Class Hierarchy - Data design in an RPG game where classes overlap (VB.Net)

Question

This is a followup to the question I asked here:

http://stackoverflow.com/questions/3445398/class-hierarchy-data-design-in-an-rpg-game-vb-net

I understand the answer in the post above, which is absolutely amazing, by the way. It's about implementing interfaces with a class. However, what if a class needs to share features with another class?

Yes, that class can an Interface. However, let's use this sample definition.

An ITEM can be USED or EQUIPPED
An EQUIPPED ITEM can be either ARMOR or a WEAPON
A USED ITEM either heals the team, casts MAGIC, or damage the opposing team.
Certain EQUIPPED ITEMS can function as a USED ITEM.
Certain EQUIPPED ITEMS can cast magic.

Or, in other words:
An equippable item can perform acts outside of its typical usage of a shield or weapon. But not all items can act as a sheid or weapon.

I mean, I could create a class that implements IWeapon, IShield, IMagic, IUseableItem, etc. But there should be a better way than returning NULL when those interfaces are called.

Answer 1

But there should be a better way than returning NULL when those interfaces are called.

It's called "not implementing them on objects that don't support them". Or so I would have thought.

One option is to have IEquippable, and IUsable, and any item that can be both equipped and used implements both, while other items only implement the applicable interface.

Answer 2

To be honest, I would choose to use a single Item class here. You're going to have lots of permutations of functionality and are going to end up having to query manually for interface existence or the object type anyway. So trying to fix the permutations at design-time seems like the wrong thing to do.