CS1617: Invalid option ‘6’ for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default

Question

Kept getting that error when running a fresh ASP.NET MVC application (straight out of VS 2015 Community) on a Windows 2008 Server machine.

Answer 1

Lots of people are running into it, so I wanted to post here .

Found a crazy easy workaround thanks to this guy: Expertdebugger.

Simply downgrade your language version from C# 6 to C# 5, like so:

<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.VisualBasic.VBCodeProvider, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" warningLevel="4" compilerOptions="/langversion:14 /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+">
          <providerOption name="CompilerVersion" value="v4.0"/>
      </compiler>
    </compilers>
</system.codedom>

Change the langversion:6 towards the bottom from 6 to 5 - langversion:5

Fixed it for me. Thank him.

Answer 2

If you'd like to use C# 6.0:

1. Make sure your project's .NET version is higher than 4.5.2.
2. And then check your .config file to perform the following modifications.

Look for the system.codedom and modify it so that it will look as shown below:

<system.codedom>
 <compilers>
  <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701" />
  <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>

Answer 3

In my case the error was:

Error CS1617 Invalid option 'latest' for /langversion; must be ISO-1, ISO-2, Default or an integer in range 1 to 6.

I opened my .csproj file with notepad and I saw this line:

<PropertyGroup>
    <LangVersion>latest</LangVersion>
</PropertyGroup>

I changed the latest for an integer in range 1 to 6

<LangVersion>6</LangVersion>

The error disappeared!

Answer 4

I found that the direct cause for the error in my case was:

this makes sense since the error is stating that there is an invalid option for language.

but, this was working fine before - so it must've been selected. what changed? turns out a member on my team upgraded to vs 2017, while i was still using 2015. after he made changes to the project, the language version was changed and i received that change over source control. but the version selected was not available to my version of vs, so it was blank - hence the error. after selecting a value in the language drop down (i chose default), a new error popped up. the new error was causing a build failure on any lines of code which used the newer version of c#. i changed the code to perform the same functions, but with my c# version syntax and problem solved.

so while the direct cause of the error was indeed an invalid selection of Language Version, the root cause was due to different vs/c# versions conflicting.

Answer 5

in my case (project create in another system):

1. clean project(right click on project in solution explorer and click clean item).
2. then build project(right click on project in solution explorer and click build item).

I can run this project.