Error Invalid option '6' for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default
C#.Netasp.net Mvc-5.2.Net 4.6C# Problem Overview
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.
C# Solutions
Solution 1 - C#
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"
Solution 2 - C#
Update the following NuGet packages (whichever installed) to resolve the problem:
- Microsoft.CodeDom.Providers.DotNetCompilerPlatform
- Microsoft.Net.Compilers
Solution 3 - C#
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">https://youtu.be/IP8feQeWqkk</a>Solution 4 - C#
Open NuGet Package Manager console and run this following command
Update-Package Microsoft.CodeDom.Providers.DotNetCompilerPlatform -r
Solution 5 - C#
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
Solution 6 - C#
I had similar issue when I was loading application for the first time in visual studio 2019. Updating "Microsoft.CodeDom.Providers.DotNetCompilerPlatform" nuget to later version did the trick for me.
Solution 7 - C#
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.