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 constructs in VB.NET.

I get helpful pointers saying "Visual Basic 10.0 does not allow string interpolation", which was what I was trying to use, but I failed to find a way to raise the language level.

I have the tips in this related question How to change the VB.NET language version in Visual Studio 2015, but they did not help in this context.

In ReSharper properties, I could set the "VB Language Level" for each of the four different projects to "Visual Basic .NET 15". This changes a line of XML in the project's .DotSetting file, and this setting changes how ReSharper analyzes the code, but alas, this did not take away the compilation errors.

How do I enable support in Visual Studio 2017 for the latest version of VB.NET in an ASP.NET Web Site or Web Application project?

Trichloride

You need to edit Web.config to use DotNetCompilerPlatform in CodeDomProviders. – Ween

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).

<system.codedom>
  <compilers>
    <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb"
type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.8.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
warningLevel="4" compilerOptions="/langversion:15.0 /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+"/>
 </compilers>

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.

Cutlery

Answer 2

The solutions was quite easy:

• Update Nuget packages for solution, install "Microsoft.CodeDom.Providers.DotNetCompilerPlatform". That will also install Microsoft.Net.Compilers
• Then, upgrade the version of Microsoft.Net.Compilers to the latest stable version

Trichloride

You should also have NuGet install 'Microsoft.CodeDom.Providers.DotNetcompilerPlatform.WebSites', which solved my issue with this problem. – Alloway

Answer 3

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:

    <system.codedom>
      <compilers>
        <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb"
    type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.8.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
    warningLevel="4" compilerOptions="/langversion:default /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+"/>
     </compilers>
   </system.codedom>

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

    <system.codedom>
      <compilers>
        <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb"
    type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.8.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
    warningLevel="4" compilerOptions="/langversion:14.0 /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+"/>
     </compilers>
   </system.codedom>

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

Trichloride

Answer 4

This worked for me -- see /langversion:latest below

<system.codedom>
  <compilers>
    <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
    warningLevel="4" compilerOptions="/langversion:latest /nowarn:1659;1699;1701"/>
    <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb"
    type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
    warningLevel="4" compilerOptions="/langversion:latest /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+"/>
  </compilers>
</system.codedom>

Flabby

You had a typo in your text there, but not in the code: /langversion:latest (and not "lastest"). – Trichloride