github-copilot-rules: Instructions file for GitHub Copilot

Type Definitions:

• Prefer records for data types:

  // Good: Immutable data type with value semantics
  public sealed record CustomerDto(string Name, Email Email);
  
  // Avoid: Class with mutable properties
  public class Customer
  {
      public string Name { get; set; }
      public string Email { get; set; }
  }
  

• Make classes sealed by default:

  // Good: Make classes sealed by default
  public sealed class OrderProcessor
  {
      // Implementation
  }
  
  // Only unsealed when inheritance is specifically designed for
  public abstract class Repository<T>
  {
      // Base implementation
  }
  

Variable Declarations:

• Use var where possible:

  // Good: Using var for type inference
  var fruit = "Apple";
  var number = 42;
  var order = new Order(fruit, number);
  

Control Flow:

• Prefer range indexers over LINQ:

  // Good: Using range indexers with clear comments
  var lastItem = items[^1];  // ^1 means "1 from the end"
  var firstThree = items[..3];  // ..3 means "take first 3 items"
  var slice = items[2..5];  // take items from index 2 to 4 (5 exclusive)
  
  // Avoid: Using LINQ when range indexers are clearer
  var lastItem = items.LastOrDefault();
  var firstThree = items.Take(3).ToList();
  var slice = items.Skip(2).Take(3).ToList();
  

• Prefer collection initializers:

  // Good: Using collection initializers
  string[] fruits = ["Apple", "Banana", "Cherry"];
  
  // Avoid: Using explicit initialization when type is clear
  var fruits = new List<int>() {
      "Apple",
      "Banana",
      "Cherry"
  };
  

• Use pattern matching effectively:

  // Good: Clear pattern matching
  public decimal CalculateDiscount(Customer customer) =>
      customer switch
      {
          { Tier: CustomerTier.Premium } => 0.2m,
          { OrderCount: > 10 } => 0.1m,
          _ => 0m
      };
  
  // Avoid: Nested if statements
  public decimal CalculateDiscount(Customer customer)
  {
      if (customer.Tier == CustomerTier.Premium)
          return 0.2m;
      if (customer.OrderCount > 10)
          return 0.1m;
      return 0m;
  }