Composition over Inheritance and other Pithy Catch Phrases

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Love them or hate them, the ALT.NET mailing list is a source of interesting debate, commentary and insight. I can’t help myself but to participate. Debate is good. Stifling debate is bad. Period. End of debate. (see!? That was bad!)

The community itself is a young community, and as such, they are going through a period of identity forming. What are their shared values? What does it mean to be an ALT.NET-er? It's not exactly clear yet, but it is starting to form.

One thing I would caution this community is to be careful in how they define their shared principles. For example, in one thread one individual mentioned debating me and then in the same message proposed the idea of Composition over Inheritance as a shared principle.

In response, someone posted this:

You can throw the book at those people--literally. Favoring composition over inheritance is straight out of the Gang of Four book. Don't like design patterns? Fine. No problem. I have a couple of \ Don Box COM+ books that say the exact same thing.

Here was my response, which I also wanted to put in a blog post since it represents pretty well what I think.

I think ALT.NET should focus more on the principles of thinking for yourselfand a desire to improve.

> Favoring composition over inheritance is straight out of the Gang of Four book.

So is the Singleton pattern. So is the Template Method pattern.

Sorry, Appeal to Authority doesn't work for me. Look, I’m not against composition over inheritance in many cases. Perhaps most cases. What I am against is saying that it applies in all cases and that if you don’t do it, you’re not ALT.NET.

I’m against the blind application of these pithy catch phrases. Blindly applying a “best practice” is just as irresponsible as never applying a “best practice”. There is no perfect design. There is no one true way. There is no one size fits all.

Why favor composition over inheritance? What trade-offs are you making when you do so? Developers should think through these things when they make these choices. And when a developer does think through the issue, but makes a choice that differs from what you think, you should applaud that. At least the developer thought through the decision.

I don’t care that a developer doesn’t favor composition over inheritance in a specific case. I only care that the developer thought it through, had a reason for the decision, wants to improve.

The goal is not to bend developers to the will of some specific patterns, but to get them to *think about their work and what they are doing. For example, one advantage with inheritance is that it is easier to use than composition. However, that ease of use comes at the cost that it is harder to *reusebecause the subclass is tied to the parent class.

One advantage of composition is that it is more flexible because behavior can be swapped at runtime. One disadvantage of composition is that the behavior of the system may be harder to understand just by looking at the source. These are all factors one should think about when applying composition over inheritance.

So while I agree that you should favor composition over inheritance, inheritance is still necessary. After all, “the set of components is never quite rich enough in practice.”

That quote is from Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides in Design Patterns. But don’t believe it just because they said it. After all, I would hate to be guilty of an Appeal to Authority. ;)

Tags: ALT.NET , Design Patterns

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