gistfile1.txt

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    public interface IFoo
    {
        int Value { get; }
    }

    public interface IBar : IFoo
    {
        int Stuff();
    }

    public class Foo : IFoo
    {
        public int Value { get; set;  }
    }

    public class Bar : IBar
    {
        public int Value { get; set; }

        public int Stuff()
        {
            return Value;
        }
    }

    public class Container
    {
        private IList<IFoo> _list = new List<IFoo>();

        public void Add<T>(T o) where T : IFoo
        {
            _list.Add(o);
        }

        public IEnumerable<IFoo> GetList<T>() where T : IFoo
        {
            return _list.Where(e => typeof(T).IsAssignableFrom(e.GetType()));
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var c = new Container();
            c.Add(new Foo { Value = 1 });
            c.Add(new Bar { Value = 42 });
            var l = c.GetList<IBar>();
        }
    }
}