Discover how composition helps you follow the SOLID principles in C#. See practical examples for each principle, learn why composition is more flexible than inheritance, and get actionable tips for writing robust, testable, and maintainable code.
TL;DR: Interface in C# - TL;DR What Is It Definition: Pure contract defining what methods/properties a class must have Purpose: Define capabilities without implementation details Keywords: interface, class : IInterface, implement multiple Key Features No Implementation: Methods are abstract by default (C# 8+ allows default implementations) No State: Cannot have instance fields or constructors Access: All members implicitly public Inheritance: Classes can implement multiple interfaces Interface Types Method Signatures: Define what methods must exist Properties: Define required properties (get/set) Events: Define event contracts Default Methods: C# 8+ allows method implementations in interfaces When to Use Multiple contracts needed on same class Dependency injection and testing (easy mocking) Plugin/modular architecture Define capabilities across unrelated classes “Can-do” relationships (ISerializable, IComparable) Benefits Flexibility: Multiple implementation support Testability: Easy to mock for unit tests Loose Coupling: Depend on contracts, not concrete classes Polymorphism: Treat different classes uniformly Common Patterns Repository pattern (IRepository<T>) Service contracts (IEmailService, IPaymentService) Strategy pattern (different algorithms, same interface) Dependency injection containers Limitations No shared implementation (mostly) All members must be public Cannot contain instance state You’re building an e-commerce platform and need to send notifications, order confirmations, shipping updates, promotional emails. Right now you’re using an email service, but next month the marketing team wants SMS notifications too. Later, they might want push notifications or Slack alerts. ...