GetComponent returns null when using a generics class for T

Question

So I'm making a game where I have a Player.cs script that instantiates the player ship, the player ship prefab has a StandardShip component attached to it. StandardShip prefab view from the editor The StandardShip component inherits from PlayerShip which inherits from ship and so on until eventually it inherits from MonoBehaviour.

StandardShip.cs

 public class StandardShip : PlayerShip<LaserBullet> {...}

PlayerShip.cs

 public abstract class PlayerShip<T> : Ship where T : Bullet {...}

LaserBullet obviously inherits from Bullet, which in turn inherits from Monobehaviour too. Bullet.cs

 public abstract class Bullet : MonoBehaviour, IPoolableEntity {...}

The error occurs inside the Player.cs file, GetComponent returns null even thought all the components inherit from MonoBehaviour so I don't know what I'm doing wrong.

Player.cs

 string pathToPrefab = $"Prefabs/{PlayerPrefs.GetString(PlayerPrefsConstants.SELECTED_SHIP, null) ?? "StandardShip"}";
         GameObject objectToInstantiate = Resources.Load<GameObject>(pathToPrefab);
         PlayerShip<Bullet> ship = objectToInstantiate.GetComponent<PlayerShip<Bullet>>();
         //var s = objectToInstantiate.GetComponent<Rigidbody2D>();

The Resources.Load returns the correct prefab and I tried getting the RigidBody2D, it worked. So I don't know what the problem is. I would really appreciate if someone could tell me what I'm doing wrong 'cause I can't figure this out.

Answer 1

Best Answer

Answer by sacredgeometry · Dec 06, 2020 at 06:41 PM

Probably for the same reasons:

 // These work      
 var a = GetComponent<StandardShip>();
 var b = GetComponent<PlayerShip<LaserBullet>>() as StandardShip;
 
 // These do not        
 var c = GetComponent<PlayerShip<Bullet>>() as StandardShip;
 var d = (StandardShip)GetComponent<PlayerShip<Bullet>>();

i.e. When using GetComponent StandardShip is not a PlayerShip<Bullet> it's a PlayerShip<LaserBullet>because it doesn't support variance in that direction.

Reading

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/covariance-contravariance/ https://docs.microsoft.com/en-us/dotnet/standard/generics/covariance-and-contravariance

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elrohirgt · Dec 06, 2020 at 07:18 PM 0

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Thank you for the answer, I didn't know this covariance and contravariance. I didn't know Generic with a derived class can't be casted to the same generic but with a base class so I'll have to search for another way of doing this.

GetComponent returns null when using a generics class for T

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