What is the purpose of dependency injection in software modules?

Dependency Injection (DI) is used to increase the modularity and testability of software modules. Instead of creating dependencies inside a class, they are passed from outside, which makes it easy to replace implementations, simplifies maintenance, and extends the code.

For example, in C#, DI can be used to pass an implementation of an interface into a class constructor:

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.");
    }
}

This approach makes it easier to replace the logger with another (for example, a file logger) without changing the UserService class.