What is dependency injection and what are its advantages and disadvantages when used in a project?

Dependency Injection (DI) is a design pattern where objects receive their dependencies from outside rather than creating them themselves. In C#, this is often implemented through constructors, properties, or methods.

Advantages:

• Improves testability of the code, as dependencies can be easily replaced with mocks.
• Enhances modularity and flexibility, making it easier to replace components.
• Reduces coupling between classes.

Disadvantages:

• Complicates the project structure, especially when used improperly.
• Requires additional time for setup and understanding.
• May lead to excessive abstraction if used unnecessarily.

Example of constructor injection:

public interface ILogger { void Log(string message); }

public class ConsoleLogger : ILogger {
    public void Log(string message) => Console.WriteLine(message);
}

public class UserService {
    private readonly ILogger _logger;

    public UserService(ILogger logger) {
        _logger = logger;
    }

    public void CreateUser(string name) {
        // user creation logic
        _logger.Log($"User {name} created.");
    }
}

// Usage
var logger = new ConsoleLogger();
var service = new UserService(logger);
service.CreateUser("Alice");