public interface IPayable
{
   decimal GetPaymentAmount(); // calculate payment; no implementation
} // end interface IPayable
_________________________________________
public abstract class Employee
{
   // read-only property that gets employee's first name
   public string FirstName { get; private set; }

   // read-only property that gets employee's last name
   public string LastName { get; private set; }

   // read-only property that gets employee's social security number
   public string SocialSecurityNumber { get; private set; }

   // three-parameter constructor
   public Employee( string first, string last, string ssn )
   {
      FirstName = first;
      LastName = last;
      SocialSecurityNumber = ssn;
   } // end three-parameter Employee constructor

   // return string representation of Employee object, using properties
   public override string ToString()
   {
      return string.Format( "{0} {1}\nsocial security number: {2}",
         FirstName, LastName, SocialSecurityNumber );
   } // end method ToString
   
} // end abstract class Employee

__________________________________________________________

public class HourlyEmployee : Employee
{
   private decimal wage; // wage per hour
   private decimal hours; // hours worked for the week

   // five-parameter constructor
   public HourlyEmployee( string first, string last, string ssn,
      decimal hourlyWage, decimal hoursWorked )
      : base( first, last, ssn )
   {
      Wage = hourlyWage; // validate hourly wage via property
      Hours = hoursWorked; // validate hours worked via property
   } // end five-parameter HourlyEmployee constructor

   // property that gets and sets hourly employee's wage
   public decimal Wage
   {
      get
      {
         return wage;
      } // end get
      set
      {
         if ( value >= 0 ) // validation
            wage = value;
         else
            throw new ArgumentOutOfRangeException( "Wage",
               value, "Wage must be >= 0" );
      } // end set
   } // end property Wage

   // property that gets and sets hourly employee's hours
   public decimal Hours
   {
      get
      {
         return hours;
      } // end get
      set
      {
         if ( value >= 0 && value <= 168 ) // validation
            hours = value;
         else
            throw new ArgumentOutOfRangeException( "Hours",
               value, "Hours must be >= 0 and <= 168" );
      } // end set
   } // end property Hours

   // calculate earnings; override Employee’s abstract method Earnings
   public override decimal Earnings()
   {
      if ( Hours <= 40 ) // no overtime                          
         return Wage * Hours;
      else
         return ( 40 * Wage ) + ( ( Hours - 40 ) * Wage * 1.5M );
   } // end method Earnings                                      

   // return string representation of HourlyEmployee object
   public override string ToString()
   {
      return string.Format(
         "hourly employee: {0}\n{1}: {2:C}; {3}: {4:F2}",
         base.ToString(), "hourly wage", Wage, "hours worked", Hours );
   } // end method ToString                                            
} // end class HourlyEmployee
_____________________________________________

public class CommissionEmployee : Employee
{
   private decimal grossSales; // gross weekly sales
   private decimal commissionRate; // commission percentage

   // five-parameter constructor
   public CommissionEmployee( string first, string last, string ssn,
      decimal sales, decimal rate ) : base( first, last, ssn )
   {
      GrossSales = sales; // validate gross sales via property
      CommissionRate = rate; // validate commission rate via property
   } // end five-parameter CommissionEmployee constructor

   // property that gets and sets commission employee's gross sales
   public decimal GrossSales
   {
      get
      {
         return grossSales;
      } // end get
      set
      {
         if ( value >= 0 )
            grossSales = value;
         else 
            throw new ArgumentOutOfRangeException(
               "GrossSales", value, "GrossSales must be >= 0" );
      } // end set
   } // end property GrossSales

   // property that gets and sets commission employee's commission rate
   public decimal CommissionRate
   {
      get
      {
         return commissionRate;
      } // end get
      set
      {
         if ( value > 0 && value < 1 )
            commissionRate = value;
         else 
            throw new ArgumentOutOfRangeException( "CommissionRate", 
               value, "CommissionRate must be > 0 and < 1" );
      } // end set
   } // end property CommissionRate

   // calculate earnings; override abstract method Earnings in Employee
   public override decimal Earnings()
   {
      return CommissionRate * GrossSales;
   } // end method Earnings              

   // return string representation of CommissionEmployee object
   public override string ToString()
   {
      return string.Format( "{0}: {1}\n{2}: {3:C}\n{4}: {5:F2}",
         "commission employee", base.ToString(),
         "gross sales", GrossSales, "commission rate", CommissionRate );
   } // end method ToString 
___________________________________

public class BasePlusCommissionEmployee 
{
   private decimal baseSalary; // base salary per week

   // six-parameter constructor
   public BasePlusCommissionEmployee( string first, string last,
      string ssn, decimal sales, decimal rate, decimal salary )
      
   {
      BaseSalary = salary; // validate base salary via property
   } // end six-parameter BasePlusCommissionEmployee constructor

   // property that gets and sets 
   // base-salaried commission employee's base salary
   public decimal BaseSalary
   {
      get
      {
         return baseSalary;
      } // end get
      set
      {
         if ( value >= 0 )                                       
            baseSalary = value;                                  
         else                                                    
            throw new ArgumentOutOfRangeException( "BaseSalary", 
               value, "BaseSalary must be >= 0" );               
      } // end set
   } // end property BaseSalary

   // calculate earnings; override method Earnings in CommissionEmployee
   public override decimal Earnings()
   {
      return BaseSalary + base.Earnings();
   } // end method Earnings               

   // return string representation of BasePlusCommissionEmployee object
   public override string ToString()
   {
      return string.Format( "base-salaried {0}; base salary: {1:C}",
         base.ToString(), BaseSalary );
   } // end method ToString                                            
} // end class BasePlusCommissionEmployee
______________________________________________

public class SalariedEmployee : Employee
{
   private decimal weeklySalary;

   // four-parameter constructor
   public SalariedEmployee( string first, string last, string ssn,
      decimal salary ) : base( first, last, ssn )
   {
      WeeklySalary = salary; // validate salary via property
   } // end four-parameter SalariedEmployee constructor

   // property that gets and sets salaried employee's salary
   public decimal WeeklySalary
   {
      get
      {
         return weeklySalary;
      } // end get
      set
      {
         if ( value >= 0 ) // validation
            weeklySalary = value;
         else
            throw new ArgumentOutOfRangeException( "WeeklySalary",
               value, "WeeklySalary must be >= 0" );
      } // end set
   } // end property WeeklySalary

   // calculate earnings; override abstract method Earnings in Employee
   public override decimal Earnings()
   {
      return WeeklySalary;
   } // end method Earnings          

   // return string representation of SalariedEmployee object
   public override string ToString()
   {
      return string.Format( "salaried employee: {0}\n{1}: {2:C}",
         base.ToString(), "weekly salary", WeeklySalary );
   } // end method ToString                                      
} // end class SalariedEmployee

______________________________________________

public class PayrollSystemTest
{
   public static void Main( string[] args )
   {
      // create derived class objects
      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() );

      // create four-element Employee array
      Employee[] employees = new Employee[ 4 ];

      // initialize array with Employees of derived types
      employees[ 0 ] = salariedEmployee;
      employees[ 1 ] = hourlyEmployee;
      employees[ 2 ] = commissionEmployee;
      employees[ 3 ] = basePlusCommissionEmployee;

      Console.WriteLine( "Employees processed polymorphically:\n" );

      // generically process each element in array employees
      foreach ( Employee currentEmployee in employees )
      {
         Console.WriteLine( currentEmployee ); // invokes ToString

         // determine whether element is a BasePlusCommissionEmployee
         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 );
         } // end if

         Console.WriteLine(
            "earned {0:C}\n", currentEmployee.Earnings() );
      } // end foreach

      // get type name of each object in employees array
      for ( int j = 0; j < employees.Length; j++ )
         Console.WriteLine( "Employee {0} is a {1}", j,
            employees[ j ].GetType() );
   } // end Main
} // end class PayrollSystemTest