Saturday, 7 May 2016

You Rarely Need Custom Exceptions

Implementing custom exceptions usually gives a hint as to why you rarely need custom implementations. They are often nothing more than sub classes where the only difference is the type name and containing message.

In this C# example there is a lot of code for nothing. When checking logs or handling bugs you will read the message and the stack trace. The first line containing a bespoke name rarely matters. Within the code throwing the exception very little context is gained from the type of exception - instead most of the details will be present within the error message.

Each custom exception you introduce adds overhead from source lines of code (SLOC) to compilation and execution.

Alternative

Simply do not create custom exceptions except in the rarest of occasions. Instead rely on the standard library of the language you are using.

Take Python as an example [Video]. ~200,000 lines of code yet only ~165 exceptions. This works out at about one exception for ~1200 lines of code.

If battle hardened and widely used standard libraries need only a fraction of the amount of custom exceptions, what makes your tiny CRUD app so special that it needs a namespace dedicated to handfuls of bespoke implementations?

Example

Rather than throwing NoBlogPostsFoundException use a HttpException with a useful message. Instead of BlogPostConfigurationException use ConfigurationErrorsException. Trying to add a comment to a post that is not published? Use an InvalidOperationException.

The downside to this suggestion is knowledge. You need to know what exception to use and more importantly where to find it. Consulting documentation or simple digging around will often yield what you need. As a rule try and default to reusing an exception over creating a new one.

The benefit of this approach is less code, and the removal of placeholder classes where the only thing that differs is the message. To ensure nothing is lost in communicating intent, care must be taken to ensure the message is useful, concise and clear.

Custom Exceptions

There are two exceptions (see what I did there) to this rule.

  1. When you explicitly need to handle a certain scenario and you cannot allow other unhandled exceptions to trigger that code path. In this case a custom exception may be valid. As usual question whether an exception is necessary at all, it may be possible to control this with an explicit code path.
  2. When the exception has some form of behaviour. This tends to be common with frameworks where when an exception of type X changes the flow but also carries out some action such as building up an error response.

In these cases this behaviour belonging with the exception makes sense. Generally most code bases treat exceptions equally. In other words any exception triggers a failure path, meaning the type of the exception does not matter in most cases.

Lessons

  • Reuse exceptions from the standard library, chances are there is one fit for the job already.
  • Only introduce custom exceptions if the scenario is exceptional and needs to be handled uniquely.
  • Put effort into ensuring the message of an exception is useful - messages and the stack trace are the most important elements.

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