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
interface IGobbler {
    void gobble(T t);
}

// Since a QuadrupedGobbler can gobble any four-footed
// creature, it is OK to treat it as a donkey gobbler.
IGobbler dg = new QuadrupedGobbler();
dg.gobble(MyDonkey());

// Covariance
interface ISpewer {
    T spew();
}

// A MouseSpewer obviously spews rodents (all mice are
// rodents), so we can treat it as a rodent spewer.
ISpewer rs = new MouseSpewer();
Rodent r = rs.spew();

For completeness…

// Invariance
interface IHat {
    void hide(T t);
    T pull();
}

// A RabbitHat…
IHat rHat = RabbitHat();

// …cannot be treated covariantly as a mammal hat…
IHat 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 cHat = rHat;  // Compiler error
// …because…
rHat.hide(new MarshRabbit());
cHat.pull();                   // Pull a marsh rabbit out of a cottontail hat??