Is C# 4.0 Tuple covariant

Question

(I would check this out for myself, but I don\'t have VS2010 (yet))

Say I have 2 base interfaces:

IBaseModelInterface
IBaseViewInterface

Answer 1

Tuple is a class (well, a family of classes) - it's invariant by definition. As you mention later on, only interfaces and delegate types support generic variance in .NET 4.

There's no ITuple interface that I'm aware of. There could be one which would be covariant, as the tuples are immutable so you only get values "out" of the API.

Answer 2

You can inherit from tuple for create your own Covariant Tuple. This way you avoid to have to rewrite your own equalities logic.

public interface ICovariantTuple<out T1>
{
    T1 Item1 { get; }
}
public class CovariantTuple<T1> : Tuple<T1>, ICovariantTuple<T1>
{
    public CovariantTuple(T1 item1) : base(item1) { }
}

public interface ICovariantTuple<out T1, out T2>
{
    T1 Item1 { get; }
    T2 Item2 { get; }
}
public class CovariantTuple<T1, T2> : Tuple<T1, T2>, ICovariantTuple<T1, T2>
{
    public CovariantTuple(T1 item1, T2 item2) : base(item1, item2) { }
}

etc.... for 3, 4, 5, 6, 7, 8 items

Compile Fail

Tuple<Exception> item = new Tuple<ArgumentNullException>(null);

Compile Success

ICovariantTuple<Exception> item = new CovariantTuple<ArgumentNullException>(null);

There is no base Tuple after 8 items, but it should be enough.