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Covariance and contravariance real world example Ask Question

tag-icon • tag-icon c# c#-4.0 covariance
Covariance and contravariance real world example Ask 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 old array example.

object[] objectArray = new string[] { "string 1", "string 2" };

It would be nice to see an example that would allow me to use it during my development if I could see it being used elsewhere.

ベストアンサー1

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

public interface ICovariant<out T> { }
public interface IContravariant<in T> { }

public class Covariant<T> : ICovariant<T> { }
public class Contravariant<T> : IContravariant<T> { }

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

public class TheInsAndOuts
{
    public void Covariance()
    {
        ICovariant<Fruit> fruit = new Covariant<Fruit>();
        ICovariant<Apple> apple = new Covariant<Apple>();

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

    public void Contravariance()
    {
        IContravariant<Fruit> fruit = new Contravariant<Fruit>();
        IContravariant<Apple> apple = new Contravariant<Apple>();

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

    public void Covariant(ICovariant<Fruit> fruit) { }

    public void Contravariant(IContravariant<Apple> apple) { }
}

tldr

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

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