Moving Away from Exceptional Code

I once took a training course taught by Ron Jeffries and Chet Hendrickson on Test Driven Development. One of the participants in the course asked both Ron and Chet what are their recommendations on testing exceptional code paths. At first I didn’t think much of the question, since all the unit testing frameworks I’ve used have built-in ways to test exceptional code paths.

• VSTS Unit Testing (.NET) has an attribute that you can put on a test to say that it is expecting an exception:

  [TestMethod, ExpectedException( typeof ( ApplicationException ) ) ]
  public void CreationOfFooWithNullParameterShouldThrowApplicationException()
  {
     var foo = new Foo( null ); //should throw an ApplicationException
  }

• Google Test (C++) places a macro around the method call that is expecting an exception to be thrown:

   ASSERT_THROW(Foo(5), bar_exception);

• unittest (Python) has a special assert to test if an exception was raised, similar to Google Test:

  def testsample(self):
     self.assertRaises(ValueError, random.sample, self.seq, 20)

The answer that Ron gave wasn’t expected, but led me to think about the way that we use exceptions within our codebase.

From what I remember, Ron said that he simply doesn’t test exceptional cases. How? He doesn’t have exceptional cases. He recommended using the Null Object pattern to get away from most of the exceptional cases.

Developers often have to pollute their code with checks to see if a value is null, like so:

XmlNode* userNode = xmlParser.GetNode( "user" );
return ( userNode == NULL ) ? "" : userNode->Text();

In the case above, the call to GetNode would search the XML document for a node with a tagName of “user”. If it finds it, it will return a pointer to the data. If not, then it will return NULL. This makes a lot of sense to developers, but it ruins some of the value of abstraction and good object-oriented programming.

First, any client who makes this call to GetNode has to know how GetNode works (they have to know that GetNode will return NULL if it can’t find the node). Second, there is a type check going on here, basically a runtime check to see if the type returned from GetNode is an actual XmlNode or if it is not.

One way to refactor away from this is to use the Null Object pattern. Martin Fowler talks about the Null Object pattern in his book, Refactoring: Improving the Design of Existing Code. In the section entitled, Introduce Null Object, there is a story told by Ron Jeffries describing how he has used Null Object to treat missing objects as actual objects.

The story goes over how if a record didn’t exist in the database, they would return an object like MissingPersonRecord, which would be derived from PersonRecord. All calls to methods on a MissingPersonRecord would simply return some default behavior and not complain. For example, if the MissingPersonRecord is asked for their name, they would respond with a blank string.

This allows any caller of the methods to not have to worry about missing people in the database. See how the code has changed:

XmlNode* userNode = xmlParser.GetNode( "user" );
return userNode->Text();

There is now no change in behavior if the node doesn’t exist, and there is also no knowledge about how GetNode works (besides the fact that the caller knows that it will never return NULL 😉 ).

One library that I have used that follows this pattern is the TinyXml library in C++. When a caller asks for an XML node, there is no checking to see if that node really exists before making the call. This allows you to write code like so:

TiXmlNode* userNode = xmlParser.GetNode( "user" );
return userNode->FirstChild()->FirstChild()->Text();

A disadvantage to this is that when a Null Object has to be used, the error in the code can take a while to find. You may not know that there is a problem until a user object is displayed on screen without a required field like a “name”.

To solve this, you would have to debug and find the Null Object to determine why it is not a real object. Another approach could be to have the Null Object log a message in the Event Log when specific methods are called on it, but this can create too much noise, especially if you are using the Null Object pattern in the way that TinyXml does.

So, that is a little introduction on the Null Object pattern. To summarize, you can use the Null Object pattern to improve abstraction and polymorphism, and to reduce the lines of code per method.