Compiler Error CS1617

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Invalid option 'option' for /langversion; must be ISO-1, ISO-2 or Default

This error occurs if you used the /langversion command line switch or project setting but didn't specify a valid language option. To resolve this error, check the command line syntax or project setting and change it to one of the listed options.

For example, compiling with csc /langversion:ISO will generate error CS1617.