SOLID: Single Responsibility Principle

swiss-army-knifeA blog about software craftsmanship cannot be complete without mentioning the SOLID principles. I discovered these principles about a year ago and it completely changed the way I write software and think about it. By using SOLID I feel like my code is more robust than before, more understandable, more maintainable and easier to test. Will them solve all your problems? Of course not but, in my opinion, it can definitely help you writing better crafted software. There are 5 object-oriented programming principles in SOLID:

  • Single Responsibility Principle (SRP)
  • Open Close Principle (OCP)
  • Liskov Substitution Principle (LSP)
  • Interface Segregation Principle (ISP)
  • Dependency Inversion Principle (DIP)

And today I will present the first one, the Single Responsibility Principle, by giving an example and explaining my understanding of this principle.

A class should have only one reason to change.

This is the rule of this first principle. Is that all? Yes it is, so now I will give details about the meaning of this rule and what it implies. If a class has more than one reason to change it means that this class certainly have more than one responsibility. If you are not able to describe the goal of your class with a short sentence you are most likely violating the SRP. And if your short sentence contains words like “or”, “and” or “then” you probably violates this principle as well.

I will use a basic example to demonstrate the concept of this principle. I created the following helper class:

class EmailHelper
    public void Validate(string email)
        // some validation logic
    public void Send(string email, string message)
        // some sending logic

So, what does this class do? I can answer by the following sentence: “This class helps manipulating email addresses”. I did use a short sentence to describe my class and without saying “and” or “or”. True, but in my opinion this is definitely imprecise and it reveals that the SRP might be violated. To describe this class I would much likely say: “This class validates email addresses and sends a message to a given address”. This is much more accurate and it shows that the class has more than one responsibility.

To respect the Single Responsibility Principle, I will create two different classes having each its own responsibility:

class EmailValidator
    public void Validate(string email)
        // some validation logic
class EmailSender
    public void Send(string email, string message)
        // some sending logic

I now have a class that validates a given email address and another that sends a message to an email address. I can now use the validation logic without having the sending logic in the same “area”. I know that my example might look mundane and sometimes it is much more difficult to tell if the SRP is violated or not. I can only advise you to keep the rule in mind to tell if you are respecting the SRP.

I hope you like this introduction to the Single Responsibility Principle from SOLID. And as always do not hesitate to share your opinion on this topic.

If you want to know more about the SRP and the SOLID principles I recommend to acquire the “Agile Principles, Patterns and Practices in C#” book by Robert “Uncle Bob” Martin and Micah Martin.

See you next time!

Are you a software Boy Scout ?


If you are a developer like me, you probably worked on a legacy code based application. If not, don’t worry (or do) it will certainly happen… We all know a project that we fear to open because it is just a huge mess without a single test and nobody really understand how it works, it just does. Every software team has to work in order to prevent the effects of technical debt.

Refactoring a whole application to make the code “cleaner” can take an enormous amount of time, it can be weeks or sometimes months, depending on the project size and complexity. Some organization will incorporate refactoring phases during the development life-cycle. But since during these periods, the team does not provide any new features (i.e. no value), having these refactoring phases is a hard sell. Time to become a Boy Scout then !

What the boys scouts have to do with software development ? The answer is found in their rule :

“Always leave the campground cleaner than you found it.”

This simple rule can be applied to code and especially legacy code, and it becomes :

“Leave the code better than you found it.”

I present you the Boy Scout Rule (BSR) of programming. Making the code “cleaner” can be done at any moment, and it can be done piece by piece, no need to wait for a “Big Bang Refactoring” phase.

If you have a legacy project it is likely that you will have some improvements or bug fixing to do in it. This is the perfect time to embrace the boy scout philosophy. Of course after your passage the application will still be legacy but a bit less, and the next time you will improve it again. The code will become better with time and one day you’ll stop considering the project as a legacy one.

For example it is possible to rename a variable with a more meaningful name. Given the following code to calculate a triangle area :

var res = b / 2 * h

After the BSR applied :

var triangleArea = base / 2 * height

This is not much but it will help the next developer (maybe you) to understand the code and its purpose. And next time you will see that this piece of code is duplicate in several parts of the code. Time to create a method then :

public int CalculateTriangleArea(int base, int height)
    return base / 2 * height;

You now have a method that can replace your duplicate code and that can be easily tested with your favorite automated testing framework ! I know that this example is really simple but I’m sure you’ll find these kinds of easy “cleaning” in your applications.

There is a type of code refactoring I often use to make my code more understandable and to ease maintainability : moving repeated magic number value into a single constant variable. For example, a few years back, the french VAT was equal to 19.6% and now it is 20%. I let you imagine the pain it could have been to change every “19.6” in some projects where it could have been far easier to use a single constant with a meaningful name.

There are a lot of refactoring techniques to improve your code base, Martin Fowler gives you a list of some of them here.

A software craftsman does not fear legacy code, by following the Boy Scout Rule he will improve his projects.

See you next time !

Meet your community


About a year ago I watched a video made by Scott Hanselman and Rob Conery entitled “Get Involved !”. The purpose of this video is to convince software developers to become active within their community. I really enjoyed this video and it made me want to participate more. This blog is the first example of the things I did toward this goal.

If you want to find more information about this subject and watch the video : Become a Social Developer !

There is a whole chapter related to user groups which allowed me to discover Meetup. This network helps you find user groups in your area in almost any domain you like. This way you can meet people who share common hobbies and interests. And if there is a subject you like without a related meetup in your neighborhood, nothing stop your from creating it !

I recently attended my first meetup and it was an extraordinary experience, I came across a lot of interesting people I could not have been able to meet otherwise. It was all about debating, discussing and sharing in a funny and respectful way. The subject of this meetup was Software Craftsmanship and you can find a summary I wrote on my company’s blog here.

I can only highly recommend Meetup to find communities you might want to socialize with.

See you next time !

The Clean Coder : How it changed me

“The Clean Coder” is not a book about the code, it is about the coder. A software developer does not only write code for himself, he writes code to solve problems, to add value to his company. In this book Robert “Uncle Bob” Martin shares his experience on the mistakes he did and how he changed his behavior in order to act as a professional.

I really enjoyed “The Clean Coder” because it made me think of my own behavior. Do I behave as a professional ? What can I improve ? What should I stop doing ?

I discovered a definition of professionalism that I was not following, I learned to do “no harm”. I discovered what I implied when I said yes and that I should not be afraid to say no. I discovered how to stay focus while coding. I discovered the benefits of Test Driven Development (TDD), the benefits of Acceptance Testing and the benefits of having a good Testing Strategy. I discovered that practicing my skills is key to achieve mastery. I discovered how to manage my time in order to stay productive, how to avoid unnecessary pressure and making concrete estimations. I discovered that collaboration is key to build excellent software and that I have to work closely with my team to complete my projects. I discovered a whole new world of apprenticeship and mentoring : Software Craftsmanship.

This software development ideology suits me, it gave me a path to follow. I decided to sign the software craftsmanship manifesto to be committed and to ask more of myself. Being a craftsman is not an easy task, it is an attitude that has to be learnt. It is challenging and that’s fine to me. I like to be challenged, it allows me to improve.

I hope you liked this journey through “The Clean Coder”, see you next time !

The Clean Coder : Mentoring, Apprenticeship and Craftsmanship


Software development is a relatively new profession, new technologies and discoveries make it changes constantly. Thereby it has not been codify yet, there are several methodologies, principles, practices and patterns. And this is a good thing, we still have a lot to explore to master and contribute to our craft. Yet, this lack of codification allows us to do whatever we want and sometimes in a bad way. We’ve all seen teams that defined themself as “agile” because they do not have any single methodology and use the term “agile” as an excuse for chaos ! We’ve all seen a two or more years project that does not have a single test ! We’ve all seen programs that become unmaintainable after 6 months of coding ! We’ve all seen projects that have more bugs than lines of code ! Somehow this is the cost of our new non-codify profession. Fortunately there are a lot of well designed, covered with tests, maintainable software too. It is up to us to share our knowledge and best practices with each other, it is up to us to codify our own craft.

Even if software development is new, we all had mentors to teach us programming. We learned through teachers, colleagues, books, videos, articles or even friends. Forty years ago when programming was starting all these resources were scarce or non-existent. Uncle Bob (“The Clean Coder” author) had to learn programming the hard way, without all the resources we can find one mouse’s click away. If we work with senior developers or any experienced professional we can ask them to share their knowledge to teach us how to behave as a professional. And we can of course share our own experience with the younger ones.

After graduation, a medical student is not thrown into an operating rooms to perform brain surgery or open heart surgery even if he as the theoretical knowledges to do it. The medical profession oversees education through intense mentoring. Medical students spend a lot of their education time working with professionals to sharpen their skills. It takes a decade and thousands hours of practice to become a professional doctor. Are we shocked by this approach? Of course not, we cannot even conceive it otherwise. Their work is highly important and we expect them to act as professionals.

In the software industry, things are “slightly” different from the medical system. It is no surprise to see “teams” formed with freshly graduated programmers that are asked to build software even critical ones. Of course creating programs is not as crucial as surgery, there is no life at stake. But bad software can lead to colossal monetary loss, Sony is one example of many. Graduating in Computer Science (CS) gives us enough skills to work in the domain but schools can’t teach us everything about programming. Software development is a complex world that evolves day by day and offers us an environment with constant learning. Creating a doctor-like system for our professions is essential to avoid making the same mistakes over and over.

Software apprenticeship can be a three steps journey : starting from apprentice and moving to journeyman before becoming a master.

Masters have more than 10 years of experience and have worked on different systems, technologies and programming languages. They are able to lead and coordinate several teams. They are responsible for the technical aspects of the projects.

Journeymen are trained and competent programmers, they are professionals. They learn to work as teams and to become team leaders. Their experience levels vary among them, there are former apprentices with little experience and there are burgeoning masters.

Apprentices are programmers that just begin their career. They are closely supervised by journeymen in order to improve their skills and knowledges, pair programming is heavily recommended to do so. They learn how to behave as professionals.

This system is similar of the guilds organization during the medieval era. In the real world this system seems to exist, graduates are supervised by young team-leads who are supervised by project-leads. But most of the time there is almost no technical supervision.

When we build software, we are crafting them, programmers are craftsmen. Craftsmanship is the mindset help by craftsmen, it contains values, disciplines, techniques, attitudes and answers. A craftsman is able to work quickly but without rushing, he also know when to say “no” and to meet the commitments. A craftsman is a professional !

If craftsmanship is your way of life keep in mind that you cannot force other programmers to become craftsmen, and convincing them is difficult. If you want them to become craftsmen you’ll have to show them how it is done and the benefits of it. Then maybe they will join the movement.

This was the final chapter of “The Clean Coder : A Code of Conduct for Professional Programmers” by Robert C. Martin. My next article will be a conclusion about the book, explaining what I’ve learned.