How to implement Inherited interfaces in C#

I am not sure what this is called so I most likely calling it wrong when I say Inherited interface. Here is what I am trying to achieve.

I have an interface like this

 public interface INotificationEngine
    {
        bool UsingDbMail();

        bool UsingSMTP();

        bool UsingSMS();
    }

My class looks like this

 public class NotificationEngine
    {
        public class Send : INotificationEngine
        {
            public bool UsingDbMail(string para)
            {
                throw new NotImplementedException();
            }

            public bool UsingSMTP()
            {
                throw new NotImplementedException();
            }

            public bool UsingSMS()
            {
                throw new NotImplementedException();
            }
        }
    }

That allows me to do something like the following

NotificationEngine.Send sendRequest = new NotificationEngine.Send();
sendRequest.UsingDbMail("hello");

What I want to achieve instead is the following

NotificationEngine engine = new NotificationEngine();
engine.UsingDbMail("hello").Send;

Any idea how can I do that with interfaces or any other way?

>Solution :

Start with your common interface, it probably needs to look something like this:

interface IMailerService
{
    bool Send ():
}

You’ll want an implementation that uses dbmail, another for SMTP, and another for SMS. Construct an instance of these classes in each of your "UsingXyz" methods (note: no need for a nested class here):

public IMailerService UsingDbMail(...)
{
    return new DbMailerService(...);
}

public IMailerService UsingSmtp(...)
{
    return new SmtpMailerService(...);
}

public IMailerService UsingSms(...)
{
    return new SmsMailerService(...)
}

Now when you call a UsingXyz method, you get an object that exposes a Send method that it implements as needed:

engine.UsingSms(...).Send(); // sends a SMS message