你用Enum做过这些吗？

2012-03-17

今天我们来讲一下枚举。

enum 关键字用于声明枚举，即一种由一组称为枚举数列表的命名常量组成的独特类型。

The enum keyword is used to declare an enumeration, a distinct type that consists of a set of named constants called the enumerator list.（in MSDN）

Example1：

namespace ConsoleApplication1
{
    enum Days : byte
    {
        Sunday = 1,
        Monday,
        Tuesday,
        Wednesday,
        Thursday,
        Friday,
        Saturday
    }

    class Program
    {
        static void Main(string[] args)
        {
        }
    }
}

　　上面一段代码，声明了一个名字为Days的枚举类型，每个枚举值的类型指定为byte。枚举支持类型byte, sbyte, short, ushort, int, uint, long, or long。同时，我们改变了它的初始化值（默认为0）。但是其实.Net推荐我们使用默认值。

Example2：

namespace ConsoleApplication1
{
    enum Days : byte
    {
        Sunday = 1,
        Monday,
        Tuesday,
        Wednesday,
        Thursday,
        Friday,
        Saturday
    }

    class Program
    {
        static void Main(string[] args)
        {
            Days day = (Days)9;
            Days day2 = (Days)6;
            Console.WriteLine(day);
            Console.WriteLine(day2);
        }
    }
}

稍作修改。我们发现对day的赋值是一个越界的值，但是编译器没有报错。我们看一下输出结果：

9

Friday

因为是一个越界的值，没有正常输出，而是输出一个9，而第二个正常输出Friday。这样的赋值编译器没有报错，所以将错误隐藏到了使用的时候。可以假设，如果我们用赋了越界值的枚举变量去做判断，如果我们直接使用它时，结果我们不得而知。

Example3：

namespace ConsoleApplication1
{
    [Flags]
    enum Days
    {
        None = 0x0,
        Sunday = 0x1,
        Monday = 0x2,
        Tuesday = 0x4,
        Wednesday = 0x8,
        Thursday = 0x10,
        Friday = 0x20,
        Saturday = 0x40
    }

    class Program
    {
        static void Main(string[] args)
        {
            // Initialize with two flags using bitwise OR.
            Days meetingDays = Days.Tuesday | Days.Thursday;

            // Set an additional flag using bitwise OR.
            meetingDays = meetingDays | Days.Friday;

            Console.WriteLine("Meeting days are {0}", meetingDays);
            // Output: Meeting days are Tuesday, Thursday, Friday

            // Remove a flag using bitwise XOR.
            meetingDays = meetingDays ^ Days.Tuesday;
            Console.WriteLine("Meeting days are {0}", meetingDays);
            // Output: Meeting days are Thursday, Friday

            // Test value of flags using bitwise AND.
            bool test = (meetingDays & Days.Thursday) == Days.Thursday;
            Console.WriteLine("Thursday {0} a meeting day.", test == true ? "is" : "is not");
            // Output: Thursday is a meeting day.
        }
    }
}

使用enum作为位标志，然后可以使用位运算做一个操作，使enum更能让人能够理解它的用途用意。