Resolved Trouble with polymorphism

Hello everyone! I'm trying to make a beekeeping simulator but I'm having some trouble with polymorphism. I want different species of bees (honey, carpenter, miner) to harvest different types of items (nectar, wood, rock), with all those items being able to be stored in one inventory.

So far I have class Item

Code (CSharp):

1. public class Item
2. {
3.     public string name;
4.     public Sprite sprite;
5.  
6.     public virtual void SetName(){}
7.  
8.     public void SetSprite(){}
9.  
10.     public virtual void DepositItem(BeeController bee, HiveController hiveController)
11.     {
12.         hiveController.otherInventory.Add(this);
13.         bee.inventory.Remove(this);
14.     }
15.  
16. }
17.  

and Nectar, inheriting from Item

Code (CSharp):

1. public class Nectar : Item
2. {
3.     public Colors color;
4.     public int stamina;
5.  
6.     public override void SetName()
7.     {
8.         name = color.ToString() + " Nectar";
9.         Debug.Log(name);
10.     }
11.  
12.     public override void DepositItem(BeeController bee, HiveController hiveController)
13.     {
14.         hiveController.nectarInventory.Add(this);
15.         bee.inventory.Remove(this);
16.     }
17.  
18.     public void DrinkNectar(BeeController bee)
19.     {
20.         bee.currentStamina += stamina;
21.         if(bee.currentStamina > 100){bee.currentStamina = 100;}
22.     }
23.  
24. }
25.  

Then I have an abstract class

Code (CSharp):

1. public abstract class ItemController
2. {
3.  
4.     public abstract Item GetItem();
5. }
6.  

which is to be inherited by the different object types the bees can interact with, IE this FlowerController

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5.  
6. public class FlowerController : ItemController
7. {
8.  
9.     public Nectar item;
10.     public Colors color;
11.     public int stamina;
12.     public FlowerSO flowerSO;
13.     // Start is called before the first frame update
14.     void Start()
15.     {
16.         item = new Nectar("Nectar");
17.         item.color = color;
18.         item.stamina = stamina;
19.     }
20.  
21.     public override Nectar GetItem()
22.     {
23.         return item;
24.     }
25. }
26.  

But this gives me the following error

Assets\Scripts\FlowerController.cs(21,28): error CS8830: 'FlowerController.GetItem()': Target runtime doesn't support covariant return types in overrides. Return type must be 'Item' to match overridden member 'ItemController.GetItem()'


I thought that the whole point of my inheritance here is that items of class Nectar are ALSO of class Item. There must be some other way to declare things to get the result I want but I can't figure it out. Thanks for reading, I know it's a bit of a wall of text but I wanted to make sure to include all the details.

 

The problem is that Item is not always Nectar.

I can't write code that generically uses an ItemController unless I know what GetItem returns. It needs to work the same regardless of what the inherited type is, or there is no point in having a base class in the first place.

 

You can't override a method and return a different type of value. Your

GetItem()

method needs to return an instance of type

Item

.

The point of polymorphism is to allow derived types to be treated as their parent type. But your base type can be any number of derived types, potentially.

 

Try this:

Code (CSharp):

1.  
2. public abstract class ItemController<T> where T: Item
3. {
4.     public abstract T GetItem();
5. }
6.  

Code (CSharp):

1.  
2. using System.Collections;
3. using System.Collections.Generic;
4. using UnityEngine;
5.  
6. public class FlowerController : ItemController<Nectar>
7. {
8.     public Nectar item;
9.     public Colors color;
10.     public int stamina;
11.     public FlowerSO flowerSO;
12.     // Start is called before the first frame update
13.     void Start()
14.     {
15.         item = new Nectar("Nectar");
16.         item.color = color;
17.         item.stamina = stamina;
18.     }
19.     public override Nectar GetItem()
20.     {
21.         return item;
22.     }
23. }
24.