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

Here's what I put together to help me understand the difference

public interface ICovariant { }
public interface IContravariant { }

public class Covariant : ICovariant { }
public class Contravariant : IContravariant { }

public class Fruit { }
public class Apple : Fruit { }

public class TheInsAndOuts
{
    public void Covariance()
    {
        ICovariant fruit = new Covariant();
        ICovariant apple = new Covariant();

        Covariant(fruit);
        Covariant(apple); //apple is being upcasted to fruit, without the out keyword this will not compile
    }

    public void Contravariance()
    {
        IContravariant fruit = new Contravariant();
        IContravariant apple = new Contravariant();

        Contravariant(fruit); //fruit is being downcasted to apple, without the in keyword this will not compile
        Contravariant(apple);
    }

    public void Covariant(ICovariant fruit) { }

    public void Contravariant(IContravariant apple) { }
}

tldr

ICovariant apple = new Covariant(); //because it's covariant
IContravariant fruit = new Contravariant(); //because it's contravariant