C# inheritance in generics question

Solution 1:

The feature you are looking for is what's referred to as generics variance (covariance and contravariance). There is limited support for this starting in .net framework 4. Here's an interesting post: How is Generic Covariance & Contra-variance Implemented in C# 4.0?

And here's the MSDN entry on Covariance and Contravariance in Generics.

Solution 2:

Nope it's a covariance issue. If you could do:

Dictionary<int, A> dict = new Dictionary<int, B>();

It would be possible without a compiler error to put an object of Type A in dict.

The problem is that dict looks like: Dictionary<int, A> but it is really type Dictionary<int, B>() (so placing an object of Type A would throw a runtime error because of an invalid cast), so you shouldn't be allowed to even try to place an object of Type A in dict, which is why you can't do :

Dictionary<int, A> dict = new Dictionary<int, B>();

It's protecting you from making a runtime mistake.
You want to check out Eric Lippert's blog on the subject: Link

It's one of his favorite topics to talk about, so it's quite thorough.