Invalid variance: The type parameter 'T' must be contravariantly valid on 'UserQuery.IItem.ItemList'. 'T' is covariant [duplicate]

Answer 1

You get the compiler error because you have a property getter (get) and a setter (set). The property getter has the T in it's output so out works, but the property setter will have the T in its input so it would need the in modifier.

Because you have out on T you need to remove the setter and it will compile:

public interface IItem where T : IFoo
{
    // IEnumerable> GetList(); // works
    IEnumerable> ItemList { get; } // also works
}

If your T is an in generic argument then the following would work:

public interface IItem where T : IFoo
{
    IEnumerable> ItemList { set; } 
}

But you cannot have both (out,in) at the same time so you cannot have a co/contravariant property with a getter and a setter.