async和await異步操作

class Program
    {
        static void Main(string[] args)
        {
            // 同步方式
            Console.WriteLine("異步方式測試開始！");
            AstbcMethod(35);
            Console.WriteLine("異步方式結束！");
            Console.ReadKey();
        }

        private static async void AstbcMethod(int input)
        {
            Console.WriteLine("進入異步操作！");
            var result = await AsyncWork(input);
            Console.WriteLine("最終結果{0}", result);
            Console.WriteLine("退出異步操作！");
        }

        private static async Task<int> AsyncWork(int val)
        {
            for (int i = val; i < 40; i++)
            {
                Console.WriteLine("耗時操作{0}", i);
                var result = await Task.Factory.StartNew((Func<object, int>)Fn, i);
                Console.WriteLine("耗時操作j結果{0}", result);
                val++;  
            }
            return val;
        }

        //斐波那契數列
        private static int Fn(object n)
        {
            var i = (int) n;
            if (i <= 0)
            {
                throw new ArgumentOutOfRangeException();
            }
            if (i == 1 || i == 2)
            {
                return 1;
            }
            return checked(Fn(i - 1) + Fn(i - 2)); // when n>46 memory will  overflow
        }
    }