ASP.NET Core 2 Dependency Injection Shared Singleton Service Collection Example

Program.cs

using System;
using Microsoft.AspNetCore;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.DependencyInjection;

namespace ServiceCollectionExample
{
    public class Foo { }

    [Route("api/[controller]")]
    public class FooController : Controller
    {
        private readonly Foo _foo;

        public FooController(Foo foo) => _foo = foo;

        [HttpGet]
        public string Get() => $"foo.GetHashCode() is {_foo.GetHashCode()}";
    }

    public class Startup
    {
        public void ConfigureServices(IServiceCollection services) => services.AddMvc();

        public void Configure(IApplicationBuilder app, IHostingEnvironment env) => app.UseMvc();
    }

    public class Program
    {
        public static void Main(string[] args)
        {
            var services = new ServiceCollection().AddSingleton<Foo>();
            var provider = services.BuildServiceProvider();

            var webHost = WebHost.CreateDefaultBuilder(args)
                .ConfigureServices(services2 =>
                {
                    foreach (var service in services)
                    {
                        if (service.Lifetime == ServiceLifetime.Singleton)
                            services2.AddSingleton(service.ServiceType, provider.GetService(service.ServiceType));
                    }
                })
                .UseStartup<Startup>()
                .Build();

            Console.WriteLine($"provider.GetService<Foo>().GetHashCode() is {provider.GetService<Foo>().GetHashCode()}");

            webHost.Run();
        }
    }
}

• Same singleton Foo instance
• Three different ServiceCollection instances
• Transfer of registrations between ServiceCollection instances in .ConfigureServices(serviceCollection => and ConfigureServices(IServiceCollection services) done by WebHostBuilder

services2.AddSingleton(service.ServiceType, provider.GetService(service.ServiceType));

Doesn't provider.GetService cause the instance to be created?

Also, I read that GetHashCode isn't guaranteed to be unique. Perhaps a debug statement or guid generator in the ctor would prove these are the same instances.