Object Mentor Blog: Getting a SOLID start.

Getting a SOLID start.

Uncle Bob — Thu, 12 Feb 2009 20:43:35 -0600

I am often asked: “How should I get started with SOLID principles?” Given the recent interest and controversy about the issue, it’s probably time I gave a written answer.

First things first.

You can read about the Solid principles here. This is a paper I wrote nearly a decade ago. It uses a somewhat dated form of UML, and examples are in C++. Also the concepts are severely abbreviated. This is the Cliff Notes. Still, it should give you an initial notion of the names, definitions, and concepts.

There are quite a few papers here that explain the principles in much more detail. Just click on the “Design Principles” topic to see them all. Though I think you’ll find the other topics pretty interesting too.

Finally, the principles are definitively described in two books: Agile Software Development: Principles, Patterns, and Practices, and Agile Principles Patterns, and Practices in C#. You can see descriptions of these books here

What do I mean by “Principle”

The SOLID principles are not rules. They are not laws. They are not perfect truths. The are statements on the order of “An apple a day keeps the doctor away.” This is a good principle, it is good advice, but it’s not a pure truth, nor is it a rule.

The principles are mental cubby-holes. They give a name to a concept so that you can talk and reason about that concept. They provide a place to hang the feelings we have about good and bad code. They attempt to categorize those feelings into concrete advice. In that sense, the principles are a kind of anodyne. Given some code or design that you feel bad about, you may be able to find a principle that explains that bad feeling and advises you about how to feel better.

These principles are heuristics. They are common-sense solutions to common problems. They are common-sense disciplines that can help you stay out of trouble. But like any heuristic, they are empirical in nature. They have been observed to work in many cases; but there is no proof that they always work, nor any proof that they should always be followed.

Following the rules on the paint can won’t teach you how to paint.

This is an important point. Principles will not turn a bad programmer into a good programmer. Principles have to be applied with judgement. If they are applied by rote it is just as bad as if they are not applied at all.

Having said that, if you want to paint well, I suggest you learn the rules on the paint can. You may not agree with them all. You may not always apply the ones you do agree with. But you’d better know them. Knowledge of the principles and patterns gives you the justification to decide when and where to apply them. If you don’t know them, your decisions are much more arbitrary.

So how do I get started?

“There is no royal road to Geometry” Euclid once said to a King who wanted the short version. Don’t expect to skim through the papers, or thumb through the books, and come out with any real knowledge. If you want to learn these principles well enough to be able to apply them, then you have to study them. The books are full of coded examples of principles done right and wrong. Work through those examples, and follow the reasoning carefully. This is not easy, but it is rewarding.

Search your own code, and the code of others, for applications and violations of the principles. Determine whether those applications and violations were justified. Improve the designs by applying one or more principles, and see if the result is actually better. If you’d like to study a code base that was written by people who’ve been immersed in these principles for years, then download the source for FitNesse from fitnesse.org.

Conduct or join a discussion group at work. Do brown-bags, lunch-n-learns, reading-groups, etc. People who learn in groups learn much more, and much more quickly, than people who study alone. Never underestimate the power of the other guy’s viewpoint.

By the same token, join a user group with people from different companies. Attend monthly meetings and listen to the speakers, or participate in the discussions. Again, you’ll learn an awful lot that way.

Practice, practice, practice, practice. Be prepared to make lots of mistakes.

And, of course, apply what you’ve learned on the job, and ask your peers to review your work. Pair program with them if at all possible. “As iron sharpens iron, so one man sharpens another.” (Proverbs 27:17)

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