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How do you implement a thread-safe Singleton pattern in modern C# using Lazy<T>? How does the Factory Method pattern work in C#, and what problem does it solve? How would you implement the Strategy pattern in C# using either interfaces or delegates? How would you implement the Decorator pattern in C# to add behavior to an object without modifying its original class? How would you implement the Observer pattern in C# using events, versus using the IObservable<T>/IObserver<T> interfaces from System? How would you implement the Unit of Work pattern alongside the Repository pattern to coordinate multiple repository changes within a single transaction? How would you implement the CQRS (Command Query Responsibility Segregation) pattern's basic structure in a C# application using MediatR-style handlers? How would you implement the Builder pattern using method chaining (a fluent API) to construct a complex object step by step? How would you implement the Chain of Responsibility pattern in C# to process a request through a series of independent handlers? What are design patterns, and how are the Singleton, Adapter, and Strategy patterns typically implemented in C#?
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How does the Repository pattern abstract data access, and what interface typically defines its contract?

Intermediate

Answer

The Repository pattern defines an interface (like IRepository<T>) exposing domain-focused CRUD operations, hiding the underlying data access technology (Entity Framework, raw SQL, an external API) behind that abstraction — letting business logic depend on the abstraction rather than a specific persistence mechanism.
public interface IRepository<T> {
  Task<T?> GetByIdAsync(int id);
  Task<IEnumerable<T>> GetAllAsync();
  Task AddAsync(T entity);
}
public class UserRepository : IRepository<User> {
  // implementation using EF Core, backed by AppDbContext
}
Real-world example Decoupling business logic from Entity Framework specifics, making it possible to swap the data layer or mock it easily in tests.

Common follow-ups: Is the Repository pattern still considered necessary on top of Entity Framework Core's DbSet<T>, which already provides similar abstraction?

Dependency Injection & IoC Principles
All 11 Design Patterns in C# questions