[Turskarama]

It's missing the killer feature though, which is the compiler warning you when you haven't done an exhaustive match. That is the magic which makes adding new values easy instead of hunting through code to find if you missed the new case anywhere.

[keithnz]

I used to think that.... but actually, in practice, and I've been using them for a long time now, it doesn't make much difference, good tooling will generate the cases and help you find all instances pretty quick (I use Rider).

[mycall]

I've only needed discriminated unions inside C# when I'm using pinvoke to C DLLs. I would think Rosylin supports some compiler warning for them, but I haven't checked that.