We will now develop the functional class so we can consume it to our functional program. Before that, let's prepare a new imperative class named Customer. The class will have an int property named id as a unique customer ID number. It also has four string properties to store the information about our customer--name, address, phoneNumber, and email. The class also has a flag--isActive--to indicate whether or not our customer is active. If the customer has signed a contract with us, they are regarded as an active customer. Another property is registeredCustomers, to save all registered customers we have, regardless of the active customer. We will make the registeredCustomers member become static so we can fill it from outside the class and can keep the list of the Customer class.
Besides these properties, our class will also have four methods to access the list of our properties. They will be the following methods:
GetActiveCustomerNames(): This can be used to get...