Single Responsibility Principle (or SRP for short), states that:
A class should have one and only one reason to change, meaning that a class should have only one job.
This doesn't mean that a class should contain only one method or one property, but its members must relate to its responsibility.
For this example, we're going to create an UserService, its responsibility, is to manage User data in a system.
public class UserService
{
public void CreateUser(string email, string password)
{
if(string.isNullOrEmpty(email)
throw new NullArgumentException("E-mail cannot be null or empty.");
if(string.IsNullOrEmpty(password)
throw new NullArgumentException("Password cannot be null or empty.");
User user = new User(email, password);
var smtpClient = new SmtpClient("smtp.gmail.com")
{
Port = 587,
Credentials = new NetworkCredential("email", "password"),
EnableSsl = true,
};
smtpClient.Send("our.app@gmail.com", email, "Welcome!", "Welcome to our system!");
}
}
The example above violates the SRP principle because the CreateUsermethod is responsible for two things, creating an user and sending an e-mail.
Let's change it so it respects the SRP principle.
public class UserService
{
private readonly IEmailService _emailService;
public UserService(IEmailService emailService)
{
_emailService = emailService;
}
public void CreateUser(string email, string password)
{
if(string.isNullOrEmpty(email)
throw new NullArgumentException("E-mail cannot be null or empty.");
if(string.IsNullOrEmpty(password)
throw new NullArgumentException("Password cannot be null or empty.");
User user = new User(email, password);
_emailService.SendEmail(email, "Welcome!", "welcome to our system!");
}
}
Now the UserService is only responsible to create the user, and it delegates the e-mail to another class.
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