Hi and welcome back to Weekly Dev Tips. I’m your host Steve Smith, aka Ardalis.

This is episode 51, on the Open/Closed principle.

Open/Closed Principle

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Show Notes / Transcript

My goal is to have every other tip for the next few weeks focus on one of the SOLID principles. If you're listening in order, which I recommend, you'll note that episode 49 was on Single Responsibility. Thus, here we are with episode 51, talking about the O in SOLID, the Open/Closed Principle.

The name of this principle is somewhat oxymoronic. It sometimes reminds me of the classic Star Wars scene where the stormtroopers are chasing Han Solo, yelling "close the blast doors, close the blast doors!" only to find themselves blocked by the doors moments later, crying "open the blast doors, open the blast doors!" What does the principle of being both open and closed mean, really?

The full principle says that software entities (no, not those entities - we mean classes, modules, functions, etc. here) should be open for extension but closed for modification. Ok, so what does each of these mean?

Open for extension means that it's possible to add to the behavior of the class or method. We can make it do new or different things.

Closed for modification means that the class or method itself should not be changed. Imagine that it's inside of a nuget package or its source is otherwise unavailable to modify.

If we think about open for extension all by itself, it's a pretty easy one. It's saying some class or method should be open to changes in how it works. We do that all the time. We need to fix a bug or add a feature, we find where the code is that does that work, and we add a few lines to change the behavior. Often this just means adding an if statement or something similar, to account for the new behavior required. This is the typical, obvious way to add behavior to our systems, and generally there's nothing wrong with this approach. But it doesn't follow the open/closed principle, because such code is clearly not closed for modification.

Now imagine that the code with the behavior you need to modify is not available to you to edit. How might you change its behavior, then? This might sound like a trick or impossible question, but we do this all the time, too. You change the behavior of classes and functions by changing their inputs. Passing in arguments or modifying data accessed by the class or method, such as configuration files or data stores, are all examples of ways in which you can modify system behavior without modifying source code. And there are some parts of your application that will benefit from being closed to modification, especially the parts that are depended on the most. By following OCP for your most fundamental classes, you'll make changes to them far more rare, which in turn will result in far fewer issues downstream due to updates to them.

If you'd like to learn more about OCP, check out my courses on Pluralsight on the SOLID principles. I just produced a revised course for 2019, but my original SOLID principles of object-oriented design course is out there as well and uses different examples, so you might get benefit from seeing both of them. You'll find links to both in this episode's show notes.

Show Resources and Links

• devBetter
• SOLID Principles for C# Developers
• SOLID Principles of Object Oriented Design -and the DRY Principle
• Refactoring Fundamentals

That’s it for this week. If you want to hear more from me, go to ardalis.com/tips to sign up for a free tip in your inbox every Wednesday. I'm also streaming programming topics on twitch.tv/ardalis most Fridays at noon Eastern Time. Thank you for subscribing to Weekly Dev Tips, and I'll see you next week with another great developer tip.

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