MarketPriceLookup/MarketPriceLookup.Common/Helpers/HandbookHelper.cs

using System;
using System.IO;
using System.Linq;
using System.Text.Json;

namespace MarketPriceLookup.Common.Helpers
{
    public static class HandbookHelper
    {
        private static Handbook _handbook;

        public static Handbook Items
        {
            get
            {
                if (_handbook == null)
                {
                    var itemsFilePath = $"{Directory.GetCurrentDirectory()}\\Assets\\handbook.json";
                    if (!File.Exists(itemsFilePath))
                    {
                        throw new Exception($"Missing handbook.json under assets ({itemsFilePath})");
                    }

                    var handbookJson = File.ReadAllText(itemsFilePath);
                    _handbook = JsonSerializer.Deserialize<Handbook>(handbookJson);
                }

                return _handbook;
            }
        }

        public static HandbookItem GetItem(string id)
        {
            return Items.Items.FirstOrDefault(x => x.Id == id);
        }

        public static HandbookCategory GetCategory(string id)
        {
            return Items.Categories.FirstOrDefault(x => x.Id == id);
        }
    }

    public class Handbook
    {
        public HandbookCategory[] Categories { get; set; }
        public HandbookItem[] Items { get; set; }
    }
}