Design Principles 101 – Writing Code That Stands the Test of Time

Writing maintainable and scalable software isn’t just about choosing the right framework or language — it’s about mastering the principles that make your codebase resilient to change. In this post, we’ll explore what design principles are, why they matter, and how they’re different from design patterns. We’ll then introduce the most important principles every developer should know.

📘 What Are Design Principles?

Design principles are guidelines that help developers make decisions about structure, behavior, and responsibility in their software systems. Unlike design patterns — which are reusable solutions to common problems — principles are more abstract and apply universally.

🔑 Top Principles to Master

• SOLID Principles: The gold standard for object-oriented design (each will be covered in future posts):
  • Single Responsibility Principle (SRP)
  • Open/Closed Principle (OCP)
  • Liskov Substitution Principle (LSP)
  • Interface Segregation Principle (ISP)
  • Dependency Inversion Principle (DIP)

  These principles help create code that is easy to maintain, extend, and test. They promote good practices like separation of concerns and reducing coupling between components.

• DRY – Don’t Repeat Yourself

  This principle emphasizes the importance of reducing code duplication. If you find yourself copying and pasting code, it’s a sign that you need to refactor.

• KISS – Keep It Simple, Stupid

  This principle emphasizes simplicity in design. Complex solutions are harder to maintain and understand, so aim for the simplest solution that works.

• YAGNI – You Aren’t Gonna Need It

  This principle advises against adding functionality until it is necessary. Premature optimization can lead to complex code that is hard to maintain.

• Separation of Concerns

  This principle states that different concerns or functionalities should be separated into distinct sections of code. This makes it easier to manage, test, and maintain each part independently.

• Law of Demeter

  Also known as the Principle of Least Knowledge, this principle suggests that an object should only talk to its immediate friends and not to strangers. This reduces dependencies and makes the code more modular.

• Composition Over Inheritance

  This principle advocates for using composition to achieve code reuse instead of inheritance. It allows for more flexible and maintainable code structures.

• Program to an Interface, Not Implementation

  This principle suggests that you should depend on abstractions (interfaces) rather than concrete implementations. This leads to more flexible and testable code.

🚨 Real-World Consequences of Ignoring Them

Let’s say you ignore SRP and cram multiple responsibilities into a single class. What happens when requirements change? You risk breaking functionality that wasn’t supposed to change. Ignoring these principles leads to:

• Code duplication and higher maintenance cost
• Tightly coupled components
• Difficulty in testing or extending functionality
• Increased bugs and regressions

👨‍💻 Examples in C# and Python

Let’s look at a small example that violates and then follows the Single Responsibility Principle (SRP):

// ❌ Violates SRP
public class ReportGenerator {
    public string GenerateReport() {
        return "Report content";
    }
    public void SaveToFile(string content) {
        File.WriteAllText("report.txt", content);
    }
}

// ✅ SRP Applied
public class ReportGenerator {
    public string GenerateReport() {
        return "Report content";
    }
}

public class FileSaver {
    public void SaveToFile(string content) {
        File.WriteAllText("report.txt", content);
    }
}

# ❌ Violates SRP
class ReportGenerator:
    def generate_report(self):
        return "Report content"

    def save_to_file(self, content):
        with open("report.txt", "w") as f:
            f.write(content)

# ✅ SRP Applied
class ReportGenerator:
    def generate_report(self):
        return "Report content"

class FileSaver:
    def save_to_file(self, content):
        with open("report.txt", "w") as f:
            f.write(content)

🧪 Quiz Yourself

🔗 What’s Next?

In the upcoming blogs, we’ll deep-dive into each principle starting with the Single Responsibility Principle. Each post will feature real-world code, bad practices, and how to fix them with visual explanations and quizzes.