class Program
{
    static void Main(string[] args)
    {
        IServiceCollection serviceCollection = new ServiceCollection()
            .AddSingleton<IFoobar, Foo>()
            .AddSingleton<IFoobar, Bar>();

        IServiceProvider serviceProvider = serviceCollection.BuildServiceProvider();
        Console.WriteLine("serviceProvider.GetService(): {0}", serviceProvider.GetService<IFoobar>());

        IEnumerable<IFoobar> services = serviceProvider.GetServices<IFoobar>();

        Console.WriteLine("serviceProvider.GetServices():");
        foreach (IFoobar foobar in services)
        {
            Console.WriteLine("{0}",  foobar);
        }
        Console.ReadKey();
    }
}
public interface IFoobar { }
public class Foo : IFoobar { }
public class Bar : IFoobar { }