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.
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