T must be contravariantly valid

Question

What is wrong with this?

interface IRepository where T : IBusinessEntity
{
    IQueryable GetAll();
    void Save(T t);
    void Delete         


        

Answer 1

Consider what would happen if the compiler allowed that:

interface IR
{
    void D(T t);
}

class C : IR
{
    public void D(Mammal m)
    {
        m.GrowHair();
    }
}
...
IR x = new C(); 
// legal because T is covariant and Mammal is convertible to Animal
x.D(new Fish()); // legal because IR.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.