如何保持私有财产的可扩展性？

Question

I'm working on the following class. I've shortened the list of properties as an example.

public class Paycheck 
{
   public decimal Gross { get; private set; }
   public decimal FederalWithholdingTax { get; private set; }
   public decimal StateWithholdingTax  { get; private set;  }
   // ...
}

Now my idea was to create an interface to calculate all the taxes...

public interface IPaycheckDeductionCalculator
{
   void Calculate();
}

The idea after that is to create methods outside of the Paycheck class that can perform calculations/deductions. With the way laws and payroll processes work, how deductions are performed and what deductions are performed are constantly changing, so I want to keep extensibility easy.

My dilemma is that I want to keep the Paycheck property setters private so that just anyone can change those values. Keeping that data constant after a paycheck is processed is critical.

I considered the use of delegates, but I'm not so sure that's my answer either, as I still wouldn't be able to develop the desired methods outside of the Paycheck class.

Is there something I'm not thinking of or is this a "you can't have your cake and eat it too" kind of situation?

Answer 1

Have the interfaces that calculate these values return the values that they are calculating. Pass in instances of these interfaces to Paycheck , which can the perform the calculations (without needing to know anything about the implementation of the interface) and then have it set the values of the appropriate properties.