Quickduck

Now that I’m developing in .NET 4.0 I thought all my covariance problems were behind me. This was until I tried to use the following:

public class  Animal {
    public void AttackOtherAnimals(IList<Animal> animals) { }
}

public class Dog : Animal { }

Dog angryDog = new Dog();
List dogsToAttack = new List<Dog>();
angryDog.AttackOtherAnimals(dogsToAttack);

Turns out, IEnumerable<T> has support for covariance, IList<T> does not. I’d like to thank Marc Gravell for his excellent writeup on this.

One way around this (instead of dogsToAttack.Cast<Animal>().ToList()) is to use generics to accept any IList<T> where T is of type Animal.

public void AttackOtherAnimals(IList<T> animals) where T : Animal { }