OnStructureUpgrade

using System.Linq;
using System;
using System.Collections.Generic;
using Newtonsoft.Json;
using Oxide.Core;

namespace Oxide.Plugins
{
    [Info("UpgradeGradeLimiter", "RustPluginGenerator", "1.0.0")]
    [Description("Restricts which building grades players may upgrade to based on permissions")]
    public class UpgradeGradeLimiter : RustPlugin
    {
        private const string PermBypass = "upgradegradelimiter.bypass";

        private PluginConfig config;

        private class PluginConfig
        {
            [JsonProperty("Allowed grades without permissions")]
            public List<string> AllowedGrades { get; set; } = new List<string> { "Twigs", "Wood" };
        }

        protected override void LoadDefaultConfig() => config = new PluginConfig();

        protected override void LoadConfig()
        {
            base.LoadConfig();
            try
            {
                config = Config.ReadObject<PluginConfig>();
                if (config == null) throw new Exception();
            }
            catch
            {
                PrintWarning("Configuration file is invalid, using defaults");
                LoadDefaultConfig();
            }
            SaveConfig();
        }

        protected override void SaveConfig() => Config.WriteObject(config);

        protected override void LoadDefaultMessages()
        {
            lang.RegisterMessages(new Dictionary<string, string>
            {
                ["NotAllowed"] = "You are not allowed to upgrade to {0}.",
                ["AllowedList"] = "Allowed grades: {0}"
            }, this);
        }

        private string Msg(string key, string userId = null, params object[] args) => string.Format(lang.GetMessage(key, this, userId), args);

        private void Init()
        {
            permission.RegisterPermission(PermBypass, this);
            foreach (var grade in Enum.GetNames(typeof(BuildingGrade.Enum)))
            {
                if (grade == "None") continue;
                if (config.AllowedGrades.Contains(grade)) continue;
                permission.RegisterPermission(PermPrefix + grade.ToLower(), this);
            }
        }

        private const string PermPrefix = "upgradegradelimiter.";

        private object OnStructureUpgrade(BuildingBlock buildingBlock, BasePlayer player, BuildingGrade.Enum type, ulong skin)
        {
            if (buildingBlock == null || player == null) return null;
            if (type == BuildingGrade.Enum.None) return null;
            if (player.IsAdmin || permission.UserHasPermission(player.UserIDString, PermBypass)) return null;

            string gradeName = type.ToString();
            if (config.AllowedGrades.Contains(gradeName)) return null;

            string perm = PermPrefix + gradeName.ToLower();
            if (permission.UserHasPermission(player.UserIDString, perm)) return null;

            player.ChatMessage(Msg("NotAllowed", player.UserIDString, gradeName));
            return true;
        }
    }
}