do you put your calculations on your sets or your gets .

public class Order
{
   private double _price;
   private double _quantity;

  public double TotalCash
  {
   get
   {
    return _price * _quantity;
   }
}

public class Order
{

   private double _totalCash;
   private double _price;
   private double _quantity;

  private void CalcCashTotal()
 {
   _totalCash = _price * _quantity
 }

  public double Price
  {
   set
   {
     _price = value;
     CalcCashTotal();
   }
  }

  public double Quantity
  {
   set
   {
     _price = value;
     CalcCashTotal();
   }
  }


  public double TotalCash
  {
   get
   {
    return _totalCash;
   }
}

public class Order
  {
     private double _totalCash;
     private double _price;
     private double _quantity;
     private _IsDirtyTotalCash = true;

  private void CalcCashTotal()
  {
    _totalCash = _price * _quantity
  }

  public double Price
  {
   set
   {
     _price = value;
     _IsDirtyTotalCash = true;
   }
  }

  public double Quantity
  {
   set
   {
     _price = value;
     _IsDirtyTotalCash = true;
   }
  }

  public double TotalCash
  {
   get
   {
        if(_IsDirtyTotalCash)
    {
      _totalCash = CalcTotalCost();
       _isDirtyTotalCash = false;
     }
     return _totalCash;
   }
  }

}

double subTotal = myOrder.TotalCash;
double tax = subTotal * 0.05;
double shipping = subTotal > 100 ? 0 : 5.95;
double grandTotal = subTotal + tax + shipping;
OutputToUser(subTotal, tax, shipping, grandTotal);

double tax = myOrder.TotalCash * 0.05;
double shipping = myOrder.TotalCash > 100 ? 0 : 5.95;
double grandTotal = myOrder.TotalCash + tax + shipping;
OutputToUser(myOrder.TotalCash, tax, shipping, grandTotal);

class Wall {
    public decimal Width {
       get {
           ...
       }
       set {
           ValidateChanges();
           width = value;
           CreateBays();
       }
    }

    public Bays[] Bays {
       get {
           ...
       }
       set {
           ValidateChanges();
           ...
       }
    }

    private void CreateBays() {
        // Delete all the old bays.
        ...
        // Create a new bay per spacing interval given the wall width.
        ...
    }
}