How to access derived class members from an interface?

Question

I have three classes; Stamp, Letter and Parcel that implement an interface IProduct and they also have some of their own functionality.

public interface IP         


        

Answer 1

Why can't I access the additional members of derived classes from a List ?

That is because, IProduct interface does not know about UnitPrice, Destination etc of the derived class properties.

Are you trying add the intelligence to calculate the Amount to each of the derived class objects Stamp, Letter, Parcel ?

Then, I would say you need to redesign a bit and use the Decorator design pattern.

DerivedClass::Amount()
{
  Base::Amount() + 
  //Amount logic based on derived class
}