change return type of virtual method

Question

Hey, I have a Database which contains every item in the game.

I have a base class Item

         public class Item: ScriptableObject 
         {
         private string itemName;
         public virtual Item ReturnItem() 
         {
             return this;
         }
         }

Several classes like Weapon derive from Item.

             public class Weapon: Item
             {
             private int damage;
             public override Item ReturnItem() 
             {
                 return this;
             }
             }

Is it possible to make the override method ReturnItem return the childclass(Weapon) instead of just the item?

Or is there any other way to determine what childclass the Item belongs to aside from casting it to a weapon and checking if it threw a error

Answer 1

Best Answer

Answer by ShadyProductions · Mar 08, 2018 at 10:06 PM

That's not possible, but you can know which type it is:

 List<Item> items = new List<Item>();
 items.Add(new Weapon());
 items.Add(new Item());
 
 foreach (var item in items)
 {
     Debug.Log(item.GetType().Name);
 }

If you made Item an abstract class you could do something like this instead:

 public abstract class Item
 {
     public virtual T GetItem<T>()
     {
         if (GetType() == typeof(T))
             return (T)Convert.ChangeType(this, typeof(T));
         return default(T);
     }
 }

and then you could use it like this without having to overwrite the method in other classes (still optional thats why I left it virtual):

 List<Item> items = new List<Item>
 {
     new Weapon(),
     new Potion()
 };
 
 for (int i=0; i < items.Count; i++)
 {
     var potion = items[i].GetItem<Potion>();
     if (potion == null) continue;
     Console.WriteLine("Item at index " + i + " is a potion!");
 }

This way you get the potion object as a potion type aswel.

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Bunny83 · Mar 09, 2018 at 12:30 AM 2

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There's no need for a virtual method or reflection based casting. You can simply do:

 public abstract class Item
 {
     public T GetItem<T>() where T : Item
     {
         return this as T;
     }
 }

Though this would make the method itself pretty pointless ^^. So your line:

 var potion = items[i].GetItem<Potion>();

could be replaced with

 var potion = items[i] as Potion;

and you get the same behaviour. Also note that your reflection based type check does not work with inheritance. For example if you have an inheritance chain like this Item <-- Weapon <-- AssaultRifle when using

 GetItem<Weapon>()

It would not return an AssaultRifle even it is a Weapon. If you do a reflection based type check in almost all cases you want to use IsAssignableFrom like this:

 if (typeof(T).IsAssignableFrom(GetType()))

JanWdm Bunny83 · Mar 09, 2018 at 01:46 AM 0

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Thanks that looks a little cleaner and easier to understand then the previous answer. Just another short question is there any benefit of using var instead of Potion? Or is it just a convenience thing since its faster to write.

Bunny83 · Mar 09, 2018 at 12:41 AM 1

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Oh and if you have a collection of "Item"s (an array or List) you can just use the Linq extension method OfType. It will filter the collection and only return the elements which could be casted into the specified type. So a simple foreach loop like this would work:

 foreach(Potion p in items.OfType<Potion>())
 {
     
 }

If the array items does not contain any Potion the loop won't run at all. Of course if you need an element index this doesn't work since we just work with IEnumerables here. To use the OfType extension method you have to add a using System.Linq; at the top.

JanWdm · Mar 09, 2018 at 01:13 AM 0

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Thanks that is exactly what i needed!

change return type of virtual method

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