I have done all the searching I can think of, and I don't see anything stating which version of C# is available for Script, Expression, or other nodes where C# syntax is used.
So I keep trying to use things like string interpolation or declaring out values inline as arguments, and they work fine where I first try them, in VS, but of course the nodes error-out once I copy-paste and try to run them in flowgear proper.
If someone in-the-know could answer here, that would work almost as well as if the actual Flowgear Help Center had that information.