{     
           "aircraftNo": null,
            "billNo": null,
            "checkArea": null,
            "checkCustomsCode": "3104"
}       

定义：
JsonSerializerSettings jsonSetting = new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore };
 
或者
 
setting.DateFormatHandling = Newtonsoft.Json.DateFormatHandling.MicrosoftDateFormat;
 //空值处理  
     setting.NullValueHandling = NullValueHandling.Ignore;  
 
使用：
 string messageOut = JsonConvert.SerializeObject(实体类, Formatting.Indented,jsonSetting);