Question [18 marks] In the real world, the notification management system consists of two parts, one is to allow clients to subscribe/unsubscribe the notification, and another part is to facilitate administrator(s) to publish the notification. This assignment simplifies the problem, and merges two parts as one. In this assignment, you are asked to implement a C# application to mimic notification management system. You are required to use delegate.

Your app facilitates clients to subscribe/unsubscribe notification as well as send notification to all subscribers. Your App needs to make sure that the provided email address is valid and provided cell phone number is followed the specified format. After the app has been launched, following GUI (or similar one) should be presented. As there is no subscriber when the app just launches, Publish Notification button is disabled. After Manage Subscription button clicked, following GUI (or similar one) should be popped up to allow clients to subscribe or unsubscribe notification. If an invalid email address has been provided, error message should be provided. After Publish Notification button clicked, following GUI is presented to facilitate notification publish.

Please make sure: 1. No duplication subscription is allowed; in other words, if I try to subscribe using my email multiple times, your app should not allow it.

2. Your app should be able to deal with multiple subscribers. [hint: use collections to hold the subscribers information . You are required to use delegate.