> if I write it the way that I understand you're suggesting, I would have to consider the behavior of all of the ICartDiscountStrategy implementations. No, you

> if I write it the way that I understand you're suggesting, I would have to consider the behavior of all of the ICartDiscountStrategy implementations. No, you would need to consider only the business requirements. > Some time later, I implement a new AcmeCorpDiscountStrategy which results in a different discount being returned. Why did you implement that? Is it a business requirement change, that "existing customers should get this different discount"? If yes, then your tests break - but for a good reason. If it's not about existing customers, but new sort of customers, then your tests should be unaffected and you just need to write new tests that target these new sorts of customers and validate that the new discount strategy is applied.