Obtain the service resolution scope.
    using var serviceProvider = services.BuildServiceProvider();
    using var serviceScope = serviceProvider.CreateScope();
    // Application is the "Resolution Root" of the app.
    var app = serviceScope.ServiceProvider.GetRequiredService<Application>();
    // Run all.
    app.Run();
}

public class Application
{
    private readonly IFoo _foo;

    // IFoo is to be injected by DI
    public Application(IFoo foo) => _foo = foo;

    public void Run()
    {
        _foo.HelloWorld();
    }
}

public interface IFoo
{
    void HelloWorld();
}

public class Foo : IFoo
{
    public void HelloWorld() => Console.WriteLine("Hello, world!");
}