Enforce language version for C# 6

[Neme12]

No description provided.

[grokys]

Thanks @Neme12 - this is a great idea. Should probably also be done for the projects in tests/ but they're in the middle of a refactor (#1721) anyway, so we can do that after the refactor.

[jcansdale]

My only reservation is that if/when we merge #1726, we'll be able to use VS 2017 exclusively for development and move to a higher language version.

If we do decide to require VS 2017, what language version do you think we should target?

[Neme12]

If we do decide to require VS 2017, what language version do you think we should target?

Is that a question for me? It depends on what version of VS 2017 you're happy mandating. Unless there's need for some of the ref or in features for performance somewhere, it's mostly just minor convenience features, so probably not a huge benefit.

That said, I've gotten quite used to some of them, for example non-trailing named arguments for clarity in C# 7.2:

M("argument", allowEmpty: true, cancellationToken) // as opposed to
M("argument", allowEmpty: true, cancellationToken: cancellationToken)

I think 7.2 would probably be a good compromise, it came out with 15.5 last year. 7.3 came out very recently with 15.7 (though I would definitely recommend everyone to update, the performance improvements are really awesome) and some tools like ReSharper might not support it yet.

[grokys]

I'm not sure why it's even a question to be honest ;) As you say, we should set it at the highest lang version that the version of VS we use supports. That version is probably going to be dictated by our CI.