IList and List Conversions with Interfaces

Question

I generally understand interfaces, inheritance and polymorphism, but one thing has me puzzled.

In this example, Cat implements IAnimal

Answer 1

C# doesn't support this kind of variance on IList for type-safety reasons.

If C# did support this, what would you expect to happen here?

IList cats = new List();

cats.Add(new Dog());         // a dog is an IAnimal too
cats.Add(new Squirrel());    // and so is a squirrel

In C#4 you're able to do something like this:

IEnumerable cats = new List();

This is because the IEnumerable interface does support variance of this kind. An IEnumerable is a read-only sequence, so there's no way that you could subsequently add a Dog or a Squirrel to an IEnumerable that's actually a list of Cat.