T must be contravariantly valid

c# .net generics covariance contravariance
Eduardo picture Eduardo · Feb 18, 2011 · Viewed 12k times · Source

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: The type parameter 'T' must be contravariantly valid on 'MyNamespace.IRepository.Delete(T)'. 'T' is covariant.

Answer

Eric Lippert picture Eric Lippert · Feb 18, 2011

Consider what would happen if the compiler allowed that:

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.

The "out" means "T is only used in output positions". You are using it in an input position.

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