www.hans-eric.com

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

When it comes to the question of whether methods should be virtual by default or not, there are two schools of thought. Anders Hejlsberg, lead architect of C#, describes them in an interview from 2003.

The academic school of thought says, “Everything should be virtual, because I might want to override it someday.” The pragmatic school of thought, which comes from building real applications that run in the real world, says, “We’ve got to be real careful about what we make virtual.”

As I told you about in my post from last week, I have left the “pragmatic school of thought” to join the “academic” camp. The main reason was unit-testing, that – in my opinion – calls for a more flexible object model than the one of C#. When unit-testing, I often want to use components in unusual ways, all in the name of dependency breaking, and therefore I like their methods to be virtual.

But, it wasn’t – and still isn’t – an easy pick; Virtual by default does bring some serious problems to the table. Again from the interview with Anders:

Every time you say virtual in an API, you are creating a call back hook. As an OS or API framework designer, you’ve got to be real careful about that. You don’t want users overriding and hooking at any arbitrary point in an API, because you cannot necessarily make those promises.

Whenever they ship a new version of the Java class libraries, breakage occurs. Whenever they introduce a new method in a base class, if someone in a derived class had a method of that same name, that method is now an override—except if it has a different return type, it no longer compiles. The problem is that Java, and also C++, does not capture the intent of the programmer with respect to virtual.

C# captures the intent better and avoids versioning problems, and Java offers the flexibility needed for unit-testing. Which is better? The answer seems to be: it depends. But, do we really have to choose? Why can’t we have both? Well, I think we can, and Java has shown the way to achieve it.

Java annotations is a powerful language feature. (The concept was rightfully stolen from C# where it is called custom attributes.) With it one can attach metadata to parts of a program, to give additional information to the compiler or other external agents. In other words, annotations can be used to extend the programming language without the need for changing the language itself.

A good example is the @override annotation.

class SomeClass extends SomeBaseClass {

  @override
  void someMethod() { … }

}

From the Java documentation:

[@override indicates] that a method declaration is intended to override a method declaration in a superclass. If a method is annotated with this annotation type but does not override a superclass method, compilers are required to generate an error message.

The use of the @override annotation takes care of the problem when name changes to virtual methods silently breaks behavior of descending classes; Or, which is more common, when misspelled names of intended overrides aren’t captured at compile-time. Now, with the introduction of the @override annotation, you can help the compiler help you to fail fast. It is now possible to show your intention better than what was possible in the pre annotation days.

Unfortunately Java doesn’t take this concept all the way. They could have introduced an @virtual annotation as a complement to @override, and thus reach the same level of expressiveness as C#, but without forgoing the flexibility of the Java object model. It would be the perfect middle-way, and provide the best of both worlds.

Class SomeBaseClass {

  @virtual
  void someMethod() { … }

}

The benefit of an annotation (or custom attribute) based solution is that it’s configurable. It would be possible to alter the compiler’s behavior based on context or environment. For instance one could enforce the use of @virtual and @override in production code. Additionally, one could relax the controls when necessary, like in test projects or legacy code, to mere warnings or complete silence.

Wouldn’t that be better than the all or nothing solutions of today?

Cheers!

Note: This article was written before D got full closure support.

Delegates are something that gives me mixed feelings. While I used to embrace them in Delphi and Object Pascal, I now find them difficult to get used to in C#. I guess that is because they introduce a little of the functional programming paradigm into an otherwise object oriented environment.

Don’t get me wrong, I do find delegates powerful and I use them whenever I see fit. It’s just that they bug me, like a badly-pitched note in an otherwise perfect chord. In that respect I am much more fond of the inner classes of Java.

The D Programming Language, my favorite pet at the moment, has support for both delegates and inner classes, thus providing the best of both worlds. And since D is a multi paradigm language with support for functional programming, it’s delegates are justified.

The semantics of D delegates are somewhat different from that of C#. For instance, C# delegates may bind to both static and non-static methods, while D may bind to non-static methods only. But on the other hand, D delegates may also bind to inner functions, which makes them look a lot like closures.

bool retry(int times, bool delegate() dg)
{
  int tries = 0;
  while (tries++ < times) {
    if(dg())
      return true;
  }
  return false;
}

// Example usage
retry(3, {
  writefln("Password: ");
  char[] buf = readln();
  return (buf == "secretn");
});

In order to be a real closure a delegate is required to keep variables that are bound from the surrounding environment alive, even if the program has left their scope. For example, consider the following program.

int delegate() countdown(int from)
{
  int current = from;
  int countdown_helper() {
    return current\-\-;
  }

  return &countdown_helper;
}

int delegate() dg = countdown(10);
dg(); // should return 10;
dg(); // should return 9;
dg(); // should return 8;

The countdown function returns a delegate that refers to the inner function countdown_helper. Counter_helper binds the variable current from it’s parent by returning its value and decreasing it by one.

So, if D’s delegates were real closures the first invocation of the delegate in the above example should return 10, the second 9, followed by 8, 7, and so on. But in reality they produce bogus values like 4202601 or 4202588. The reason is this: D doesn’t implement real closures. The variable current is invalid because the program is no longer executing the countdown function when countdown_helper is invoked via the delegate.

So D doesn’t have real closures, is that a big deal? No! In my opinion real closures are not important for languages that are not pure functional. The creators of D shouldn’t waste their resources on this feature since it brings little value but lots of implementation complexity. The energy is better spent on features like multicast delegates. Those would bring value in the form of convenience, and I hope future versions of D will incorporate them.

Complete versions of the code samples in this post can be found here and here.

C# entered the field of battle some seven years ago as a better version of Java. The one feature that many people referred to when claiming C# superior, was custom attributes. Now Java has them too, but they go under the name of annotations.

The ability to attach meta data to various parts of your program is a powerful way to integrate with frameworks or configure external utilities. Mike Kaufman, for example, used annotations beautifully to configure an Obfuscator. He has posted the details on his blog.