{Dev Tricks}

A short introduction to Covariance and Contravariance in C# 3 preparing you to an article about that in C# 4.
So what is covariance ?
Covariance is basically using a method which returns something derived from the expected type.

An exemple ? It’s safe to have a method returning a cat when you expect it to return an animal.

In C sharp it’s

public class Animal
{
}

public class Cat : Animal
{
}

public class Dog : Animal
{
}

// It's safe to say that something returns a Animal when in fact this thing returns a Cat
class Covariance
{
        void test()
        {
                Func<Animal> a = Method; // OK
                Func<Animal> b = delegate { return new Cat(); }; // OK
                Func<Animal> c = () => new Cat(); // OK
        }

        Cat Method()
        {
                return new Cat();
        }
}

Download this code: covariance.cs

So Funcs a, b, and c are returning animals which in fact are cats, which is true.

And, what is contravariance ?
Contravariance is basically using a method which takes something which is a parent of the expected type.
An exemple ? It’s safe to have a method taking an animal when you expect it to take a cat

// It's safe to say that something can take a Cat if in fact this thing can take any Animal
class Contravariance
{
        static void test()
        {
                Action<Cat> a = Method; // OK
                Action<Cat> b = delegate(Animal value) { }; // ERROR
                // From C#3 Specification :
                // $7.14.1 Anonymous function signatures :
                // [...] contra-variance of anonymous function parameter types is not supported.
                Action<Cat> d = (Animal value) => { }; // idem... anonymous... not supported.
        }

        public static void Method(Animal value)
        {
        }
}

Download this code: contravariance.cs

So Action a take Cats, but in fact can take any Animals, so it’s safe.

This entry was posted on Monday, March 9th, 2009 at 9:19 pm and is filed under C#, Code. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.