Error Invalid option '6' for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default

Question

I am trying to target .NET 4.6 and also take advantage of the latest C# version by changing the C# language version to 6.

However during compilation I got this error:

Error Invalid option '6' for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default

If I update the /langversion:6 in Web.Config setting to 5 it works,

  <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">

But how do I resolve this without resorting to lower language version?

I am using Visual Studio 2015 Community Edition, and I have also installed .NET Framework 4.6 manually just in case it was not installed by default by Visual Studio 2015.

The project is a standard ASP.NET MVC template project created by Visual Studio 2015.

Answer 1

Pay attention to compiler "type" in the Web.Config file, when changing Framework version:

for 4.5 and C#5 -

type="Microsoft.CSharp.CSharpCodeProvider...

for 4.6 and C#6 -

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

More info in this forum post

Answer 2

Update the following NuGet packages (whichever installed) to resolve the problem:

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

Answer 3

1.Go to Project and select your Project properties.

2 Select Build and Click Advanced Button.

3.Select Language Version default.

4.Save .

https://youtu.be/IP8feQeWqkk

Answer 4

I met such behavior if project was not fully upgraded to VS2015. In case if you open solution from 2015 in VS2013 - you will get this error

Answer 5

See an answer to ASP.NET strange compilation error!.

Downgrading your C# language to 5 only masks the problem, as soon as someone uses a C# 6 feature your website/project will break and you will still spend time fixing it again. Try the solution in the link.