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Question by haruna9x · Mar 11, 2018 at 03:26 AM · c#convertgeneric

Generic type conversion issue

I'm writing a small pool, which is used by the spawners, allowing spawn enemies in my game.

   public class Pool<T> where T : Pooling
     {
         public Pool(T prefab, int capacity)
         {
                 T instance = Object.Instantiate(prefab);
                 instance.owner = (this) as Pool<Pooling>;  // this does not work
         }
     }

The Pooling class contains several functions that allow me to reset the enemy state when it is spawning. As well as letting me call gameobject.SetActive (false) to despawn without resorting to a manager.

However, I can not convert type from Pool< EntityPooling> to Pool< Pooling>.

     public abstract class Pooling : MonoBehaviour
     {
         public Pool<Pooling> owner;
     }
     
     public class EntityPooling : Pooling
     {
     }
     
     public class Spawner : MonoBehaviour
     {
         private Pool<EntityPooling> pool;
     }
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Answer by Bunny83 · Mar 11, 2018 at 09:01 AM

This should work:

 public class Pool<T> where T : Pooling<T>
 {
     public Pool(T prefab, int capacity)
     {
         T instance = UnityEngine.Object.Instantiate(prefab);
         instance.owner = (Pool<T>)this;
     }
 }
 
 public abstract class Pooling<T> : MonoBehaviour where T : Pooling<T>
 {
     public Pool<T> owner;
 }
 
 public class EntityPooling : Pooling<EntityPooling>
 {
 }
 
 public class Spawner : MonoBehaviour
 {
     private Pool<EntityPooling> pool;
 }


Your Pooling basc class requires a self type reference in order to be compatible with the generic type. Like hexagonius said just because two concrete classes are based on the same generic class doesn't make tnem compatible in any way. Imagine this list:

 List<MonoBehaviour> list = new List<Spawner>();

If this was possible one could do

 public EntityPooling someObj;
 
 list.Add(someObj);

However since or actual list is a list of "Spawners" we can't add a EntityPooling in that list. Covariance and Contravariance do only apply in very rare cases. Generally for generics only one of the two applies and it depends on the in / out restriction of the type parameter. The generic List has a type parameter that is both an "in" and "out" type so neither apply here.


Generics are a quite complicated beast. Since they are compiled at compile time but the type parameter is bound at runtime a lot restrictions apply.

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avatar image Bunny83 · Mar 11, 2018 at 09:15 AM 0
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Note that a generic self-reference restrictions brings some other problems. While it is possible to derive another class from the "EntityPooling" class and even put those derived instances in a Pool<EntityPooling>, you can not create a Pool with the derived class. So this wouldn't work:

 public class $$anonymous$$yDerived : EntityPooling { }
 
 public Pool<$$anonymous$$yDerived> myDerivedPool; // <-- doesn't work as $$anonymous$$yDerived does not satisfy Pooling<$$anonymous$$yDerived>

So a self reference basically blocks further inheritance when you want to use it in the pool. Though it doesn't generally prevent inheritance. $$anonymous$$yDerived has this chain of inheritance:

 $$anonymous$$yDerived --> EntityPooling --> Pooling<EntityPooling> --> $$anonymous$$onoBehaviour --> ...

So a "$$anonymous$$yDerived" is an EntityPooling and is a Pooling<EntityPooling> but not a Pooling<$$anonymous$$yDerived>

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Answer by hexagonius · Mar 11, 2018 at 06:45 AM

each genetic class that gets concrete is a class on it's own. even though the underlying type might be derived, doesn't make the genetic classes derive.
since you're only interested in Poolings (everything pooled derives from it), use one pool for all Poolings, meaning the Spawner class also has a Pooling type Pool. if you want one pool for each type even though derived the base needs the reference to it's pooland the spawned one reference each.

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