When to throw an exception?

Question

My personal guideline is: an exception is thrown when a fundamental assumption of the current code block is found to be false.

Example 1: say I have a function which is supposed to examine an arbitrary class and return true if that class inherits from List<>. This function asks the question, "Is this object a descendant of List?" This function should never throw an exception, because there are no gray areas in its operation - every single class either does or does not inherit from List<>, so the answer is always "yes" or "no".

Example 2: say I have another function which examines a List<> and returns true if its length is more than 50, and false if the length is less. This function asks the question, "Does this list have more than 50 items?" But this question makes an assumption - it assumes that the object it is given is a list. If I hand it a NULL, then that assumption is false. In that case, if the function returns either true or false, then it is breaking its own rules. The function cannot return anything and claim that it answered the question correctly. So it doesn't return - it throws an exception.

This is comparable to the "loaded question" logical fallacy. Every function asks a question. If the input it is given makes that question a fallacy, then throw an exception. This line is harder to draw with functions that return void, but the bottom line is: if the function's assumptions about its inputs are violated, it should throw an exception instead of returning normally.

The other side of this equation is: if you find your functions throwing exceptions frequently, then you probably need to refine their assumptions.

Because they're things that will happen normally. Exceptions are not control flow mechanisms. Users often get passwords wrong, it's not an exceptional case. Exceptions should be a truly rare thing, UserHasDiedAtKeyboard type situations.

My little guidelines are heavily influenced by the great book "Code complete":

• Use exceptions to notify about things that should not be ignored.
• Don't use exceptions if the error can be handled locally
• Make sure the exceptions are at the same level of abstraction as the rest of your routine.
• Exceptions should be reserved for what's truly exceptional.

It is NOT an exception if the username is not valid or the password is not correct. Those are things you should expect in the normal flow of operation. Exceptions are things that are not part of the normal program operation and are rather rare.

EDIT: I do not like using exceptions because you can not tell if a method throws an exception just by looking at the call. Thats why exceptions should only be used if you can't handle the situation in a decent manner (think "out of memory" or "computer is on fire").