[TestFixture]
    public class when_determining_the_tax_code_for_an_employee_earning_99_for_the_last_year
    {
        private IPerson person;
        private string result;
        [SetUp]
        public void given()
        {
            person = MockRepository.GenerateStub<IPerson>();
            person.Stub(x => x.earnings_for_the_last_year).Return(99m);
            result = person.calculate_code_rate();
        }
        [Test]
        public void it_should_get_the_tax_code_GHYK9()
        {
            Assert.That(result, Is.EqualTo("GHYK9"));
        }
        [Test]
        public void it_should_ask_the_employee_for_earnings_for_the_last_year()
        {
            person.AssertWasCalled(x => x.earnings_for_the_last_year);
        }
    }
 
    public interface IPerson
    {
        decimal earnings_for_the_last_year { get; }
    }
    public static class PersonExtensions
    {
        public static string calculate_code_rate(this IPerson person)
        {
            if (person.earnings_for_the_last_year > 100)
                return "EK9";
            else            
                return "GHYK9";            
        }
    }

 
    public static class ComparisonExtensions
    {
        public static bool implements_interface<TInterface>(this object object_to_compare)
        {
            return typeof (TInterface).IsAssignableFrom(object_to_compare.GetType());
        }
        public static bool is_type_of<TType>(this object object_to_compare)
        {
            return object_to_compare.GetType() == typeof(TType);
        }
    }