避免输入验证中的重复代码

Suppose you have a subsystem that does some kind of work. It could be anything. Obviously, at the entry point(s) to this subsystem there will be certain restrictions on the input. Suppose this subsystem is primarily called by a GUI. The subsystem needs to check all the input it recieves to make sure it's valid. We wouldn't want to FireTheMissles() if there was invalid input. The UI is also interested in the validation though, because it needs to report what went wrong. Maybe the user forgot to specify a target or targetted the missles at the launchpad itself. Of course, you can just return a null value or throw an exception, but that doesn't tell the user SPECIFICALLY what went wrong (unless, of course, you write a separate exception class for each error, which I'm fine with if that's the best practice). Suppose you have a subsystem that does some kin