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

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I ve updated the Microsoft Net Compilers to version 2 0 or higher  see this

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In my case I was using the wrong MSBuild exe  the one found in   C  Windows Microsoft NET Framework64 v4 0 30319   To resolve the error  I updated my PATH environment variable to start using the Visual Studio 2017 MSBuild exe   C  Program Files  x86  Microsoft Visual Studio 2017 Community MSBuild 15 0 Bin MSbuild exe   Please see this link for details   Error CS1056  Unexpected character   39    39  running the msbuild on a tfs continuous integration process

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I just the changed the langversion to default and it worked for me  VS 2015   lt system codedom gt   lt compilers gt     lt 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 default  nowarn 1659 1699 1701  gt       lt providerOption name    CompilerVersion    value    v4 0   gt     lt  compiler gt     lt 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   amp quot Web  amp quot   optionInfer     gt         lt providerOption name    CompilerVersion    value    v4 0   gt     lt  compiler gt   lt  compilers gt

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In my case  I was downloading a library with sample code of keycloak implementation by mattorg from GITHUB  https   github com mattmorg55 Owin Security Keycloak tree dev samples The solution was quite easy  as I used  Net Framework 4 6 1  but the project begged me in the beginning to use 4 6 2  Although I downloaded it  it was first actively chosen  when restartet all instances of Visual Studion  or better close all instances   The project was manipulated to 4 6 1  although I wished not and chose so   So after I chose the configuration again to choose  Net Framework 4 6 1 the error vanished immediately

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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

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Upgrade the Microsoft Net Compilers Nuget package to the latest version  at least 2 x

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In my case the error message was   ASPNETCOMPILER   error CS1617  Invalid option  7 3  for  langversion  must be ISO-1  ISO-2  Default or an integer in range 1 to 6   As stated in this GitHub issue  and this VS Developer Community post it seems to be a bug in an older Microsoft CodeDom Providers DotNetCompilerPlatform NuGet package  After upgrading this NuGet package to 3 6 0 the error still persisted in my web application  Solution I found out that I had to delete an old  quot bin Roslyn quot  folder in my Web Application to make this work  It seems that the newer Microsoft CodeDom Providers DotNetCompilerPlatform NuGet package  3 6 0 in my case  does not bring a its own  quot Rosyln quot  folder anymore  and if present  that old  quot Roslyn quot  folder took precedence during compilation

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If you d like to use C  6 0    Make sure your project s  NET version is higher than 4 5 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    lt system codedom gt    lt compilers gt     lt 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    gt     lt 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   amp quot Web  amp quot   optionInfer     gt    lt  compilers gt   lt  system codedom gt

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Instead of changing the language version from 6 to 5  change the  type  attribute on the compiler tag from  Microsoft CSharp CSharpCodeProvider  System  Version 4 0 0 0  Culture neutral  PublicKeyToken b77a5c561934e089   to  Microsoft CodeDom Providers DotNetCompilerPlatform CSharpCodeProvider  Microsoft CodeDom Providers DotNetCompilerPlatform  Version 1 0 0 0  Culture neutral  PublicKeyToken 31bf3856ad364e35

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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   lt system codedom gt       lt compilers gt         lt compiler language  quot c  cs csharp quot  extension  quot  cs quot  type  quot Microsoft CSharp CSharpCodeProvider  System  Version 4 0 0 0  Culture neutral  PublicKeyToken b77a5c561934e089 quot  warningLevel  quot 4 quot  compilerOptions  quot  langversion 6  nowarn 1659 1699 1701 quot  gt           lt providerOption name  quot CompilerVersion quot  value  quot v4 0 quot   gt         lt  compiler gt         lt compiler language  quot vb vbs visualbasic vbscript quot  extension  quot  vb quot  type  quot Microsoft VisualBasic VBCodeProvider  System  Version 4 0 0 0  Culture neutral  PublicKeyToken b77a5c561934e089 quot  warningLevel  quot 4 quot  compilerOptions  quot  langversion 14  nowarn 41008  define  MYTYPE   amp quot Web  amp quot   optionInfer  quot  gt             lt providerOption name  quot CompilerVersion quot  value  quot v4 0 quot   gt         lt  compiler gt       lt  compilers gt   lt  system codedom gt   Change the langversion 6 towards the bottom from 6 to 5 - langversion 5 Fixed it for me  Thank him

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This happened to me when I pulled down my repo after moving it to a new location  The solution was to unload and then reload each project that was showing this error  I tried a few other options but nothing worked for me  Also in this situation my config files were set to use languageversion 6  so setting them to default did not fix the issue

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If above all options are not working and you have used nuget packages like Microsoft Net Compilers and CodeDom and still not working then there is issue with your project file open project file  Project file is using one of the compiler option which not support your selected language  Open project file with notepad   and remove the following line  Orignal Project File  lt Project ToolsVersion  quot 14 0 quot  DefaultTargets  quot Build quot  xmlns  quot http   schemas microsoft com developer msbuild 2003 quot  gt     lt Import Project  quot    packages Microsoft Net Compilers Toolset 3 7 0 build Microsoft Net Compilers Toolset props quot  Condition  quot Exists     packages Microsoft Net Compilers Toolset 3 7 0 build Microsoft Net Compilers Toolset props   quot    gt     lt Import Project  quot    packages Microsoft Net Compilers 3 7 0 build Microsoft Net Compilers props quot  Condition  quot Exists     packages Microsoft Net Compilers 3 7 0 build Microsoft Net Compilers props   quot    gt     lt Import Project  quot    packages Microsoft Net Compilers 1 0 0 build Microsoft Net Compilers props quot  Condition  quot Exists     packages Microsoft Net Compilers 1 0 0 build Microsoft Net Compilers props   quot    gt     lt Import Project  quot   MSBuildExtensionsPath32  Microsoft VisualStudio v  VisualStudioVersion  TypeScript Microsoft TypeScript Default props quot  Condition  quot Exists    MSBuildExtensionsPath32  Microsoft VisualStudio v  VisualStudioVersion  TypeScript Microsoft TypeScript Default props   quot    gt     lt  --Don t delete below one-- gt     lt Import Project  quot   MSBuildExtensionsPath    MSBuildToolsVersion  Microsoft Common props quot  Condition  quot Exists    MSBuildExtensionsPath    MSBuildToolsVersion  Microsoft Common props   quot    gt   Remove The following lines    lt Import Project  quot    packages Microsoft Net Compilers Toolset 3 7 0 build Microsoft Net Compilers Toolset props quot  Condition  quot Exists     packages Microsoft Net Compilers Toolset 3 7 0 build Microsoft Net Compilers Toolset props   quot    gt     lt Import Project  quot    packages Microsoft Net Compilers 3 7 0 build Microsoft Net Compilers props quot  Condition  quot Exists     packages Microsoft Net Compilers 3 7 0 build Microsoft Net Compilers props   quot    gt     lt Import Project  quot    packages Microsoft Net Compilers 1 0 0 build Microsoft Net Compilers props quot  Condition  quot Exists     packages Microsoft Net Compilers 1 0 0 build Microsoft Net Compilers props   quot    gt     lt Import Project  quot   MSBuildExtensionsPath32  Microsoft VisualStudio v  VisualStudioVersion  TypeScript Microsoft TypeScript Default props quot  Condition  quot Exists    MSBuildExtensionsPath32  Microsoft VisualStudio v  VisualStudioVersion  TypeScript Microsoft TypeScript Default props   quot    gt

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The easiest solution for me was upgrading the  Net Compilers via Package Manager Install-Package Microsoft Net Compilers and then changing the Web Config lines to this  lt system codedom gt    lt compilers gt     lt compiler language  quot c  cs csharp quot  extension  quot  cs quot  type  quot Microsoft CodeDom Providers DotNetCompilerPlatform CSharpCodeProvider  Microsoft CodeDom Providers DotNetCompilerPlatform  Version 1 0 0 0  Culture neutral  PublicKeyToken 31bf3856ad364e35 quot  warningLevel  quot 4 quot  compilerOptions  quot  langversion 6  nowarn 1659 1699 1701 quot    gt     lt compiler language  quot vb vbs visualbasic vbscript quot  extension  quot  vb quot  type  quot Microsoft CodeDom Providers DotNetCompilerPlatform VBCodeProvider  Microsoft CodeDom Providers DotNetCompilerPlatform  Version 1 0 0 0  Culture neutral  PublicKeyToken 31bf3856ad364e35 quot  warningLevel  quot 4 quot  compilerOptions  quot  langversion 14  nowarn 41008  define  MYTYPE   amp quot Web  amp quot   optionInfer  quot    gt    lt  compilers gt   lt  system codedom gt

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Turns out this was a problem  because the ASP NET MVC 4 project was referencing a specific version of the Microsoft Net Compilers package  Visual Studio was using the compiler from this specific package  and not the compiler that was installed otherwise on the computer   A warning or something would have been nice from VS2019  -   The solution then is to update the Microsoft Net Compilers package to a newer version   Version 1 x is for C  6 Version 2 x is for C  7 Version 3 x is for C  8 How I got to solve this was not immediately obvious  Visual Studio could have suggested or hinted that by me selecting a new version in the project settings that setting now conflicted with the package installed into the project    I ended up turning on Diagnostics level MSBuild logging to find out which CSC EXE the IDE is really trying to use   https   developercommunity visualstudio com content problem 519531 c-7x-versions-do-not-seem-to-work-in-vs2019 html

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in  my case  project create in another system     clean project right click on project in solution explorer and click     clean item   then build project right click on project in solution     explorer and click build item     I can run this project

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Clean Solution and Build again In my case  whatever the previous settings were blocking due to mismatch  I imported a new project and build it  tried changing versions and all  clean the solutions and build worked for me

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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    lt PropertyGroup gt       lt LangVersion gt latest lt  LangVersion gt   lt  PropertyGroup gt    I changed the latest for an integer in range 1 to 6   lt LangVersion gt 6 lt  LangVersion gt    The error disappeared

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

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