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Introducing SOLID Principles

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

This is episode 47, in which we'll introduce the SOLID principles. I'll spend a little time reviewing these principles in the upcoming episodes.

What are the SOLID principles of object-oriented design?

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Show Notes / Transcript

Depending on how long you've been programming, you may have heard of the SOLID principles. These are a set of 5 principles that have been around for several decades, and about 15 years ago someone - I think it was Michael Feathers - had the idea to arrange them in such a way that they formed the macronym SOLID. Prior to that, I think the first time they were all published together was in Robert C. Martin's 2003 book, Agile Software Development: Principles, Patterns, and Practices in which their sequence spelled SOLDI - so close! This same sequence was used in the 2006 book Agile Principles, Patterns, and Practices in C#.

So what are the SOLID principles? As I mentioned, SOLID is a macronym, meaning it is an acronym formed by other acronyms. In this case, these are SRP, OCP, LSP, ISP, and DIP. All those Ps at the end of each acronym stand for principle, of course. Listing each principle, we have:

  • Single Responsibility
  • Open/Closed
  • Liskov Substitution
  • Interface Segregation
  • Dependency Inversion

You may already be familiar with these principles. If you're a developer who's using a strongly typed language like C# or Java, you should be extremely familiar with them. If you're not, I recommend digging into them more deeply. Applying them can make a massive difference in the quality of code you write. How do I define quality? Well, that's probably a topic I could devote an episode to, but the short version is that quality code is code that is easy to understand and easy to change to suit new requirements. It's easily and quickly tested by automated tests, which reduces the need for expensive manual testing. And it's loosely coupled to infrastructure concerns like databases or files.

How do these principles help you to write quality code? They provide guidance. You need to write code that solves a problem, first and foremost. But once you have code that does that, before you call it done and check it in, you should evaluate its design and see if it makes sense to spend a few moments cleaning anything up. Back in Episode 6 - you are listening to these in sequential, not reverse, order, right? - I talked about Kent Beck's approach of Make It Work, Make It Right, Make It Fast. SOLID principles should generally be applied during the Make It Right step. Don't apply them up front, but as I discussed in Episode 10, follow Pain Driven Development. If you try to apply every principle to every part of your codebase from the start, you'll end up with extremely abstract code that could do anything but actually does nothing. Don't do that.

Instead, build the code you need to solve the problem at hand, and then evaluate whether that code has any major code smells like I discussed in episode 30. One huge code smell is code that is hard to unit test, meaning it's hard to write an automated test that can just test your code, without any external infrastructure or dependencies like databases, files, or web servers. Code that is easy to unit test is generally easy to change, and code that has tests is also easier to refactor because when you're done you'll have some degree of confidence that you haven't broken anything.

In upcoming episodes, I'll drill into each principle a bit more. I've published two courses on SOLID at Pluralsight where you can obviously learn a lot more and see real code as opposed to just hearing me through a podcast. The first one was published in 2010 and so the tools and look were a bit dated. The more recent one is slimmed down and uses the latest version of Visual Studio and .NET Core. There are links to both courses in the show notes - the original one also covers the Don't Repeat Yourself principle. Let me wrap this episode up with a very brief overview of each principle.

The Single Responsibility Principle is generally applied to classes and suggests that classes should have only one responsibility, which can also be thought of as one reason to change. Responsibilities include things like business logic, ui logic, data access, and more. Following this principle, you'll tend to have smaller, more focused classes.

The Open/Closed Principle suggests that you should be able to change the behavior of your system without changing its source code. This generally relies on some kind of parameter or plug-in capability to provide new behavior to an existing class or service.

The Liskov Substitution Principle cautions against creating inheritance hierarchies in which child types are not 100% substitutable for their base types. When violated, this can result in messy code and bugs.

The Interface Segregation Principle suggests that classes that use interfaces should use all or most of the interface's exposed members. This then leads to interfaces that are small, focused, and cohesive, much like SRP.

Finally, the Dependency Inversion Principle recommends that low-level concerns depend on high level concerns, not the other way around. This means for example that business layer code shouldn't directly depend on data access code, but rather an abstraction should exist that the business code works with and that the data access code implements. At runtime, the data access code will be provided as an implementation of the interface the business code is written to work with, providing loose coupling and more testable code.

Show Resources and Links

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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