You cannot cast a Generic<Derived> to a Generic<Base>.
Just imagine if you could. You have a List<Wolf> and cast it to a List<Animal>. Now you could .Add() a Sheep to your List<Animal>. But wait... now your List<Wolf> contains a Sheep. What a mess.
This would only work out if you could make sure that the thing you cast to is read-only in all possible forms. This is was co- and contravariance is all about. It only works for interfaces though.
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