core-principles

Core Principles & Code Organization

Purpose: Fundamental principles guiding production code development and project structure. Focus: SOLID, DRY, KISS, design patterns, project organization.

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When to Use This Skill

• Reviewing code for SOLID principle compliance
• Refactoring code for maintainability
• Choosing appropriate design patterns
• Teaching or evaluating engineering standards
• Setting up a new project structure
• Refactoring monolithic codebases
• Implementing dependency injection
• Organizing modules for team collaboration
• Choosing between architectural patterns

Prerequisites

• Basic OOP programming knowledge

Decision Tree

Architecture or code quality concern?
+-- New project setup? -> Apply Clean Architecture layers (Api/Core/Infrastructure)
+-- Class doing too much? -> Apply SRP - split by responsibility
+-- Need to extend behavior? -> Apply OCP - use interfaces, not if/else chains
+-- Deep inheritance tree? -> Favor composition over inheritance
+-- Hard to test? -> Apply DIP - inject abstractions, not concretions
+-- Code duplicated? -> Extract shared logic (DRY), but avoid premature abstraction
+-- Complex solution? -> Simplify (KISS) - can a junior dev understand it?
+-- Building speculative features? -> Stop (YAGNI) - build only what is needed now

SOLID Principles

Single Responsibility (SRP)

Each class has one reason to change.

// [FAIL] Multiple responsibilities
public class User
{
 public string Name { get; set; }
 public void SaveToDatabase() { } // Persistence
 public void SendEmail() { } // Communication
}

// [PASS] Single responsibility
public class User
{
 public string Name { get; set; }
}
public class UserRepository
{
 public void Save(User user) { }
}
public class EmailService
{
 public void SendEmail(User user) { }
}

Open/Closed (OCP)

Open for extension, closed for modification.

// [PASS] Extend via abstraction
public interface IPaymentProcessor
{
 Task<PaymentResult> ProcessAsync(decimal amount);
}

public class CreditCardProcessor : IPaymentProcessor { }
public class PayPalProcessor : IPaymentProcessor { }

public class PaymentService
{
 public async Task ProcessPaymentAsync(IPaymentProcessor processor, decimal amount)
 {
 return await processor.ProcessAsync(amount);
 }
}