Why are Exceptions said to be so bad for Input Validation


I understand that "Exceptions are for exceptional cases" [a], but besides just being repeated over and over again, I've never found an actual reason for this fact.

Being that they halt execution, it makes sense that you wouldn't want them for plain conditional logic, but why not input validation?

Say you were to loop through a group of inputs and catch each exception to group them together for user notification… I continually see that this is somehow "wrong" because users enter incorrect input all the time, but that point seems to be based on semantics.

The input is Not what was expected and hence is exceptional. Throwing an exception allows me to define exactly what was wrong like StringValueTooLong or or IntegerValueTooLow or InvalidDateValue or whatever. Why is this considered wrong?

Alternatives to throwing an exception would be to either return (and eventually collect) an error code or far worse an error string. Then I would either show those error strings directly, or parse the error codes and then show corresponding error messages to the user. Wouldn't a exception be considered a malleable error code? Why create a separate table of error codes and messages, when these could be generalized with the exception functionality already built into my language?

Also, I found this article by Martin Fowler as to how to handle such things – the Notification pattern. I'm not sure how I see this as being anything other than Exceptions that don't halt execution.

a: Everywhere I've read anything about Exceptions.

— Edit —

Many great points have been made. I've commented on most and +'d the good points, but I'm not yet completely convinced.

I don't mean to advocate Exceptions as the proper means to resolve Input Validation, but I would like to find good reasons why the practice is considered so evil when it seems most alternate solutions are just Exceptions in disguise.

Best Solution

Reading these answers, I find it very unhelpful to say, "Exceptions should only be used for exceptional conditions". This begs the whole question of what is an "exceptional condition". This is a subjective term, the best definition of which is "any condition that your normal logic flow doesn't deal with". In other words, an exceptional condition is any condition you deal with using exceptions.

I'm fine with that as a definition, I don't know that we'll get any closer than that anyway. But you should know that that's the definition you are using.

If you are going to argue against exceptions in a certain case, you have to explain how to divide the universe of conditions into "exceptional" and "non-exceptional".

In some ways, it's similar to answering the question, "where are the boundaries between procedures?" The answer is, "Wherever you put the begin and end", and then we can talk about rules of thumb and different styles for determining where to put them. There are no hard and fast rules.

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