使用Newtonsoft.Json.dll 序列化为json时主动将NULL值过滤掉。详细做法：

var jSetting = new JsonSerializerSettings {NullValueHandling = NullValueHandling.Ignore};

var json = JsonConvert.SerializeObject（response， Formatting.Indented， jSetting）;

之前转换的JSON为：

{"header":{"responseCode":"200"，"responseDesc"":"运行成功"，"responseDescErrorMsg":""。"responseDescTime":"2015-07-07T13:40:48.7162196+08:00"}。"body":{"order":{"orderid":"S1507070001"}，"product":null}}

參加代码后转换出来的为：

{"header":{"responseCode":"200"，"responseDesc":"运行成功"，"responseDescErrorMsg":""。"responseDescTime":"2015-07-07T13:40:48.7162196+08:00"}，"body":{"order":{"orderid":"S1507070001"}}}