Mastering Software Design Principles: SOLID, DRY, KISS, and More

Any fool can write code that a computer understands; a good programmer writes code that humans can understand — Martin Fowler

If you are new to programming, you might wonder why design principles are needed. The answer is not to appear pretentious; simple programs may not require them, and a one‑off complex program that will never be changed also does not benefit.

In reality, most software systems have a certain level of complexity and are continuously modified, so we need code that is understandable both to machines and to people. Human‑readable code is maintainable code, and it rests on two core ideas: high cohesion and low coupling.

One principle that helps achieve high cohesion and low coupling is Separation of Concerns (SoC). Concerns are distinct parts of a system’s functionality, such as business logic or presentation. SoC advocates decomposing a system into non‑overlapping features—for example, keeping business logic in the domain layer rather than splitting it between stored procedures and the UI.

Robert C. Martin later refined these ideas into the five SOLID principles, which are widely adopted:

Single Responsibility Principle (SRP): a class should be as simple as possible and focus on a single core task.

Open/Closed Principle (OCP): software entities should be open for extension but closed for modification.

Liskov Substitution Principle (LSP): subclasses must be replaceable for their base classes.

Interface Segregation Principle (ISP): interfaces should remain minimal, avoiding bloated contracts.

Dependency Inversion Principle (DIP): high‑level modules should not depend on low‑level modules; both should depend on abstractions. For instance, in ASP.NET MVC a controller should depend on service or repository interfaces rather than concrete implementations, typically using an IoC container such as Unity or Autofac.

Additional practical principles include:

DRY (Don’t Repeat Yourself)

Tell, Don’t Ask

KISS (Keep It Simple, Stupid) – inspired by a French poet who said perfection is not adding more, but removing the unnecessary, helping avoid over‑design.

YAGNI (You Aren’t Gonna Need It) – discourages implementing features that are not explicitly required, preventing hidden problems.

Writing good code is not innate; it requires a process of continual refactoring. In practice, refactoring happens daily, not because the code stops working, but to satisfy non‑functional requirements such as readability, maintainability, testability, extensibility, or performance.

Common refactoring actions include:

Extract Method

Extract Interface

Encapsulate Field

Rename

Replace old components with new ones