I believe that's because of covariance.
In short:
aDictionary
will be a Dictionary<SomeEnum, SomeClass>
, but in the current context it is known as Dictionary<Enum, SomeClass>
.
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<SomeEnum, SomeClass> aDictionary =
new Dictionary<SomeEnum, SomeClass>();