Generic ObjectPool class can use in Unity.

ObjectPool.cs

using System.Collections.Generic;

public class ObjectPool<T> where T : UnityEngine.Object
{
    private Queue<T> queue = null;
    private T original = null;
    private int capacity = 0;

    public ObjectPool(T original, int capacity = 10)
    {
        this.capacity = capacity;
        queue = new Queue<T>(capacity);
        this.original = original;
        Enqueue(original, capacity);
    }

    public T Dequeue()
    {
        if (queue.Count <= 0)
        {
            Enqueue(original, capacity);
        }

        T obj = queue.Dequeue();
        TrySetActive(obj, true);
        return obj;
    }

    public bool Return(T obj)
    {
        if (obj == null)
        {
            return false;
        }

        Enqueue(obj);
        return true;
    }

    private void Enqueue(T original, int capacity)
    {
        for (int i = 0; i < capacity; i++)
        {
            T obj = UnityEngine.Object.Instantiate<T>(original);
            Enqueue(obj);
        }
    }

    private void Enqueue(T obj)
    {
        queue.Enqueue(obj);
        TrySetActive(obj, false);
    }

    private static void TrySetActive(T obj, bool value)
    {
        if (obj is UnityEngine.GameObject)
        {
            var gameObject = obj as UnityEngine.GameObject;
            gameObject.SetActive(value);
        }
        else if (obj is UnityEngine.Component)
        {
            var component = obj as UnityEngine.Component;
            component.gameObject.SetActive(value);
        }
    }
}