copilot-instructions object-calisthenics.instructions.md

Object Calisthenics Rules

⚠️ Warning: This file contains the 9 original Object Calisthenics rules. No additional rules must be added, and none of these rules should be replaced or removed. Examples may be added later if needed.

Objective

This rule enforces the principles of Object Calisthenics to ensure clean, maintainable, and robust code in the backend, primarily for business domain code.

Scope and Application

• Primary focus: Business domain classes (aggregates, entities, value objects, domain services)
• Secondary focus: Application layer services and use case handlers
• Exemptions:
  • DTOs (Data Transfer Objects)
  • API models/contracts
  • Configuration classes
  • Simple data containers without business logic
  • Infrastructure code where flexibility is needed

Key Principles

1. One Level of Indentation per Method:

   • Ensure methods are simple and do not exceed one level of indentation.

   // Bad Example - this method has multiple levels of indentation
   public void SendNewsletter() {
         foreach (var user in users) {
            if (user.IsActive) {
               // Do something
               mailer.Send(user.Email);
            }
         }
   }
   // Good Example - Extracted method to reduce indentation
   public void SendNewsletter() {
       foreach (var user in users) {
           SendEmail(user);
       }
   }
   private void SendEmail(User user) {
       if (user.IsActive) {
           mailer.Send(user.Email);
       }
   }
   
   // Good Example - Filtering users before sending emails
   public void SendNewsletter() {
       var activeUsers = users.Where(user => user.IsActive);
   
       foreach (var user in activeUsers) {
           mailer.Send(user.Email);
       }
   }
   

2. Don't Use the ELSE Keyword:

   • Avoid using the else keyword to reduce complexity and improve readability.
   • Use early returns to handle conditions instead.
   • Use Fail Fast principle
   • Use Guard Clauses to validate inputs and conditions at the beginning of methods.