If you are running msbuild.exe via command line and get this error:

CSC : error CS1617: Invalid option '7.3' for /langversion; must be ISO-1, ISO-2, Default, Latest or a valid version in range 1 to 7.1.

e.g. from also from a build agent such as jenkins, the solution may be as easy as to upgrade Microsoft Build Tools 2015.

You can do that via choco install microsoft-build-tools or manually via the official Microsoft Build Tools 2015 or by updating your Visual Studio 2017 installation.

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