ManualPool.cs

using System;
using System.Collections.Generic;

namespace Unity.VisualScripting
{
    public static class ManualPool<T> where T : class
    {
        private static readonly object @lock = new object();
        private static readonly Stack<T> free = new Stack<T>();
        private static readonly HashSet<T> busy = new HashSet<T>();

        public static T New(Func<T> constructor)
        {
            lock (@lock)
            {
                if (free.Count == 0)
                {
                    free.Push(constructor());
                }

                var item = free.Pop();

                busy.Add(item);

                return item;
            }
        }

        public static void Free(T item)
        {
            lock (@lock)
            {
                if (!busy.Contains(item))
                {
                    throw new ArgumentException("The item to free is not in use by the pool.", nameof(item));
                }

                busy.Remove(item);

                free.Push(item);
            }
        }
    }
}