JsonUtility.ToJson() に IEnumerable な要素を渡してみた

SerializeIEnumerable.cs

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

namespace Monry
{
    public class SerializeIEnumerable : MonoBehaviour
    {
        private void Start()
        {
            Debug.Log(JsonUtility.ToJson(new Items()));    // {"itemList":[{"index":1},{"index":2},{"index":3},{"index":4},{"index":5}]}
            Debug.Log(JsonUtility.ToJson(new ItemList())); // {}
        }

        [Serializable]
        private class Item
        {
            [SerializeField] private int index;

            public Item(int index)
            {
                this.index = index;
            }
        }

        [Serializable]
        private class ItemList : List<Item>
        {
            public ItemList()
            {
                AddRange(
                    new List<Item>
                    {
                        new Item(1),
                        new Item(2),
                        new Item(3),
                        new Item(4),
                        new Item(5),
                    }
                );
            }
        }

        [Serializable]
        private class Items : IEnumerable<Item>
        {
            [SerializeField] private List<Item> itemList = new List<Item>
            {
                new Item(1),
                new Item(2),
                new Item(3),
                new Item(4),
                new Item(5),
            };

            public IEnumerator<Item> GetEnumerator()
            {
                return itemList.GetEnumerator();
            }

            IEnumerator IEnumerable.GetEnumerator()
            {
                return GetEnumerator();
            }
        }
    }
}