C# phantom types

gistfile1.cs

interface IUnit {}
interface IMeter : IUnit {}
interface ICentimeter : IUnit {}
interface ILiter : IUnit {}

struct Quantity<TUnit> where TUnit : IUnit {
  public readonly double Value;
  public Quantity(double l) { Value = l; }
  
  public override string ToString() { return string.Format("{0}", Value); }
  public static Quantity<TUnit> operator +(Quantity<TUnit> me, Quantity<TUnit> other) {
    return new Quantity<TUnit>(me.Value + other.Value);
  }
}

static class QuantityExtensions {
  public static Quantity<IMeter> ToMeter(this Quantity<ICentimeter> l) {
    return new Quantity<IMeter>(l.Value / 100);
  }
}

static class LiterOperations {
  public static object FillObject(this object obj, Quantity<ILiter> liters) {
    Console.WriteLine("Filled the object with {0} liters.", liters.Value);    
    return obj;
  }
}

interface IAuthenticationState {}
interface IAuthenticated : IAuthenticationState {}
interface IUnauthenticated : IAuthenticationState {}

interface ISession<out TAuthenticationState> where TAuthenticationState : IAuthenticationState {}

static class APIOperations {
  private class Session<TAuthenticationState> : ISession<TAuthenticationState>
    where TAuthenticationState : IAuthenticationState {}
  private static readonly ISession<IAuthenticationState> Authenticated = new Session<IAuthenticated>();  
  private static readonly ISession<IAuthenticationState> Unauthenticated = new Session<IUnauthenticated>();

  public static ISession<IAuthenticationState> Authenticate(string password) {
    return (new Random()).NextDouble() > 0.5
    ? Authenticated
    : Unauthenticated;
  }
  
  public static string GetSecret(this ISession<IAuthenticated> session) {
    return "42";
  }
}

void Main()
{
  var maybeAuth = APIOperations.Authenticate("so_secret") as ISession<IAuthenticated>;
  if (maybeAuth != null) {
    Console.WriteLine("Shhh, the secret is {0}.", maybeAuth.GetSecret());
  } else {
    Console.WriteLine("No way!");
  }
  
  var _100cm = new Quantity<ICentimeter>(100);
  var _5cm   = new Quantity<ICentimeter>(5);
  var _1l    = new Quantity<ILiter>(1);
  
  Console.WriteLine("{0}m", _100cm.ToMeter());
  Console.WriteLine("{0}cm", _5cm + _100cm);
  (new object()).FillObject(_1l);
}