Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace PayrollSystem
{
    class Program
    {
        static void Main(string[] args)
        {
            SalariedEmployee salariedEmployee = new SalariedEmployee("John", "Smith", "111-11-1111", 800.00M);
            HourlyEmployee hourlyEmployee = new HourlyEmployee("Karen", "Price", "222-22-2222", 16.75M, 40.0M);
            CommissionEmployee commissionEmployee = new CommissionEmployee("Sue", "Jones", "333-33-3333", 10000.00M, .06M);
            BasePlusCommissionEmployee basePlusCommissionEmployee = new BasePlusCommissionEmployee("Bob", "Lewis", "444-44-4444", 5000.00M, .04M, 300.00M);
            Console.WriteLine("Employees processed individually:\n");

            Console.WriteLine("{0}\nearned: {1:C}\n", salariedEmployee, salariedEmployee.Earnings());
            Console.WriteLine("{0}\nearned: {1:C}\n", hourlyEmployee, hourlyEmployee.Earnings());
            Console.WriteLine("{0}\nearned: {1:C}\n", commissionEmployee, commissionEmployee.Earnings());
            Console.WriteLine("{0}\nearned: {1:C}\n", basePlusCommissionEmployee, basePlusCommissionEmployee.Earnings());

            Employee[] employees = new Employee[4];
            employees[0] = salariedEmployee;
            employees[1] = hourlyEmployee;
            employees[2] = commissionEmployee;
            employees[3] = basePlusCommissionEmployee;

            foreach (var currentEmployee in employees)
            {
                Console.WriteLine(currentEmployee);
                if (currentEmployee is BasePlusCommissionEmployee)
                {
                    //downcast Employee reference to BasePlusCommissionEmployee reference
                    BasePlusCommissionEmployee employee = (BasePlusCommissionEmployee)currentEmployee;
                    employee.BaseSalary *= 1.10M;
                    Console.WriteLine("new base salary with 10% increase is: {0:C}", employee.BaseSalary);
                }
                Console.WriteLine("earned{0:C}\n", currentEmployee.Earnings());
            }
            for (int j = 0; j < employees.Length; j++)
                Console.WriteLine("Employee {0} is a {1}", j,employees[j].GetType());
            Console.ReadLine();

        }
    }
}