Why are Exceptions said to be so bad for Input Validation?

Question

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.

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Answer 1

When using exceptions, the error handling code is separated from the code causing the error. This is the intent of exception handling - being an exceptional condition, the error can not be handled locally, so an exception is thrown to some higher (and unknown) scope. If not handled, the application will exit before any more hard is done.

If you ever, ever, ever throw exception when you are doing simple logic operations, like verifying user input, you are doing something very, very very, wrong.

The input is Not what was expected and hence is exceptional.

This statement does not sit well with me at all. Either the UI constrains user input (eg, the use of a slider that bounds min/max values) and you can now assert certain conditions - no error handling required. Or, the user can enter rubbish and you expect this to happen and must handle it. One or the other - there is nothing exception going here whatsoever.

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?

I consider this beyond - closer to evil. You can define an abstract ErrorProvider interface, or return a complex object representing the error rather than a simple code. There are many, many options on how you retrieve error reports. Using exceptions because the are convenient is so, so wrong. I feel dirty just writing this paragraph.

Think of throwing an exception as hope. A last chance. A prayer. Validating user input should not lead to any of these conditions.