Explain the principle of dependency injection in programming code and its advantages.

Dependency Injection (DI) is a design pattern where an object receives its dependencies from outside rather than creating them itself. This enhances modularity and testability of the code.

Advantages of DI:

• Easier testing — dependencies can be easily replaced with mocks or stubs.
• Loose coupling — classes do not depend on specific implementations, only on abstractions.
• Increased flexibility — it's easy to change dependency implementations without modifying the code that uses them.

Example in C# using 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) // dependency injected via constructor
    {
        _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");