Because people might throw a lot of weird things at my class and I want to make sure that no invalid values are set on those properties. I might use the propert

Because people might throw a lot of weird things at my class and I want to make sure that no invalid values are set on those properties. I might use the property or class member later on. Let's assume my app uses stock prices and because of some issue, the price for a given equity comes through as Null. And then I don't want someone to be able to do Share.Price = API.Price. I might want to have a method called SetPrice(APIPrice price) and there I do a check to see if it's a valid value as far as my code is concerned. I had a situation like this handling pricing information taken from a variety of indices but I want a calculation to still work out to something usable. Also, most business users don't understand what NULL * 3.5 would mean, but they would understand 0.0 * 3.5. That's the main one off the top of my head.