HowTo: Use latest C# features in MVC5 Razor views (.cshtml)

Having recently updated an ASP.net MVC web app from MVC4 to MVC5 and from .NET 4.5 to .NET 4.7.2 I was expecting Razor views (.cshtml files) to use the latest C# compiler, especially since at Properties/Build/Advanced option for the web project one read “C# latest major version (default)”.

However that was not the case and trying to use newer C# language features like the ?. ternary conditional operator or interpolated strings (or nameof etc.) would show errors like

Feature ‘interpolated strings’ is not available in C# 5. Please use language version 6 or greater.

Luckily there is a workaround for using the latest C# compiler in MVC5. Just need to add the NuGet package https://www.nuget.org/packages/Microsoft.CodeDom.Providers.DotNetCompilerPlatform/ to one’s project as explained at https://dusted.codes/using-csharp-6-features-in-aspdotnet-mvc-5-razor-views. Alternatively one could move their project to ASP.net Core, which is a more drastic move though.

After doing it I started seeing Intellisense issues in .cshtml like:

The type ‘Expression<>’ is defined in an assembly that is not referenced. You must add a reference to assembly ‘System.Core …

Tried to add the System.Core assembly to the project, but wasn’t allowed (it said the Build system was adding it). Adding System.Core as a NuGet package would mean moving to .NET Core which I wasn’t ready to try with that project yet.

Seems there was an easy solution to that, just closed and reopened the Visual Studio solution and did a Rebuild and all was fine after that.