C＃泛型/ Unity3d-如何提供GameObject约束？

Question

I'm just learning to use C# Generics and Unity has thrown me a curve ball. I have this method into which I intend to be able to pass either a list or an array and return an item of a variable object type (could be GameObject, ParticleSystem etc depending on the array contents):

private T RetrieveItem<T>(IList<T> list) where T : Component
{

    for (int i = 0; i < list.Count; i++)
    {

        if (!list[i].GetComponent<GameObject>().activeInHierarchy)
        {
            return list[i];
        }
    }

    return list[0];
}

This compiles fine, but when I try to use it:

GameObject obj = RetrieveItem<GameObject>(myListOrArray);

I get the error "The type 'UnityEngine.GameObject' cannot be used as type parameter 'T' in the generic type or method 'J_ObjectPool.RetrieveItem(IList)'. There is no implicit reference conversion from 'UnityEngine.GameObject' to 'UnityEngine.Component'."

I've tried using Monobehaviour as the constraint and get the same error (can't cast GameObject to Monobehaviour) and I've tried using UnityEngine.Object but get the same error and additionally I can't use the GetComponent method. Finally I've tried using GameObject as the constraint but the compiler complains that "GameObject is not a valid constraint".

Any ideas how I could get this to work? Thanks.

Answer 1

While list[i].GetComponent<GameObject>() may compile, it will not make sense during runtime.

GetComponent<T>() returns a Component of type T. GameObject is not a Component, so GetComponent<GameObject>() will always return null.

GameObjects and Components are two completely seperate things. A GameObject is basically a container that contains any number of Components. You can get the GameObject that holds the Component like this:

Component component = ...;
GameObject containingObject = component.gameObject;

So your code should probably read

private T RetrieveItem<T>(IList<T> list) where T : Component
{
    for (int i = 0; i < list.Count; i++)
    {
        if (!list[i].gameObject.activeInHierarchy)
        {
            return list[i];
        }
    }
    return list[0];
}

You might ask, why GetComponent<T>() does not contrain T to be a Component. That is to allow GetComponent to be used with Interface types, too, which of course can not be derived from Component, while Components can still implement them.