Purpose of Interfaces

void Main()
{
    Person person = new Person("Person1","North Avenue","New City");
    Manager manager = new Manager("Manager","South Avenue","Old City", "Central Area");
    Company company1 = new Company("Fast Couriers","East Avenue","City Extension","Courier");
    
    Console.WriteLine ("---------");
    Console.WriteLine (person.GetAddressLabel());
    Console.WriteLine ("---------");
    Console.WriteLine (manager.GetAddressLabel());
    Console.WriteLine ("---------");
    Console.WriteLine (company1.GetAddressLabel());
}
class Person : IAddress {
    public string Name {get;  set;}
    public string Street {get; set;}
    public string City {get; set;}
    
    public Person (string name, string street, string city){
    	Name = name;
    	Street=street;
    	City=city;
    }
}
class Manager : Person {
    public string AreaManaged {get; set;}
    public Manager(string name, string street, string city, string areaManaged) :
    	base(name,street,city){
    	AreaManaged = areaManaged;
    }
}
class Company : IAddress {
    public string BusinessType {get; set;}
    public string Name {get;  set;}
    public string Street {get; set;}
    public string City {get; set;}
    
    public Company (string name, string street, string city, string businessType){
    	Name = name;
    	Street=street;
    	City=city;
    	BusinessType = businessType;
    }
}

interface IAddress
{
    string Name {get; set;}
    string Street {get; set;}
    string City {get; set;}
}

//This extension method is introduced later, to get address labels
//for all classes that implement IAddress interface
//A method can be introduced in IAddress interface, but it requires
//modifying all the classes that implement IAddress
static class ExtensionMethods {
    static public string GetAddressLabel(this IAddress address){
    	return string.Format("{0}\n{1}\n{2}",
    		address.Name,address.Street,address.City);
    }
}
//Output from the above sample code:
//---------
//Person1
//North Avenue
//New City
//---------
//Manager
//South Avenue
//Old City
//---------
//Fast Couriers
//East Avenue
//City Extension