using System;
using System.Linq;
using System.Collections.Generic;

public class Test
{
	interface IId {int Id {get;}}
	class A : IId {
		public int Id {get;set;}
		public override string ToString() {
			return "A"+Id;
		}
	}
	class B : IId {
		public int Id {get;set;}
		public override string ToString() {
			return "B"+Id;
		}
	}
	public static void Main() {
		var a = new List<A> {new A {Id=5}, new A {Id=6}};
		var b = new List<B> {new B {Id=7}, new B {Id=8}};
		var all = new List<IEnumerable<IId>> {a, b};
		var item3 = all.SelectMany(list => list).FirstOrDefault(item => item.Id == 3);
		var item5 = all.SelectMany(list => list).FirstOrDefault(item => item.Id == 5);
		var item7 = all.SelectMany(list => list).FirstOrDefault(item => item.Id == 7);
		Console.WriteLine("3:'{0}' 5:'{1}' 7:'{2}'", item3, item5, item7);
	}
}