Compiler Error CS1617

Invalid option 'option' for LangVersion. Use <LangVersion>?</LangVersion> to list supported values.

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.

Valid values for -langversion

The valid values for the language versions depend on the .NET version you are using. See the language version rules for more information on which language version is available with which version of .NET. If you are receiving this error while attempting to use a newer language version, either downgrade to a lower language version or update your .NET SDK to a version that supports the language version.

The following table specifies the current valid values for -langversion:

Value	Meaning
preview	The compiler accepts all valid language syntax from the latest preview version.
latest	The compiler accepts syntax from the latest released version of the compiler (including minor version).
latestMajor or default	The compiler accepts syntax from the latest released major version of the compiler.
12.0	The compiler accepts only syntax that is included in C# 12 or lower.
11.0	The compiler accepts only syntax that is included in C# 11 or lower.
10.0	The compiler accepts only syntax that is included in C# 10 or lower.
9.0	The compiler accepts only syntax that is included in C# 9 or lower.
8.0	The compiler accepts only syntax that is included in C# 8.0 or lower.
7.3	The compiler accepts only syntax that is included in C# 7.3 or lower.
7.2	The compiler accepts only syntax that is included in C# 7.2 or lower.
7.1	The compiler accepts only syntax that is included in C# 7.1 or lower.
7	The compiler accepts only syntax that is included in C# 7.0 or lower.
6	The compiler accepts only syntax that is included in C# 6.0 or lower.
5	The compiler accepts only syntax that is included in C# 5.0 or lower.
4	The compiler accepts only syntax that is included in C# 4.0 or lower.
3	The compiler accepts only syntax that is included in C# 3.0 or lower.
ISO-2 or 2	The compiler accepts only syntax that is included in ISO/IEC 23270:2006 C# (2.0).
ISO-1 or 1	The compiler accepts only syntax that is included in ISO/IEC 23270:2003 C# (1.0/1.2).