Description

Decouple an abstraction from its implementation so that the two can vary independently.

Exemple

ILogger.cs

public interface ILogger
{
    void WriteLine(string text);
}

ConsoleLogger

public class ConsoleLogger : ILogger
{
    public void WriteLine(string text)
    {
        Console.WriteLine(text);
    }
}

IUserService.cs

public interface IUserService
{
    void ChangePassword(int userId, string newPassword);
}

UserService.cs

public class UserService : IUserService
{
    private ILogger logger;

    public UserService(ILogger logger)
    {
        this.logger = logger;
    }

    public void ChangePassword(int userId, string newPassword)
    {
        logger.WriteLine($"Password has been change for user {userId}.");
    }
}

IUserService userService = new UserService(new ConsoleLogger());
userService.ChangePassword(1, "0000");