Is C# 4.0 Tuple covariant
c#, covariance, tuples
Solution
`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.
Problem
(I would check this out for myself, but I don't have VS2010 (yet)) Say I have 2 base interfaces: ``` IBaseModelInterface IBaseViewInterface ``` And 2 interfaces realizing those: ``` ISubModelInterface : IBaseModelInterface ISubViewInterface : IBaseViewInterface ``` If I define a `Tuple<IBaseModelInterface, IBaseViewInterface>` I would like to set that based on the result of a factory that returns `Tuple<ISubModelInterface, ISubViewInterface>`. In C# 3 I can't do this even though the sub interfaces realize the base interfaces. And I'm pretty sure C# 4 lets me do this if I was using `IEnumerable<IBaseModelInterface>` because it's now defined with the `in` keyword to allow covariance. So does `Tuple` allow me to do this? From what (little) I understand, covariance is only allowed on interfaces, so does that mean there needs to be an `ITuple<T1, T2>` interface? Does this exist?