You are writing C# like it's F#. In F# you can write union types that are also structs:

    [<Struct>]
    type Entry = 
      | Message
      | Term
      | Comment

And like in Rust, you have pattern matching on them:

    let computeFooForEntry entry = 
      match entry with
      | Message -> 0
      | Term -> 1
      | Comment -> 2