Trampolining

Bounce.cs

public sealed class Bounce<T1, T2, TResult>
{
  public T1 Arg1 { get; private set; }
  public T2 Arg2 { get; private set; }
  public TResult Result { get; private set; }
  public bool HasResult { get; private set; }

  private Bounce() { }

  public static Bounce<T1, T2, TResult> Continue(T1 arg1, T2 arg2)
  {
    return new Bounce<T1, T2, TResult>() 
    { 
      Arg1 = arg1,
      Arg2 = arg2
    };
  }

  public static Bounce<T1, T2, TResult> End(TResult result)
  {
    return new Bounce<T1, T2, TResult>() 
    { 
      Result = result, 
      HasResult = true 
    };
  }
}

client.cs

public class  Client
{

public BigInt factorial(BigInt n)
{
  return Trampoline.Start<BigInt, BigInt, BigInt>(iteration, 1, n);
}

 private Bounce<BigInt, BigInt, BigInt> iteration(BigInt current, BigInt n)
 {
  return n == 0 ? Bounce<BigInt, BigInt, BigInt>.End(current) : 
    Bounce<BigInt, BigInt, BigInt>.Continue(current * n, n - 1);
 }
}

Trampoline.cs

public static class Trampoline
{
  public static TResult Start<T1, T2, TResult>(Func<T1, T2, Bounce<T1, T2, TResult>> action, 
    T1 arg1, T2 arg2)
  {
    TResult result = default(TResult);
    Bounce<T1, T2, TResult> bounce = Bounce<T1, T2, TResult>.Continue(arg1, arg2);

    while (true)
    {
      if (bounce.HasResult)
      {
        result = bounce.Result;
        break;
      }

      bounce = action(bounce.Arg1, bounce.Arg2);
    }

    return result;
  }
}