This is copied from this SO answer.
There is a difference between error codes and error return values. An error code is for the user and help desk. An error return value is a coding technique to indicate that your code has encountered an error.
One can implement error codes using error return values, but I would advice against that. Exceptions are the modern way to report errors, and there is no reason why they should not carry an error code within them.
This is how I would organize it (Note that points 2-6 are language agnostic):
- Use a custom exception type with an additional
ErrorCodeproperty. The catch in the main loop will report this field in the usual way (log file / error pop-up / error reply). Use the same exception type in all of your code. - Do not start at 1 and don't use leading zeros. Keep all error codes to the same length, so a wrong error code is easy to spot. Starting at 1000 usually is good enough. Maybe add a leading 'E' to make them clearly identifiable for users (especially useful when the support desk has to instruct users how to spot the error code).
- Keep a list of all error codes, but don't do this in your code. Keep a short list on a wiki-page for developers, which they can easily edit when they need a new code. The help desk should have a separate list on their own wiki.
- Do not try to enforce a structure on the error codes. There will always be hard-to-classify errors and you don't want to discuss for hours whether an error should be in the 45xx group or in the 54xx group. Be pragmatic.
- Assign each throw in your code a separate code. Even though you think it's the same cause, the help desk might need to do different things in different cases. It's easier for them to have "E1234: See E1235" in their wiki, than to get the user to confess what he has done wrong.
- Split error codes if the help desk asks for it. A simple
if (...) throw new FooException(1234, ".."); else throw new FooException(1235, "..");line in your code might save half an hour for the help desk.
And never forget that the purpose of the error codes is to make life easier for the help desk.