Quick demo of how to use delegates and events in C#.

In our class called UserProcessor.cs we need to set up three things:

1. The delegate public delegate void UserProcesserEventHandler(object source, EventArgs args);

2. The Event: public event UserProcesserEventHandler UserProcessed;

3. The Event Handler notice the virtual keyword

 protected virtual void OnUserProcessed()
        {
            if (UserProcessed != null)
            {
                UserProcessed(this, EventArgs.Empty);
            }
        }

This allows us to fire off notifications by simply calling OnUserProcessed()

When we want to create a subscriber to the above events we need to abide by the contract defined in the delegate we created on our UserProcessor.cs class For instance our EmailNotificationService class has a method called OnUserProcessed with a signature matching the delegate:

public void OnUserProcessed(object sender, EventArgs args)
        {
            SendEmail();
        }

All that's left to do now is have our services subscribed to the events. To see this in action we jump over to our Program.cs file where we have the following code:

  var processor = new UserProcessor();
  var emailService = new EmailNotificationService();
  var textService = new TextMessageNotificationService();
  processor.UserProcessed += emailService.OnUserProcessed;
  processor.UserProcessed += textService.OnUserProcessed;

As you can see we're creating a new instance of everything we need and creating the subscriptions to the events.

Now when we run the application, not only does the message from the UserProcessor.cs show up in the console, but also any message sent by subscribers to the UserProcessed get sent to the console as well.