Enum as Dictionary keys

Question

Suppose to have

enum SomeEnum { One, Two, Three };

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

Answer 1

I believe that's because of covariance.

In short:

aDictionary will be a Dictionary, but in the current context it is known as Dictionary.

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

aDictionary.Add(someValueFromAnotherEnumUnrelatedToSomeEnum, aValue);

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

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

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

Dictionary aDictionary = 
    new Dictionary();