public static readonly object obj = new object();

        /// <summary>
        /// 由于用到了全局变量
        /// 
        /// 
        /// 必须要避免线程竞争
        /// </summary>
        private string itemSellNo;

        public string ItemSellNo
        {
            get
            {
                lock (obj)
                {
                    return itemSellNo;
                }
            }
            set
            {
                lock (obj)
                {
                    itemSellNo = value;
                }
            }
        }