Enum定义位域， 即可以通过位操作来产生未命名的值

通过FlagsAttribute可以实现。

// A bit field or flag enumeration of harvesting seasons.
[Flags]
public enum Seasons
{
    None = 0,
    Summer = 1,
    Autumn = 2,
    Winter = 4,
    Spring = 8,
    All = Summer | Autumn | Winter | Spring
}

Seasons sa = Seasons.Summer | Seasons.Autumn;