public void Variance_HigherOrderFunctions()
{
//GenericHolder itself is **contravariant** on the T parameter (like an Action<T>)
GenericHolder <Animal> animalHolder = (a) => { Console.WriteLine(a.ToString()); };
GenericHolder <Mammal> mammalHolder = (m) => { Console.WriteLine(m.ToString()); };
GenericHolder <Giraffe> giraffeHolder = (g) => { Console.WriteLine(g.ToString()); };
animalHolder(new Animal());
animalHolder(new Mammal());
animalHolder(new Giraffe());
mammalHolder(new Mammal());
mammalHolder(new Giraffe());
giraffeHolder(new Giraffe());
giraffeHolder = mammalHolder = animalHolder;
//Meta is **covariant** on the input parameter of type GenericHolder
Meta <Giraffe> giraffeMeta = (GenericHolder <Giraffe> holder) => holder(new Giraffe());
Meta <Mammal> mammalMeta = (GenericHolder <Mammal> holder) => holder(new Mammal());
Meta <Animal> animalMeta = (GenericHolder <Animal> holder) => holder(new Animal());
giraffeMeta(giraffeHolder);
giraffeMeta(mammalHolder);
giraffeMeta(animalHolder);
mammalMeta(mammalHolder);
mammalMeta(animalHolder);
animalMeta(animalHolder);
animalMeta = mammalMeta = giraffeMeta;
Assert.Pass("If this line executes, means that contravariance works as expected :)");
}
private void StoreTestData(IObjectContainer container)
{
for (int i = 0; i < NumberOfItems; i++)
{
var item = new Item(i);
container.Store(ItemHolder.Create(item));
container.Store(GenericHolder.Create(item));
}
}
private void StoreTestData(IObjectContainer container)
{
for (int i = 0; i < NumberOfItems; i++)
{
var item = new Item(i);
container.Store(ItemHolder.Create(item));
container.Store(GenericHolder.Create(item));
container.Store(
CollectionHolder.Create(
item,
new Item(NumberOfItems + i),
new Item(2 * NumberOfItems + i)));
}
container.Commit();
}
