关于静态的方法用来winform传值,提示为引用实例对象我很奇怪为什么


/// <summary>
        /// 提供一个字典序来保存挂单时的数据
        /// </summary>
        public static Dictionary<string, DataTable> hungListTable;
        public static Dictionary<string, DataTable> hungListReturnTable;
        public static Dictionary<string, string> hungString;
        /// <summary>
        /// 静态方法来实现对对象的添加
        /// </summary>
        /// <param name="key">键</param>
        /// <param name="value">值</param>
        public static void AddhungListTable(string key, DataTable value)
        {
            if (hungListTable.Count == 0)
            {
                hungListTable = new Dictionary<string, DataTable>();
            }
            hungListTable.Add(key, value);
        }
        /// <summary>
        /// 静态方法来实现对对象的添加
        /// </summary>
        /// <param name="key">键</param>
        /// <param name="value">值</param>
        public static void AddhungListReturnTable(string key, DataTable value)
        {
            if (hungListReturnTable.Count == 0)
            {
                hungListReturnTable = new Dictionary<string, DataTable>();
            }
            hungListReturnTable.Add(key,value);
        }
        /// <summary>
        /// 静态方法实现字典序中的对象删除
        /// </summary>
        /// <param name="key">键</param>
        public static void RemovehungListTable(string key)
        {
           hungListTable.Remove(key);
        }
        /// <summary>
        /// 静态方法实现字典序中的对象删除
        /// </summary>
        /// <param name="key">键</param>
        public static void RemovehungListReturnTable(string key)
        {
            hungListReturnTable.Remove(key);
        }
        public static void AddString(string key, string value)
        {
            if (hungString.Count == 0)
            {
                hungString = new Dictionary<string, string>();
            }
            hungString.Add(key, value);
        }
        public static void RemoveString(string key)
        {
            hungString.Remove(key);
        }
单步调试的时候到这里就出错。。请问我需要实例化什么。。
//string datetime = System.DataTime.Now.ToString(); 
//HungList.hungString.Add("DateTime", datetime);