public void VerifyExtendedDate()
{
DiscountByDate original = new DiscountByDate(.1m, DateTime.Now);
DiscountByDate extended = original.ExtendByDays(7);
Assert.IsTrue(extended.ExpirationDate == original.ExpirationDate.AddDays(7));
}
public void TestImmutabilityWhenChangingExpirationDate()
{
DiscountByDate original = new DiscountByDate(.1m, DateTime.Now);
DiscountByDate verifier = new DiscountByDate(original);
DiscountByDate extended = original.ExtendByDays(7);
Assert.IsFalse(original.Equals(extended));
Assert.IsTrue(original.Equals(verifier));
}