Covariance and contravariance real world example

Question

I\'m having a little trouble understanding how I would use covariance and contravariance in the real world.

So far, the only examples I\'ve seen have been the same o

Answer 1

Contravariance

In the real world, you can always use a shelter for animals instead of a shelter for rabbits because every time an animal shelter hosts a rabbit it is an animal. However, if you use a rabbit shelter instead of an animal shelter its staff can get eaten by a tiger.

In code, this means that if you have an IShelter animals you can simply write IShelter rabbits = animals if you promise and use T in the IShelter only as method parameters like so:

public class Contravariance
{
    public class Animal { }
    public class Rabbit : Animal { }

    public interface IShelter
    {
        void Host(T thing);
    }

    public void NoCompileErrors()
    {
        IShelter animals = null;
        IShelter rabbits = null;

        rabbits = animals;
    }
}

and replace an item with a more generic one, i.e. reduce the variance or introduce contravariance.

Covariance

In the real world, you can always use a supplier of rabbits instead of a supplier of animals because every time a rabbit supplier gives you a rabbit it is an animal. However, if you use an animal supplier instead of a rabbit supplier you can get eaten by a tiger.

In code, this means that if you have an ISupply rabbits you can simply write ISupply animals = rabbits if you promise and use T in the ISupply only as method return values like so:

public class Covariance
{
    public class Animal { }
    public class Rabbit : Animal { }

    public interface ISupply
    {
        T Get();
    }

    public void NoCompileErrors()
    {
        ISupply animals = null;
        ISupply rabbits = null;

        animals = rabbits;
    }
}

and replace an item with a more derived one, i.e. increase the variance or introduce covariance.

All in all, this is just a compile-time checkable promise from you that you would treat a generic type in a certain fashion to keep the type safety and not get anyone eaten.

You might want to give this a read to double-wrap your head around this.