c# - 枚举的意外行为

Question

我使用 System.Drawing.RotateFlipType 实现了一个方法，并且在比较值时出现了奇怪的行为。

System.Drawing.RotateFlipType item1 = System.Drawing.RotateFlipType.RotateNoneFlipNone;
System.Drawing.RotateFlipType item2 = System.Drawing.RotateFlipType.Rotate180FlipXY;
Console.WriteLine(item1 == item2); //true

这怎么可能是真的？

Answer 1

作为canton7在 a comment 中指出,

If you think about it, it makes sense. "Specifies a 180-degree clockwise rotation followed by a horizontal and vertical flip." Do that transformation in your head, and you end up back where you started.

因此，RotateFlipType 值 RotateNoneFlipNone 和 Rotate180FlipXY 都具有相同的值(0) (API source)。

这在 reference source 中很明显(删除了 XML 注释):

public enum RotateFlipType
{
    RotateNoneFlipNone = 0,
    Rotate90FlipNone   = 1,
    Rotate180FlipNone  = 2,
    Rotate270FlipNone  = 3,
    RotateNoneFlipX    = 4,
    Rotate90FlipX      = 5,
    Rotate180FlipX     = 6,
    Rotate270FlipX     = 7,
    RotateNoneFlipY    = Rotate180FlipX,
    Rotate90FlipY      = Rotate270FlipX,
    Rotate180FlipY     = RotateNoneFlipX,
    Rotate270FlipY     = Rotate90FlipX,
    RotateNoneFlipXY   = Rotate180FlipNone,
    Rotate90FlipXY     = Rotate270FlipNone,
    Rotate180FlipXY    = RotateNoneFlipNone,
    Rotate270FlipXY    = Rotate90FlipNone
}

拥有重复项没有坏处，所以我认为枚举的使用者更容易对同一事物拥有多个定义。

事实上，没有理由不能为枚举值提供重复的数值 - 实际上是 C# 语言规范 says :

Multiple enum members may share the same associated value. The example

enum Color 
{
    Red,
    Green,
    Blue,
    Max = Blue
}

shows an enum in which two enum members—Blue and Max—have the same associated value.