Error Invalid option '6' for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default

Question

I am trying to target .NET 4.6 and also take advantage of the latest C# version by changing the C# language version to 6.

However during compilation I got this error

Answer 1

See an answer to ASP.NET strange compilation error!.

Downgrading your C# language to 5 only masks the problem, as soon as someone uses a C# 6 feature your website/project will break and you will still spend time fixing it again. Try the solution in the link.