This essay happens to be specific case about "null" but the more generalized concept is the optimal design of any public-vs-private state and how it affects the

This essay happens to be specific case about "null" but the more generalized concept is the optimal design of any public-vs-private state and how it affects the balance of code at the client site vs embedded inside the class. Unfortunately, the author's email example isn't a good case study of the principle he's trying to demonstrate. If you expose unnecessary state, your class is acting like a "dumb data struct" and you make client code more complicated and it proliferates needless and error-prone code duplication. (I made a previous comment about this.[1]) On the hand, if you hide information that makes sense for client code to know about, you increasingly make fatter and heavier class code with extraneous class methods that's idiosyncratic to an isolated piece of client code rather than being of general utility for many clients. Arguably in this case of "missing email", it may make more sense for the client code to handle an "if (customer.email == null) SendPostalMail()". You have to determine whether you can reasonably hide knowledge in a class such that client code (including all future client code) never needs it. E.g. if some future code needs to behave differently such as creating a different type of web login screen depending on the existence of email address, it probably doesn't make sense to fatten up Customer class with another method such as "CreateWebLogin()" to generate HTML -- all to avoid exposing the null status of email address. This would be a code smell. [1] https://news.ycombinator.com/item?id=11481194 https://news.ycombinator.com/item?id=11481194