F# does have [<Struct>] DUs now but I believe because of the runtime limitations you mention, it ends up as sizeof(fields of case 1 + fields of case 2 + ...). T

F# does have [<Struct>] DUs now but I believe because of the runtime limitations you mention, it ends up as sizeof(fields of case 1 + fields of case 2 + ...). They might reuse fields of the same type so if your DU cases can carry an A of string or B of string*int, the underlying struct is sizeof(string+int) not sizeof(string+string+int), but I'm not sure. Regardless F#'s biggest downfall is that to use it you basically have to be a C# expert already and learn the F# syntax and additional features on top of that. Otherwise it's a baffling standalone language because you're left wondering why there are two ways to do almost everything -- Task vs Async, struct ("tup", "les") vs ("regular", "tuples"), two kinds of extension methods for classes, Nullable<T> vs Option<T>, it goes on and on.