Use attributes for value tuples

Question

In C# 7.0, .NET introduces a new return value tuple types (functional programming), so instead of:

[NotNull]
WrapperUser Lookup(int id)

I'd like to use value tuples:

(User, Info) Lookup(int id)

And I want to use attributes for these return types:

([NotNull] User, [CanBeNull] Info) Lookup(int id)

But VS2017 doesn't allow me to do it. How can I use attributes without using a wrapper class?

Answer 1

You can't.

(User, Info) Lookup(int id)

is just syntactic sugar for

ValueTuple<User,Info> Lookup(int id)

The type parameters of ValueTuple are not valid targets for attributes. Your only option besides a wrapper class is to wrap the type parameters in a NonNullable wrapper

(NonNullable<User>,NonNullable<Info>) Lookup(int id)

which allows you to use it just like a normal ValueTuple, e.g.

(NonNullable<User>,NonNullable<Info>) Lookup(int id) => (new User(), new Info());
(User user, Info info) = Lookup(5);

Otherwise you could stick a custom attribute to the whole ValueTuple indicating which tuple elements can be null with an array, like the TupleElementNamesAttribute that is used to assign names to the tuple elements. You would have to write your own visual studio / resharper plugin that does the work though.