Help on Covariance and Contravariance in C#
Hi there! I am trying to understand Covariance and Contravariance. Now I am thinking about why In and Out permitted only generic class T in parameter and output only? I found this example of Eric Lippert on stackoverflow to be great to illustrate why T in parameter for Out is impossible
interface IR<out T>
{
void D(T t);
}
class C : IR<Mammal>
{
public void D(Mammal m)
{
m.GrowHair();
}
}
...
IR<Animal> x = new C();
// legal because T is covariant and Mammal is convertible to Animal
x.D(new Fish()); // legal because IR<Animal>.D takes an Animal
And you just tried to grow hair on a fish!
I try to come up with an equivalent to Contravariance but I couldn't. Please help me. I somehow think that Contravariance with T as output is not impossible as the case with Covariance and T in parameter. :nekoknife:
Link the original stackoverflow: https://stackoverflow.com/a/5043054/25500815Stack Overflow
T must be contravariantly valid
What is wrong with this?
interface IRepository<out T> where T : IBusinessEntity
{
IQueryable<T> GetAll();
void Save(T t);
void Delete(T t);
}
It says:
Invalid variance: Th...
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