Chapter 12: Dependency Injection

rupen · #1 (**permalink**) April 15th, 2014, 10:12 AM

Hi,

The code at page # 299 is as follows:

Code:

public interface IMessagingService
{
void SendMessage();
}
public class EmailService : IMessagingService
{
public void SendMessage() { ... }
}
public class NotificationSystem
{
private IMessagingService svc;
public NotificationSystem()
{
svc = new EmailService();
}
public void InterestingEventHappened()
{
svc.SendMessage();
}
}

I think the constructor of NotificationSystem should not create a new object of EmailService. Instead, it should be IMessagingService.
Please justify if not so.

mmorgan30 · #2 (**permalink**) April 22nd, 2014, 12:28 AM

Rupen,

you can not create an instance of IMessagingService, it was declared as an interface. Since EmailService inherits the IMessagingService interface the private member variable can hold an instance of an EmailService. Interfaces are a contract for other classes to fullfill through inheritance they do not contain concrete code and can't be instantiated.

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