Single Responsibility Principle

• A class should have one, and only one, reason to change.
• This means that a class must have only one responsibility.

Example 1

Problem

public class Order 
{
	public int OrderId { get; set; }
	public string Customer { get; set; }
	public decimal OrderAmount { get; set; } 
}

public class OrderService
{
    AppDbContext context;
    
    public void OrderService(AppDbContext context) 
    {
	    this.context = context;
    }
	
	public void CreateOrder(Order order) 
	{
		//logging
		Console.WriteLine("CreateOrder start");

		//Save Order
		context.Orders.Add(order);
		context.SaveChanges();
		
		//Send Email
		SmtpClient smtpClient = new SmtpClient("mail.MyWebsiteDomainName.com", 25);
		smtpClient.Credentials = new System.Net.NetworkCredential("info@MyWebsiteDomainName.com", "myIDPassword");
		MailMessage mail = new MailMessage();

		mail.From = new MailAddress("info@MyWebsiteDomainName", "MyWeb Site");
		mail.To.Add(new MailAddress(order.Customer));
		mail.Subject = "New Order";
		mail.Body = "Thank you for your order";

		smtpClient.Send(mail);

		Console.WriteLine("CreateOrder end");
	}
}

• Order class has the implementation detail of
  • how to send email.
  • how to log.
• If logging or send email changes, the order class should be updated.

Solution

• Logger class is responsible for logging activity
• OrderRepository is responsible for saving order
• EmailManager is responsible for sending email

public class Order 
{
	public int OrderId { get; set; }
	public string Customer { get; set; }
	public decimal OrderAmount { get; set; } 
}
public class OrderService
{
	IOrderRepository orderRepo;
I	ILogger logger;
	IEmailManager emailManager;
    
    public void OrderService(IOrderRepository orderRepo, ILogger logger, IEmailManager emailManager) 
    {
	    this.orderRepo = orderRepo;
	    this.logger = logger;
	    this.EmailManager = emailManager;
    }
    
	public void CreateOrder(Order order) 
	{
		logger.Write("CreateOrder start");

		//Save Order
		orderRepo.CreateOrder(order);
		
		//Send Email
		emailManager.Send(order.Customer, "New Order", "Thank you for your order")

		logger.Write("CreateOrder end");
	}
}

Example 2

Problem

public class UserManager 
{
	public bool AuthenticateUser(string username, string password) 
	{
		Console.WriteLine("Authenticate user");
		return true;
	}
	
	public void UpdateUserProfile(string userid, string address) 
	{
		Console.WriteLine("Update User");
	}
						  
	public void SendEmail(string email, string message) 
    {
		Console.WriteLine("Email user");
	}
}

Solution

public class UserAuthenticator
{
	public bool AuthenticateUser(string username, string password) 
	{
		Console.WriteLine("Authenticate user");
		return true;
	}
}

public class UserProfileManager
{
	public void UpdateUserProfile(string userid, string address) 
	{
		Console.WriteLine("Update User");
	}
}

public class EmailNotifier
{
	public void SendEmail(string email, string message) 
	{
		Console.WriteLine("Email user");
	}
}