Why you shouldn't use LangVersion = latest

Unless you absolutely must.

Never being one to be concerned about being controversial, this is why I don't think you should ever use the LangVersion of "latest" in your C# projects.

TLDR: "Latest" can mean different things on different machines and at different times. You avoid the potential for inconsistencies and unexpected behavior by not using it.

I've had the (let's call it) privilege of working as part of a certain distributed team on a large code base.

There was a global setting, hidden away somewhere, which set the LangVersion to latest. This was mostly fine until one member of the "team" updated the code to use some language features in version 10 on the day it was released. They committed the code, and other team members pulled down the changes. But now the other team members (who hadn't yet updated their tooling--it was, after all, only release day) were getting errors and they couldn't understand why code was suddenly breaking. The use of the new language features wasn't necessary, and the confusion and wasted time/effort trying to work out where the errors had come from could have been avoided if the actual number had been updated in the settings and they hadn't relied on a value that changes over time and from machine to machine. The rest of the team would still have had to update their tool, but at least they would have gotten a useful error message.

And then, there was another project I worked on.

We were using an open-source library and as part of some "clever" MSBuild task that they included in the package, it was doing a check for the LangVersion. At some point in the past, they needed to do a workaround for a specific version number and also added that workaround for "latest". In time, the underlying issue was fixed, and so when the package was used in a project that tried to use the "Latest" LangVersion, the package was trying to apply a fix that was no longer necessary and actually caused an exception. Yes, this was a pain to resolve. And yes, by "pain", I mean a long, frustrating waste of time and effort.

"Latest" may be useful if you're doing cutting-edge work that is only supported by that LangVersion setting. For all other cases, you should specify an actual number.

Of course, you're free to ignore these arguments. If you do, I'd love to know why.