直接看代码：

        /// <summary>添加一个属性
        /// 
        /// </summary>
        /// <param name="obj">待添加属性的对象</param>
        /// <param name="key">键名</param>
        /// <param name="value">值</param>
        /// <returns>添加属性后的对象</returns>
        public static object Add(object obj, string key, object value)
        {
            JObject jObj = JObject.Parse(JsonConvert.SerializeObject(obj));
            jObj.Add(new JProperty(key, value));
            return JsonConvert.DeserializeObject(jObj.ToString());
        }