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Posts Tagged ‘Error Handling’

The Bad Practices of Exception Handling

October 31st, 2009 12 comments

Exception handling has truly been a blessing to us software developers. Without it, dealing with special conditions and writing robust programs was a lot more painful. But, like any powerful tool, badly used it could cause more harm than good. This article name the top three on my Exception handling bad practices list, all of which I’ve been practicing in the past but now stay away from.

Swallowing Exceptions

Have you ever come across code like this?

try
{
  DoSomeNonCriticalStuff();
}
catch (Exception e)
{
  // Ignore errors
}

DoStuffThatMustBeDoneDispiteAnyErrorsAbove();

Of all the bad exception handling practices, this is the worst since its effect is the complete opposite of the programmer’s intention. The reasoning goes something like this: Catching exceptions where they don’t hurt makes my program more robust since it’ll continue working even when conditions aren’t perfect.

The reasoning could have been valid if it wasn’t for Fatal exceptions; Here described by Eric Lippert.

Fatal exceptions are not your fault, you cannot prevent them, and you cannot sensibly clean up from them. They almost always happen because the process is deeply diseased and is about to be put out of its misery. Out of memory, thread aborted, and so on. There is absolutely no point in catching these because nothing your puny user code can do will fix the problem. Just let your “finally” blocks run and hope for the best.

Catching and ignoring these fatal exceptions makes your program less robust since it will try to carry on as if nothing happened in the worst of conditions, most likely making things worse. Not very Fail fastish.

So, am I saying that ignoring exceptions is bad and should always be avoided? No, the bad practice is catching and ignoring general exceptions. Specific exceptions on the other hand is quite OK.

try
{
  DoSomeNonCriticalStuff();
}
catch (FileNotFoundException e)
{
  // So we couldn't find the settings file, big deal!
}

Bad example, I know, but you get the point.

Throwing Exception

Here’s another bad practice I come across every now and then.

throw new Exception("No connection!");

The problem is that in order to handle Exception we have to catch Exception, and if we catch Exception we have to be prepared to handle every other type of Exception, including the Fatal exceptions that we discussed in the previous section.

So, if you feel the need to throw an exception, make sure it’s specific.

throw new NoConnectionException();

If the idea of defining lots of specific exceptions puts you off, then the very least thing you should do is to define your own application exception to use instead of the basic Exception. This is nothing I recommend though, since general exceptions, like ApplicationException, violate the Be specific rule of the Three rules for Effective Exception Handling.  It’ll make you depend heavily on the message property to separate different errors, so don’t go there.

Overusing exceptions

Exceptions is a language construct to handle exceptional circumstances, situations where a piece of code discovers an error but don’t have the context necessary to handle it. It should not be used to handle valid program states. The reasons are:

  1. Performance. Throwing an exception with all that’s involved, like building a stack trace, will cost you a second or so.
  2. Annoyance. Debugging code where exceptions are a part of the normal execution flow can be frustrating.

Eric Lippert calls those exceptions Vexing exceptions, which I concider a great name given the second argument. Make sure you’ll check out his article (link at the beginning of this article).

Those were the three misuses of exception handling I concider worst. What’s on your list?

Cheers

Tools of the Effective Developer: Error Handling Infrastructure

September 3rd, 2009 2 comments

I often come across code with little or no infrastructure for error handling. This is a big mistake, one that’ll make you pay increasingly as the code base grows. Why? Because your code’ll end up with loads of this:

try
{
  ParseText(SomeText);
}
catch (Exception e)
{
  MessageBox.Show("Error while parsing.", "My Application",
         MessageBoxButtons.OK, MessageBoxIcon.Error);

  // Skipping the rest due to the error
  return;
}

It may look OK but there are several problems with the error handling code above, some more severe than others.

  • Code duplication
    A message box for displaying error messages to the user should almost always have an error icon, the application name as the title, and an OK button. Repeating this all over the code base is not very DRY and should be avoided.
  • Inconsistency
    A related problem is that when you leave programmers with only the general error handling routines of the platform chances are they’ll end up doing things differently, resulting in an inconsistent user experience; For example showing some error dialogs with an error icon and some without.
  • Information loss
    A more serious problem is the fact that the code in the example, by ignoring the exception object, drops information that could be important to track down a bug. We could of course mititgate that problem somewhat by showing the error message of the exception to the user (by concatinating e.Message), but we’d still miss an important piece of information: the stack trace.
    Since dumping a pile of function calls in the face of the users is a no no, what we’d really need is a way to put the details in a non-intrusive place, for example a log file. If the Error Handling Infrastructure doesn’t make this easy, we’re likely to leave that kind of information out completely. For shame.
  • Not automation-friendly
    But the most severe problem with a spread out usage of UI displaying methods like MessageBox is that it makes your code impossible to automate. If someone has to monitor a scheduled run of for example unit tests, and every now and then klick an OK button, it kind of beats the purpose of automation.

So here’s my advice: Implement a strategy for handling errors at the earliest possible time. That is, right after setting up continuous integration for the Hello world application.

Properties of an Error Handling Infrastructure

So how should a error handling infrastructure be like? There’s only one rule. It has to be easy!
If it isn’t simple to use, it won’t be used, and programmers end up using the general purpose message showing methods again, or worse, not doing any error handling at all.

I usually build my error handling interfaces around two use cases.

  1. Displaying error messages
  2. Logging error messages

These are often used in combination, for example

try
{
  ParseText(SomeText);
}
catch (Exception e)
{
  ApplicationEnvironment.ShowErrorMessage("Error while parsing: " + e.Message);
  ApplicationEnvironment.LogError(e);
  return;
}

Or, to avoid code duplication, combined into a single convenient method.

try
{
  ParseText(SomeText);
}
catch (Exception e)
{
  ApplicationEnvironment.HandleException("Error while parsing", e);
  return;
}

When you design your error handling interfaces don’t be afraid to add lots of convenient methods. Remember, it has to be easy to use and that’s what convenient methods are all about, as opposed to utility methods who generally need to be given more arguments. In this case I prefer the Humane interface design style instead of a minimal interface approach.

Needs to be configurable

Furthermore, the Error Handling Infrastructure needs to be configurable or support some other kind of dependency breaking technique like dependency injection. For instance, you should be able to mute all error messages intended for the user, and instead write them to the error log. This way your code will be able to run in a scripted environment, like during unit testing.

There are many ways to design an Error Handling Infrastructure. You could create your own message dialogs allowing the user to send a report right away, you could use the built in application log of the operating system or just plain text files, etc etc.

Whatever you choose to do, don’t forget to do it early and make it easy.

Cheers!

Previous posts in the Tools of The Effective Developer series:

  1. Tools of The Effective Developer: Personal Logs
  2. Tools of The Effective Developer: Personal Planning
  3. Tools of The Effective Developer: Programming By Intention
  4. Tools of The Effective Developer: Customer View
  5. Tools of The Effective Developer: Fail Fast!
  6. Tools of The Effective Developer: Make It Work – First!
  7. Tools of The Effective Developer: Whetstones
  8. Tools of The Effective Developer: Rule of Three
  9. Tools of The Effective Developer: Touch Typing