[sparkie]

C# doesn't allow you to specify both abstract and sealed on the same class - something that would be really useful for implementing closed types. The restriction is artificial and unnecessary though - CIL allows the two to coexist, and it is how the F# compiler implements discriminated unions. I think it would be wise to aim for feature-compatibility with F#, to ensure you can pattern match over types defined in F# from C#, and vice-versa.

[Locke1689]

You're right of course -- I misspoke. Marking the types internal, not sealed, would allow the compiler to guarantee that all it sees is all there is.