using System;

namespace Worx.CSharp.Basics
{
    class EnumExample
    {
        public static int Main()
        {
            WriteGreeting(TimeOfDay.Morning);
            return 0;
        }

        static void WriteGreeting(TimeOfDay timeOfDay)
        {
            switch (timeOfDay)
            {
                case TimeOfDay.Morning:
                    Console.WriteLine("good morning!");
                    break;

                case TimeOfDay.Afternoon:
                    Console.WriteLine("good afternoon!");
                    break;

                case TimeOfDay.Evening:
                    Console.WriteLine("good evening!");
                    break;

                default:
                    Console.WriteLine("hello!");
                    break;
            }
        }
    }
}


这段代码是c#高级编程第四版中的一个例子，编译时TimeOfDay下有一道绿线，不知道要怎么写才能不出错。

没太明白楼上的意思？能不能说的详细一点，谢谢。