generic list and casting

Let's say I have class A and I derive class B from class A.

class A { }

class B: class A
{ }

Then I declare variable of type List<B>.

I'm wondering why it is not possible to do cast

List<B> listB;
List<A> listA = listB as List<A>
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Refael AckermannCommented:
This is a limitation of .NET generics.
The simple explanation is you're trying to cast Lists not A or B, and List<B> is not related to List<A>.
If you're using C#3, you can do:

List<A> listA = listB.Cast<A>().ToList();

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