枚举作为字典键 [英] Enum as Dictionary keys

问题描述

假设拥有

enum SomeEnum { One, Two, Three };

SomeEnum是一个枚举，因此应该从Enum继承，所以为什么写：

SomeEnum is an enum so it is supposed to inherit from Enum so why if I write:

Dictionary<Enum, SomeClass> aDictionary = new Dictionary<SomeEnum, SomeClass>();

编译器抱怨无法将SomeEnum隐式转换为Enum？

The compiler complains that it cannot implicitly convert SomeEnum to Enum?

推荐答案

我认为这是因为协方差。

简而言之：

aDictionary 将是 Dictionary< SomeEnum，SomeClass> ，但在当前上下文中它称为字典< Enum，SomeClass> 。

aDictionary will be a Dictionary<SomeEnum, SomeClass>, but in the current context it is known as Dictionary<Enum, SomeClass>.

如果您的声明被允许，则编译器随后应允许您这样做：

Had your declaration been allowed, the compiler should afterwards let you do:

aDictionary.Add(someValueFromAnotherEnumUnrelatedToSomeEnum, aValue);

这显然与字典的实际类型不一致。

which is obviously inconsistent with respect to the actual type of the dictionary.

这就是默认情况下不允许协方差的原因，并且在有意义的情况下必须显式启用它。

That's why co-variance is not allowed by default and you have to explicitly enable it in cases where it makes sense.

结论是您必须完全指定类型：

The conclusion is that you have to specify the type exactly:

Dictionary<SomeEnum, SomeClass> aDictionary = 
    new Dictionary<SomeEnum, SomeClass>();

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