Are exceptions really for exceptional errors? [closed]

It's my understanding that common wisdom says to only use exceptions for truly exceptional conditions (In fact, I've seen that statement here at SO several times).

However, Krzysztof Cwalina says:

One of the biggest misconceptions about exceptions is that they are for “exceptional conditions.” The reality is that they are for communicating error conditions. From a framework design perspective, there is no such thing as an “exceptional condition”. Whether a condition is exceptional or not depends on the context of usage, --- but reusable libraries rarely know how they will be used. For example, OutOfMemoryException might be exceptional for a simple data entry application; it’s not so exceptional for applications doing their own memory management (e.g. SQL server). In other words, one man’s exceptional condition is another man’s chronic condition.

He then also goes on to say that exceptions should be used for:

• Usage errors
• Program errors
• System failures

Considering Krzysztof Cwalina is the PM for the CLR team at MS I ask: What do you think of his statement?

This sounds over-simplistic, but I think it makes sense to simply use exceptions where they are appropriate. In languages like Java and Python, exceptions are very common, especially in certain situations. Exceptions are appropriate for the type of error you want to bubble up through a code path and force the developer to explicitly catch. In my own coding, I consider the right time to add an exception when the error either can't be ignored, or it's simply more elegant to throw an exception instead of returning an error value to a function call etc.

Some of the most appropriate places for exceptions that I can think of offhand:

• NotImplementedException - very appropriate way of designating that a particular method or function isn't available, rather than simply returning without doing anything.
• OutOfMemory exceptions - it's difficult to imagine a better way of handling this type of error, since it represents a process-wide or OS-wide memory allocation failure. This is essential to deal with, of course!
• NullPointerException - Accessing a null variable is a programmer mistake, and IMO this is another good place to force an error to bubble to the surface
• ArrayIndexException - In an unforgiving language like C, buffer overflows are disastrous. Nicer languages might return a null value of some type, or in some implementations, even wrap around the array. In my opinion, throwing an exception is a much more elegant response.

This is by no means a comprehensive list, but hopefully it illustrates the point. Use exceptions where they are elegant and logical. As always with programming, the right tool for the right job is good advice. There's no point going exception-crazy for nothing, but it's equally unwise to completely ignore a powerful and elegant tool at your disposal.

For people who write frameworks, perhaps it's interesting.

For the rest of us, it's confusing (and possibly useless.) For ordinary applications, exceptions have to be set aside as "exceptional" situations. Exceptions interrupt the ordinary sequential presentation of your program.

You should be circumspect about breaking the ordinary top-to-bottom sequential processing of your program. The exception handling is -- intentionally -- hard to read. Therefore, reserve exceptions for things that are outside the standard scenarios.

Example: Don't use exceptions to validate user input. People make input mistakes all the time. That's not exceptional, that's why we write software. That's what if-statements are for.

When your application gets an OutOfMemory exception, there's no point in catching it. That's exceptional. The "sequential execution" assumption is out the window. Your application is doomed, just crash and hope that your RDBMS transaction finishes before you crash.

It is indeed difficult to know what exactly construes an "exceptional condition" which warrants the use of an exception in a program.

One instance that is very helpful for using communicating the cause of errors. As the quote from Krzysztof Cwalina mentions:

One of the biggest misconceptions about exceptions is that they are for “exceptional conditions.” The reality is that they are for communicating error conditions.

To give a concrete example, say we have a getHeader(File f) method that is reading some header from a file and returns a FileHeader object.

There can be several problems which can arise from trying to read data from a disk. Perhaps the file specified doesn't exist, file contains data that can't be read, unexpected disk access errors, running out of memory, etc. Having multiple means of failure means that there should be multiple ways to report what went wrong.

If exceptions weren't used, but there was a need to communicate the kind of error that occurred, with the current method signature, the best we can do is to return a null. Since getting a null isn't very informative, the best communication we get from that result is that "some kind of error happened, so we couldn't continue, sorry." -- It doesn't communicate the cause of the error.

(Or alternatively, we may have class constants for FileHeader objects which indicate FileNotFound conditions and such, emulating error codes, but that really reeks of having a boolean type with TRUE, FALSE, FILE_NOT_FOUND.)

If we had gotten a FileNotFound or DeviceNotReady exception (hypothetical), at least we know what the source of the error was, and if this was an end user application, we could handle the error in ways to solve the problem.

Using the exception mechanism gives a means of communication that doesn't require a fallback to using error codes for notification of conditions that aren't within the normal flow of execution.

However, that doesn't mean that everything should be handled by exceptions. As pointed out by S.Lott:

Don't use exceptions to validate user input, for example. People make mistakes all the time. That's what if-statements are for.

That's one thing that can't be stressed enough. One of the dangers of not knowing when exactly to use exceptions is the tendency to go exception-happy; using exceptions where input validation would suffice.

There's really no point in defining and throwing a InvalidUserInput exception when all that is required to deal in such a situation is to notify the user of what is expected as input.

Also, it should be noted that user input is expected to have faulty input at some point. It's a defensive measure to validate input before handing off input from the outside world to the internals of the program.

It's a little bit difficult to decide what is exceptional and what is not.