private static void ChildTest1()
        {
            FriendColl friends = new FriendColl();

            friends[1] = new Friend("fwq");
            friends.Add(new Friend("zxc", 100));
            foreach (Friend f in friends)
            {
                Console.WriteLine(f.Name + "     " + f.Age);
            }
            Console.WriteLine("---------------------------------------------");
            Console.WriteLine("使用yield return 返回一个枚举序列");
            foreach (int ii in WithIterator())
            {
                Console.WriteLine(ii);
            }
            var tempList = WhereListWithIterator(friends);

            foreach (var ss in tempList)
            {
                Console.WriteLine(ss);
            }
            Console.ReadKey();
        }