Can you explain the concept of three-tier architecture and its main components?

The three-tier architecture divides the application into three main layers, which improves modularity, scalability, and maintainability of the code:

1. Presentation Layer — responsible for user interaction. In C#, this can be forms, web pages, or API controllers. This layer displays data and accepts user input.

2. Business Logic Layer — contains business rules and data processing logic. It implements core operations, checks, and algorithms.

3. Data Access Layer — responsible for interacting with the database or other data sources. This layer executes queries, saves, and retrieves data.

This separation allows changing one layer without affecting others, simplifies testing and maintenance.

Example class structure:

// Data Access Layer
public class UserRepository
{
    public User GetUser(int id) { /* database query */ }
}

// Business Logic Layer
public class UserService
{
    private readonly UserRepository _repository;
    public UserService(UserRepository repository) { _repository = repository; }
    public UserDto GetUserDetails(int id) { /* business logic */ }
}

// Presentation Layer
public class UserController
{
    private readonly UserService _service;
    public UserController(UserService service) { _service = service; }
    public IActionResult GetUser(int id) { /* service call and return result */ }
}

This approach improves the application's structure and facilitates its development.