C# Error “Is not supported by the language” after migration to .Net4

Question

I\'m trying to migration our website from .Net 3.5 to 4 and I\'m encountering a very weird issue.

Code that works just fine in 3.5 does not anymore once I target .Net4,

Answer 1

For me this occurred due to a missing or out of date System.ValueTuple when using multiple return values in c# 7.

The error message is a bit misleading.