List<int> a = new List<int>() { 1, 2, 3, 6, 8, 7 };
            List<int> b = new List<int>() { 1, 2, 3, 4, 5, 6 };
            List<int> c = b.Except(a).ToList();
            foreach (int i in c)
            {
                Console.WriteLine(i); //4  5  
            }
            Console.Read();