using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<Employee> employees = new List<Employee>()

        {

            new Employee() { EIN = 1, FirstName = "John", LastName  = "Doe", Salary= 55000M },

            new Employee() { EIN = 2, FirstName = "Jane", LastName = "Doe", Salary= 55000M },

            new Employee() { EIN = 3, FirstName = "Don", LastName  = "DeLuth", Salary= 55000M },

        };

        IEnumerable<Person> people = new List<Person>()

        {

            new Person() { FirstName = "Tom", LastName  = "Jones" },

            new Person() { FirstName = "Henry", LastName = "Holy" },

            new Person() { FirstName = "Barry", LastName  = "Bonds" },

        };

        IReadOnlyCollection<Person> directory = employees;

        int numStaff = directory.Count;

        Console.WriteLine("numStaff = {0}", numStaff);

        Covariance(employees);

        //Contravariance(people);

        //IList<Person> directoryList = employees;

    }

    private static void Covariance(IEnumerable<Person> list)

    {

        IList<Person> personList = list.ToList();

        Console.WriteLine("numStaff = {0}", personList.Count);

        Console.WriteLine("numStaff = {0}", personList.FirstOrDefault().FirstName);

        Console.WriteLine("numStaff = {0}", personList.FirstOrDefault().LastName);

    }

    private static void Contravariance(IEnumerable<Employee> list)

    {

        IList<Employee> personList = list.ToList();

        Console.WriteLine("numStaff = {0}", personList.Count);

        Console.WriteLine("numStaff = {0}", personList.FirstOrDefault().FirstName);

        Console.WriteLine("numStaff = {0}", personList.FirstOrDefault().LastName);

    }

}

​

public class Person

{

    public string FirstName { get; set; }

    public string LastName { get; set; }

}

​

public class Employee : Person

{

    public int EIN { get; set; }

    public Decimal Salary { get; set; }

}

​