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

The reason why you can't access additional members from a derived class is that you are using the interface in the List<> - therefore you'll only be able to access properties on that interface.

A pattern that might help you is the double-dispatch pattern.

Example below:

public interface IHandler
{
    void Handle(Stamp stamp);
    void Handle(Letter letter);
    ...
}

public class Handler : IHandler
{
    public void Handle(Stamp stamp)
    {
       // do some specific thing here...
    }
    public void Handle(Letter letter)
    {
       // do some specific thing here...
    }
    ...
}

public interface IProduct
{
    string Name { get; }
    int Quantity { get; set; }
    float Amount { get; }
    void Handle(IHandler handler);
}

public class Stamp : IProduct
{
    public string Name { get { return "Stamp"; } }
    public int Quantity { get; set; }
    public float Amount { get; set; }
    public float UnitPrice { get; set; }
    public void Handle(IHandler handler)
    {
         handler.Handle(this);
    }
}

You can now program some specific functionality in the Handler - I'm guessing you want to calculate some kind of total price given things such as quantity * unit price or a weight & destination lookup table...