Ejemplo del mundo real de covarianza y contravarianza
Tengo algunos problemas para entender cómo usaría la covarianza y la contravarianza en el mundo real.
Hasta ahora, los únicos ejemplos que he visto han sido el mismo ejemplo de matriz anterior.
object[] objectArray = new string[] { "string 1", "string 2" };
Sería bueno ver un ejemplo que me permitiera usarlo durante mi desarrollo si pudiera verlo usado en otros lugares.
Aceptado
Esto es lo que preparé para ayudarme a comprender la diferencia.
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
// Contravariance
interface IGobbler<in T> {
void gobble(T t);
}
// Since a QuadrupedGobbler can gobble any four-footed
// creature, it is OK to treat it as a donkey gobbler.
IGobbler<Donkey> dg = new QuadrupedGobbler();
dg.gobble(MyDonkey());
// Covariance
interface ISpewer<out T> {
T spew();
}
// A MouseSpewer obviously spews rodents (all mice are
// rodents), so we can treat it as a rodent spewer.
ISpewer<Rodent> rs = new MouseSpewer();
Rodent r = rs.spew();
Por completitud…
// Invariance
interface IHat<T> {
void hide(T t);
T pull();
}
// A RabbitHat…
IHat<Rabbit> rHat = RabbitHat();
// …cannot be treated covariantly as a mammal hat…
IHat<Mammal> mHat = rHat; // Compiler error
// …because…
mHat.hide(new Dolphin()); // Hide a dolphin in a rabbit hat??
// It also cannot be treated contravariantly as a cottontail hat…
IHat<CottonTail> cHat = rHat; // Compiler error
// …because…
rHat.hide(new MarshRabbit());
cHat.pull(); // Pull a marsh rabbit out of a cottontail hat??
