Exposing Discriminaed Unions from F# to C#

Consumer.cs

using System;
using FsLibrary;

namespace CsConsumer
{
    public static class Program
    {
        public static void Main(string[] _)
        {
            var msg = Vsn.DefaultScheme.Either(
                unknown: () => "Unknown",
                serial: rev => $"r{rev}",
                semantic: (m, n, p) => $"v{m}.{n}.{p}"
            )
            
            Console.WriteLine(msg);
        }
    }
}

Library.fs

namespace FsLibrary

type Vsn = {
  Scheme : VsnScheme
  Suffix : string option
} with
  static member DefaultSchema = VsnScheme.Unknown
and VsnScheme =
  | Unknown
  | Serial of revision : uint64
  | Semantic of major : byte * minor : byte * patch : byte
  
  
// it's often useful to group the "C#-facing" API into separate components
[<Extension>]
type VsnExtensions =

  [<Extension>]
  static member Either
    (
      scheme : VsnScheme,
      unknown : Func<'T>, 
      serial : Func<uint64, 'T>, 
      semantic : FromSemanticVsn<'T>   // in addition to common delegates, using named delegates is also an option
    ) =
      // null checks elided for clarity
      match me with
      | Unknown -> unknown.Invoke()
      | Serial revision -> serial.Invoke(revision)
      | Semantic (m, n, p) -> semantic.Invoke(m, n, p)

and FromSemanicVsn<'T> = delegate of (byte * byte * byte) -> 'T