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：

1 namespace ConsoleApplication1  2 {
    3     enum Days : byte  4     {
    5         Sunday = 1,  6         Monday,  7         Tuesday,  8         Wednesday,  9         Thursday, 10         Friday, 11         Saturday 12     } 13 14     class Program 15     {
   16         static void Main(string[] args) 17         {
   18         } 19     } 20 }

　　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：

1 namespace ConsoleApplication1  2 {
    3     enum Days : byte  4     {
    5         Sunday = 1,  6         Monday,  7         Tuesday,  8         Wednesday,  9         Thursday, 10         Friday, 11         Saturday 12     } 13 14     class Program 15     {
   16         static void Main(string[] args) 17         {
   18             Days day = (Days)9; 19             Days day2 = (Days)6; 20             Console.WriteLine(day); 21             Console.WriteLine(day2); 22         } 23     } 24 }

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。