Granted the F# approach is somewhat nicer, but I don't think the C# approach with contracts is too bad (no recent experience with .Net - 6yrs ago contracts were

Granted the F# approach is somewhat nicer, but I don't think the C# approach with contracts is too bad (no recent experience with .Net - 6yrs ago contracts were painful to use), but you also could go the immutable route and via a method call easily return a new instance of User and enforce the invariant prior to creating a new instance etc.. It's a minor issue in the end, the bigger issue is that your client/stakeholder knows what they actually want and you actually capture those in-variants exhaustively and correctly (implementation is minor, F#/ocaml/haskell can't save you here)...