When discussing the upcoming ASP.NET MVC framework, one of the key benefits I like to tout is how this framework will improve testability of your web applications.

The response I often get is the same question I get when mention patterns such as Dependency Injection, IoC, etc…

Why would I want to do XYZ just to improve testability?

I think to myself in response

Just to improve testability? Isn’t that enough of a reason!

That’s how excited I am about test driven development. Testing seems enough of a reason for me!

Of course, when I’m done un-bunching my knickers, I realize that despite all the benefits of unit testable code, the real benefit of testable code is how it helps handle the software development’s biggest problem since time immemorial, managing complexity.

There are two ways that testable code helps manage complexity.

​1. It directly helps manage complexity assuming that you not only write testable code, but also write the unit tests to go along with them. With decent code coverage, you now have a nice suite of regression tests, which helps manage complexity by alerting you to potential bugs introduced during code maintenance in a large project long before they become a problem in production.

​2. It indirectly helps manage complexity because in order to write testable code, you have to employ the principle of separation of concerns to really write testable code.

Separating concerns within an application is an excellent tool for managing complexity when writing code. And writing code is complex!

The MVC pattern, for example, separates an application into three main components: the Model, the View, and the Controller. Not only does it separate these three components, it outlines the loosely coupled relationships and communication between these components.

Key Benefits of Separating Concerns

This separation combined with loose coupling allows a developer to manage complexity because it allows the developer to focus on one aspect of the problem at a time.

Martin Fowler writes about this benefit in his paper, Separating User Interface Code (pdf):

A clear separation lets you concentrate on each aspect of the problem separately—and one complicated thing at a time is enough. It also lets different people work on the separate pieces, which is useful when people want to hone more specialized skills. \

The ability to divide work into parallel tracks is a great benefit of this approach. In a well separated application, if Alice needs time to implement the controller or business logic, she can quickly stub out the model so that Bob can work on the view without being blocked by Alice. Meanwhile, Alice continues developing the business layer without the added stress that Bob is waiting on her.

Bring it home

The MVC example above talks about separation of concerns on a large architectural scale. But the same benefits apply on a much smaller scale outside of the MVC context. And all of these benefits can be yours as a side-effect of writing testable code.

So to summarize, when you write testable code, whether it is via Test Driven Development (TDD) or Test After Development, you get the following side effects.

A nice suite of regression tests. Well separated code that helps manage complexity. Well separated code that helps enable concurrent development.

Compare that list of side effects with the list of side effects of the latest pharmaceutical wonder drug for curing restless legs or whatever. What’s not to like!?