/* http://goo.gl/53fWh
* The first thing to understand is that it is a relation which is *variant.
* If it preserves the direction of assignment, then it is covariant.
*/
public Covariance()
{
// A valid assignment operation
Animal animal = new Giraffe();
// This line shows that IEnumerator<T> is covariant for type T
IEnumerable<Animal> animals = new List<Giraffe>();
}
static void CombineVariances()
{
ReturnOneAnimal pickGiraffe = new ReturnOneAnimal(ReturnGiraffe);
Giraffe giraffe = new Giraffe();
Rabbit rabbit = new Rabbit();
pickGiraffe(rabbit, giraffe);
}
static void BrokenCovariance()
{
// An example showing how the covariance is broken wrt
// Arrays in C#
Animal[] animals = new Giraffe[10];
// Although the below code doesn't throw a compile time error, it will
// fail at runtime because Rabbit != Giraffe
animals[4] = new Rabbit();
}
static void MethodParamContravariance()
{
UseSpecificAnimals useSpecificAnimals = new UseSpecificAnimals(UseUnspecificAnimals);
// While the delegate can accept the method which less derived params,
// the method itself requires that correct objects are passed.
Giraffe giraffe = new Giraffe();
Rabbit rabbit = new Rabbit();
useSpecificAnimals(giraffe, rabbit);
}
public void ArrayCovariance()
{
// Arrays are always covariant since
Animal[] animals = new Giraffe[10];
// and
object[] strings = new String[5];
// but these can be misused, since arrays are broken (http://goo.gl/guLSO)
// strings[0] = 12;
// The above code compiles fine but breaks at runtime.
}
static void ReturnTypeCovariance()
{
Giraffe giraffe = new Giraffe();
// This is an example of method group being covariant in their
// return types. The delegate definition very clearly wants an
// Animal from the method group but the definition of MakeGiraffe
// returns a giraffe.
// From the article at Eric Lippert's blog: http://goo.gl/bpDPPX
// "The caller of func is never going to get anything that they’re
// not capable of dealing with."
AnimalMaker animalMaker = new AnimalMaker(giraffe.MakeGiraffe);
}