using System;

using System.Collections.Generic;

​

//=========== The Classes =======

​

public abstract class CustomerBase

{

    public CustomerBase(string name)

    {

        Name = name;

    }

​

    public abstract string Type { get; }

    public string Name { get; }

}

​

public class GoldCustomer : CustomerBase

{

    public GoldCustomer(string name)

      : base(name)

    {

    }

​

    public override string Type => "Gold";

}

​

public class SilverCustomer : CustomerBase

{

    public SilverCustomer(string name)

      : base(name)

    {

    }

​

    public override string Type => "Silver";

}

​

// ==============================

​

public class Program

{

    public static void Main()

    {

        var customers = new List<CustomerBase> {

            new GoldCustomer("Frank"),

            new SilverCustomer("Jack")

        };

​

        foreach (CustomerBase c in customers) {

            Console.WriteLine($"{c.Name} is {c.Type}");

        }

    }

}