Tuesday, January 23, 2018

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

After setting the Language version to C# latest minor version for my project, our TFS build started to fail with the following error message:

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

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While looking through the build logs I noticed that the build server was still using MSBuild v14 instead of 15. I first thought that my build agents were not up to date, so I triggered an upgrade: http://bartwullems.blogspot.be/2017/09/team-foundation-serverupgrade-your.html

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This made no difference, the used build tasks were not able to discover the newer version of MSBuild on the build server as they didn’t use VSWhere.exe yet.

In the end I decided to do an upgrade of TFS(a TFS 2017 instance) to TFS 2017 Update 3. After doing that, a new version of the Build task was available that correctly found MSBuild v15.0 and allowed me to use the new C# features. Finally!

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