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

Answer 1

Open NuGet Package Manager console and run this following command

Update-Package Microsoft.CodeDom.Providers.DotNetCompilerPlatform -r

Answer 2

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.

Answer 3

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

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

Answer 4

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.

Answer 5

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 6

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