public class No
        {
            public int Number { get; set; }
            public bool IsUsed { get; set; } = false;
        }

        public List<No> NoManage()
        {
            List<No> _list = new List<No>();
            for (int i = 1; i <= 1000; i++)
            {
                No no = new No();
                no.Number = i;
                _list.Add(no);
            }
            return _list;
        }

        public No GetNoUsed(List<No> _list)
        {
            return _list.Where(o => !o.IsUsed).SingleOrDefault();
        }