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

IList is not a covariant interface (or it would be IList). This is because IList both takes type T as a parameter and returns it as a return value from methods, which makes covariance problematic.

For example, if in your example:

IList cats = new List();

You wanted to add a new cat, from cats, it would allow:

cats.Add(new Dog());

assuming Dog also implemented IAnimal, this is obviously incorrect and wouldn't work. Which is why IList is not a covariant or contravariant interface.