Difference between Covariance & Contra-variance

Question

I am having trouble understanding the difference between covariance and contravariance.

Answer 1

Co and Contra variance are pretty logical things. Language type system forces us to support real life logic. It's easy to understand by example.

Covariance

For instance you want to buy a flower and you have two flowers shop in your city: rose shop and daisy shop.

If you ask someone "where is the flowers shop?" and someone tells you where is rose shop, would it be okay? Yes, because rose is a flower, if you want to buy a flower you can buy a rose. The same applies if someone replied you with the address of the daisy shop.

This is example of covariance: you are allowed to cast A to A, where C is a subclass of B, if A produces generic values (returns as a result from the function). Covariance is about producers, that's why C# use keyword out for covariance.

Types:

class Flower {  }
class Rose: Flower { }
class Daisy: Flower { }

interface FlowerShop where T: Flower {
    T getFlower();
}

class RoseShop: FlowerShop {
    public Rose getFlower() {
        return new Rose();
    }
}

class DaisyShop: FlowerShop {
    public Daisy getFlower() {
        return new Daisy();
    }
}

Question is "where is the flower shop?", answer is "rose shop there":

static FlowerShop tellMeShopAddress() {
    return new RoseShop();
}

Contravariance

For instance you want to gift a flower to your girlfriend and your girlfrend likes any flowers. Can you consider her as a person who loves roses, or as a person who loves daisies? Yes, because if she loves any flower she would love both rose and daisy.

This is an example of the contravariance: you’re allowed to cast A to A, where C is subclass of B, if A consumes generic value. Contravariance is about consumers, that's why C# use keyword in for contravariance.

Types:

interface PrettyGirl where TFavoriteFlower: Flower {
    void takeGift(TFavoriteFlower flower);
}

class AnyFlowerLover: PrettyGirl {
    public void takeGift(Flower flower) {
        Console.WriteLine("I like all flowers!");
    }
}

You're considering your girlfriend who loves any flower as someone who loves roses, and giving her a rose:

PrettyGirl girlfriend = new AnyFlowerLover();
girlfriend.takeGift(new Rose());

Links

• Post about generic programming in general
• The same answer for Java programming language