active_pattern.cs

using Microsoft.FSharp.Core;
using System;
using System.IO;
using System.Runtime.CompilerServices;

[CompilationMapping(SourceConstructFlags.Module)]
public static class Program
{
  [SpecialName]
  public static FSharpChoice<Unit, Unit> \u007CEven\u007COdd\u007C(int n)
  {
    if (n % 2 == 0)
      return FSharpChoice<Unit, Unit>.NewChoice1Of2((Unit) null);
    return FSharpChoice<Unit, Unit>.NewChoice2Of2((Unit) null);
  }

  [EntryPoint]
  public static int main(string[] argv)
  {
    int result = 0;
    int n = new Tuple<bool, int>(int.TryParse(Console.ReadLine(), out result), result).Item2;
    switch (n)
    {
      case 2:
        ExtraTopLevelOperators.PrintFormatLine<Unit>((PrintfFormat<Unit, TextWriter, Unit, Unit>) new PrintfFormat<Unit, TextWriter, Unit, Unit, Unit>("2"));
        break;
      default:
        if (!(Program.\u007CEven\u007COdd\u007C(n) is FSharpChoice<Unit, Unit>.Choice2Of2))
        {
          ExtraTopLevelOperators.PrintFormatLine<Unit>((PrintfFormat<Unit, TextWriter, Unit, Unit>) new PrintfFormat<Unit, TextWriter, Unit, Unit, Unit>("Even"));
          break;
        }
        ExtraTopLevelOperators.PrintFormatLine<Unit>((PrintfFormat<Unit, TextWriter, Unit, Unit>) new PrintfFormat<Unit, TextWriter, Unit, Unit, Unit>("Odd"));
        break;
    }
    return 0;
  }
}

active_pattern.fs

let (|Even|Odd|) n =
    if n % 2 = 0 then Even
    else Odd

[<EntryPoint>]
let main argv = 
    let (_, i) = System.Int32.TryParse(System.Console.ReadLine())

    match i with
    | 2 -> printfn "2"
    | Even -> printfn "Even"
    | Odd -> printfn "Odd"

    0