How can I use the latest VB.NET language level in an ASP.NET web site project?

Question

I\'ve been tasked with modernising a web application developed in 2009. It is written in VB.NET and using ASP.NET WebForms. I would like to use the latest language construc

Answer 1

And this is the solution to get rid of the red squiggles underneath any language construct that Visual Studio 2017 thinks is unsupported.

The web.config contained this:

in which /langversion:default had to be replaced with /langversion:14.0 like this:

I found a reference to this on https://github.com/aspnet/RoslynCodeDomProvider/issues/16