C# 6.0 Features Not Working with Visual Studio 2015

This works in MVC 5 (tested 5.2.3), you just need to add the roslyn code dom Nuget package

CodeDOM Providers for .NET Compiler...

Replacement CodeDOM providers that use the new .NET Compiler Platform ("Roslyn") compiler as a service APIs. This provides support for new language features in systems using CodeDOM (e.g. ASP.NET runtime compilation) as well as improving the compilation performance of these systems.

PM> Install-Package Microsoft.CodeDom.Providers.DotNetCompilerPlatform

https://www.nuget.org/packages/Microsoft.CodeDom.Providers.DotNetCompilerPlatform/

Well, I have MVC5 and recently installed VS 2015.

I've installed CodeDOM providers package, but it didn't help... But after that I realized, that package supports framework 4.5 only, while I have target framework set to 4.6 during tests - it works with 4.5 though...

So pay attention also to target framework. If you have 4.5 - just install package Microsoft.CodeDom.Providers.DotNetCompilerPlatform. But if you have 4.5.1-4.6 as a target, you will have to change in web.config section

  <system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CSharp.CSharpCodeProvider, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701">
          <providerOption name="CompilerVersion" value="v4.0"/>
      </compiler>
      <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:14 /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+"/>
    </compilers>
  </system.codedom>

For C#, just change type to:

type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" 

I was having this same problem in Visual Studio 2015. Another answer here alluded to the solution I used, but they incorrectly specified the fix and never gave clarification.

On the the Visual Studio menu, select Project and you should see the sub-item Enable C#6 / VB 14 if you are having this problem. Select this menu sub-item. It will download the correct packages from Nuget and install them. After this, restart Visual Studio and reload your solution.

I cannot verify if this will also fix the Project Properties > Build > Advanced > Language Version selection to C# 6, so you might want to check this as well after enabling C# 6 from the menu.