S - Single Responsibility PrincipleAn element of code (such as module, class or function) should do one primary thing and nothing else. That is, do not build Swiss army knife code elements. For example, a function that changes the case of a string should not trim it as well.
O - Open/Closed PrincipleDo not alter the foundational code (such as base classes and common functions) but rather compose or extend them. Modules, classes and functions should be open for extension but closed for modification. The initial functionality should not be changed but may be extended via composition or inheritance.
L - Liskov Substitution PrincipleProgram to interfaces. Your business logic should be programmed against class interfaces (not concrete implementation of classes). A subclass should have "is a" relationship with the base class. Wherever you can use a base class, you should be able to use a derived class.
I - Interface Segregation PrincipleInterfaces should be minimal and tailored to the function they need to perform. Minimize the number of methods in a given interface. Instead, expose multiple smaller interfaces. Clients can choose to implement just the interfaces they require.
D - Dependency Inversion PrincipleDo not use concrete implementations directly. High level modules should not depend on lower level modules. Constructors for higher level modules should accept abstractions (interfaces) of lower level modules. At runtime, Dependency Container injects a concrete instance of the abstraction into the constructor.
SEPARATION OF CONCERNSSimilar to Single Responsibility Principle in SOLID but applied at a macro level (Systems, Modules, Sub-Systems). For example, a payment processing module should only perform payment processing and nothing else (i.e. it should not calculate invoices or bills).
DRY - DON'T REPEAT YOURSELFDo not implement the same logic twice. Put it in a common module, library, class or function and use it everywhere.
Factor out common code into helper classes, methods and components. This makes the testing effort more concise and helps track down bugs to a single place in code instead of several repeating code sections.
MINIMIZE THE CODE YOU WRITEMinimize the amount of code you have to write to solve a problem. The less code there is, the lesser the chance of buggy code. Code Minimization goes beyond DRY in that you may opt to look at alternate design approaches that minimize the amount of code you have to write to begin with. Remember code is a liability! The less code you have to solve the problem, the better. Examples:
- Use a well established library or software package to solve the problem instead of writing code to do the same.
- Use declarative code instead of imperative code. For example in .Net, instead of using for loops use LINQ.
- Instead of using switch/case statements to select a value or action use a table driven approach to look up a value or execute an action (a database table or an in-memory data structure should do the trick).
YAGNI - YOU AREN'T GONNA NEED ITDo not implement code that is not immediately needed now even if you think you might need it in the future (you probably won't or if you do, you will need something entirely different).
Don't anticipate features and try to adapt your design or code to the features that are not necessary to complete the stories at hand. The chances are that either the requirements will change over time or the features you anticipate are never really needed. Do the simplest thing that will work.
USE DESIGN PATTERNS
When writing code check if there is a well-known design pattern that can be used to make implementation more robust and maintainable. Design Pattern Examples
ORGANIZE YOUR CODE WELLUse polymorphism (abstract class + derived classes) to implement specific behaviors instead of conditionals sprinkled all over the code to handle differing behaviors. Use a single switch to instantiate the polymorphic derived class of the desired behavior and have the controlling logic invoke the behavior in a general fashion that is applicable to the entire family of the derived polymorphic classes.
Keep methods (functions) short and single-purposed. Complex methods should be no longer than 20 lines. Simpler methods should be no longer than 10 lines.
Do not pass too many parameters into a method. When several parameters must be passed into a method, create a request object that contains the parameters and pass the request object into the method.
Name the methods purposefully. The name should clearly reflect what the method does. A comment for the method should ideally be unnecessary. Write code that reads like prose. Comments should not be necessary when reading the code. Refactor logic into smaller methods with descriptive names. When reading the main method, the reader should be able deduce the purpose of the helper methods being called and understand what the main method is trying to accomplish without reading the implementation of the helper methods.
Do not enter who changed the code or the what the ticket or issue number was in code comments. Such information belongs in the source control check-in comments.
GET HELP FROM THE COMPILERUse strongly type variables and constructs (generics, for example) to detect problems as early as possible (at compile time instead of runtime). Use enumerations instead of string values. Use named constants instead of string literals. Pay attention to compiler warnings.
Write testable codeStructure and implement your code so that it is easily testable. This mind set will naturally help you write code that is loosely coupled and minimizes inter-dependencies. The loose coupling is essential for testing a class or a component in isolation as is necessary when writing Unit Tests.
Write automated testsBy all means write automated Unit Tests. Better yet, take the Test Driven Development (TDD) route. That is, devise and write your tests before you start developing. TDD will greatly help in the design of the system in addition to the more obvious benefit of having a more complete set of automated tests. Having good automated unit tests allow us to refactor our code confidently.
Integrate often - Fail fastDon't sit on pending changes. Check them in right away to find any incompatible changes. It is better to fail fast and correct the problem earlier in the cycle than to sit on a change and having to figure out why things don't work due to a change you made last week.
REFACTORINGGroom (clean up) code without changing the underlying functionality often. Refactoring is essential for long term maintenance of the code and to facilitate future enhancements.
Over the time, the code will naturally decay. You will see violations of DRY, Minimize Code and even YAGNI principles. It is imperative to take time to periodically refactor the code to ensure its maintainability over time. Remove and discard dead code. Such code only adds noise to your code base. The lower the noise-to-signal (code) ratio, the better.