public class Consumable : Item {
    public delegate void ConsumableDelegate(Entity user);

    private readonly ConsumableDelegate onUse;

    public Consumable(string name, ConsumableDelegate onUse) {
        this.onUse = onUse;
    }

    public void Use(Entity user) {
        onUse(user);
    }
}

public class ItemDatabase {
     private List<Item> items;

     public ItemDatabase() {
         items = new List<Item>();

         items.Add(new Consumable("Apple", Heal_10));
     }

     // List of all ConsumableDelegates //
     private void Heal_10(Entity user){
         user.Heal(10);
     }
     private void Heal_50(Entity user){
     user.Heal(50);
     }
     // etc.
}