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

To use the latest VB.NET or C# with ASP.NET Web Applications and Web Sites projects you need to install or update two Rosyln Nuget packages.

Microsoft:

When you have a solution open which has at least one web project which is targetting .NET 4.5+ and does not have the DotNetCompilerPlatform NuGet package in the Project menu you’ll see a new option, Enable C# 6 / VB 14 appear.

The following screenshot is from VS2015 but the option should be present in VS2017 too:

Alternatively, bypass the above GUI feature and just go directly to the Nuget package manager and install the latest version of the following packages into the project/site to be as up-to-date with the VB.NET language as it is possible to be:

• Microsoft.CodeDom.Providers.DotNetCompilerPlatform
• Microsoft.Net.Compilers

(NB The latest releases are demanding .NET 4.6+ is installed on the system for the compiler to run, but can compile code targeting any platform).

This will likely get you sorted for C# 6/VB 14. There is one last step to get VB 15: edit the langversion in the web.config file so that it reads 15.0 (or latest if you want to be on the newest version - this in my preferred option).

Unfortunately I have found myself having to edit the langversion after upgrading my Roslyn to C# 7.0 and then to 7.1 so it is definitely something to look out for.

By the way, this answer doesn't just apply to old projects - even new web projects created in VS 2017 need this fix if the template used doesn't reference the Nuget compiler packages. Roslyn is not mandatory and the template for e.g. a new Web Site has reportedly not been updated.