.NET 4.x Ошибка при наследовании с использованием закрытых переменных экземпляров - C#

Console.WriteLine("\n{0}:\n\n{l}",
            "Updated  employee  information  obtained  bу  ToString",
            employee);

//BasePlusCommissionEmployee.cs
 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
 
namespace UseVarExs
{
    public class BasePlusCommissionEmployee : CommissionEmployee
    {
         private decimal baseSalary;
 
        public BasePlusCommissionEmployee(string first, string last,
            string ssn, decimal sales, decimal rate, decimal salary)
            :base(first, last, ssn,sales,rate)
        {
            baseSalary = salary;
        }
 
        public decimal BaseSalary
        {
            get
            {
                return baseSalary;
            }
            set
            {
                if (value >= 0)
                    baseSalary = value;
                else
                    throw new ArgumentOutOfRangeException(
                        "BaseSalary", value, "baseSalary must be >= 0");
            }
        }
 
        public override decimal Earnings()
        {
            return BaseSalary + base.Earnings();
        }
 
        public override string ToString()
        {
            return string.Format("base-sales {0}\nbase salary: {1:C}",
                base.ToString(), BaseSalary);
        }
    }
}

// CommissionEmployee.cs
 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
 
namespace UseVarExs
{
    public class CommissionEmployee
    {
        private string firstName;
        private string lastName;
        private string socialSecurityNumber;
        private decimal grossSales;
        private decimal commissionRate;
 
        public CommissionEmployee(string first, string last, string ssn,
            decimal sales, decimal rate)
        {
            firstName = first;
            lastName = last;
            socialSecurityNumber = ssn;
            GrossSales = sales;
            CommissionRate = rate;
        }
 
        public string FirstName
        {
            get
            {
                return firstName;
            }
        }
 
        public string LastName
        {
            get
            {
                return lastName;
            }
        }
 
        public string SocialSecurityNumber
        {
            get
            {
                return socialSecurityNumber;
            }
        }
 
        public decimal GrossSales
        {
            get
            {
                return grossSales;
            }
            set
            {
                if (value >= 0)
                    grossSales = value;
                else
                    throw new ArgumentOutOfRangeException(
                        "GrossSales", value, "Grosssales must be >=0");
            }
        }
 
        public decimal CommissionRate
        {
            get
            {
                return commissionRate;
            }
            set
            {
                if (value > 0 && value < 1)
                    commissionRate = value;
                else
                    throw new ArgumentOutOfRangeException(
                        "CommissionRate", value, "CommissionRate must be > 0 & <1");
            }
        }
 
        public virtual decimal Earnings()
        {
            return CommissionRate * GrossSales;
        }
 
        public override string ToString()
        {
            return string.Format(
                "{0}: {1} {2}\n{3}: {4}\n{5}: {6:C}\n{7}: {8:F2}",
                "commission employee", FirstName, LastName, 
                "social security number", SocialSecurityNumber,
                "gross sales", GrossSales, "commission rate", CommissionRate);
        }
    }
}

//BasePlusCommissionEmployeeTest.cs
 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
 
namespace UseVarExs
{
    public class BasePlusCommissionEmployeeTest
    {
        static void Main(string[] args)
        {
            BasePlusCommissionEmployee employee =
                new BasePlusCommissionEmployee("Bob", "Lewis", "333-33-3333",
                    5000.00M, .04M, 300.00M);
 
            Console.WriteLine(
            "Employee  information  obtained  Ьу  properties  and  methods:  \n");
            Console.WriteLine("First  name  is  {0}",
            employee.FirstName);
            Console.WriteLine("Last  name  is  {0}",
            employee.LastName);
            Console.WriteLine("Social  security  number  is  {0}",
            employee.SocialSecurityNumber);
            Console.WriteLine("Gross  sales  are  {0:С}",
            employee.GrossSales);
            Console.WriteLine("Commission  rate  is  {0:F2}",
            employee.CommissionRate);
            Console.WriteLine("Earnings  are  {0:С}",
            employee.Earnings());
            Console.WriteLine("Base  salary  is  {0:С}",
            employee.BaseSalary);
 
            employee.BaseSalary = 1000.00M;
 
            Console.WriteLine("\n{0}:\n\n{l}",
            "Updated  employee  information  obtained  Ьу  ToString",
            employee);
            Console.WriteLine("earnings:  {0:С}",
            employee.Earnings());
 
            Console.ReadKey();
        }
    }
}