新手求助，，，为什莫这样啊，，谢谢啦

        static void Main(string[] args)
        {
            char a = A;
            switch (a)
            {
                case A:
                    Console.WriteLine("优秀");
                    break;
                default:
                    Console.WriteLine("错误");
                    break;

            } Console.ReadLine();

       提示 上下文不存在A      啥意思啊
------解决方案--------------------
static void Main(string[] args)
        {
            char a = 'A';
            switch (a)
            {
                case 'A':
                    Console.WriteLine("优秀");
                    break;
                default:
                    Console.WriteLine("错误");
                    break;

            } Console.ReadLine();