class Program
{
    static void Main(string[] args)
    {
        List<Person> people = new List<Person>();
        people.Add(new Person() { FirstName = "John", LastName = "Doe"});
        people.Add(new Person() { FirstName = "Jane", LastName = "Doe" });
        people.Add(new Employee() { FirstName = "John", LastName = "Smith" });
 
        // Displays 3
        Console.WriteLine("Number of People In Collection:     {0}", people.Count);
 
        // Displays 1
        Console.WriteLine("Number of Employees In Collection:  {0}", people.OfType<Employee>().Count());
        Console.ReadKey();
    }
}
 
public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}
 
public class Employee : Person
{
    public int EmployeeId { get; set; }
}