昨天提到Json.NET屬性序列化設定，接獲讀者森哥留言:
請問黑大，
針對不需要序列化的「屬性」是否可以透過程式「動態」設定或是過濾?
有預感遲早也會遇到這個靠杯火盃的考驗，決定打鐵趁熱，馬上來練習。所幸，Json.NET真的很強大，早就料想到此一需求，提供ContractResolver以實現神乎奇技的高度動態化。
我寫了一個範例，展示兩種動態決定應序列化屬性的情境:
- Serialize時傳入屬性名稱陣列作為參數，正向表列JSON應包含的屬性。
- 由物件屬性值決定屬性是否要序列化，例如: 如果是女生就不包含年齡。(這幾乎已彈性到極點，雖然實務上不常用到)
程式的做法是宣告兩個繼承自DefaultContractResolver的類別: LimitPropsContractResolver在建構時傳入string[]參數列出要序列化的屬性名稱，並覆寫CreateProperties方法，過濾base.CreateProperties()傳回的IList<JsonProperty>，只保留前述string[]有列出的屬性；HideAgeContractResolver則覆寫CreateProperty()方法，由base.CreateProperty()取得JsonProperty，JsonProperty有個ShouldSerialize屬性可以傳入Lambda運算式，逐筆處理每個要序列化的物件，在Lambda運算式中可將物件轉型為原型別進行判斷，若不要序列化就傳回false。
using System;using System.Collections.Generic;using System.Linq;using System.Reflection;using System.Text;using Newtonsoft.Json;using Newtonsoft.Json.Converters;using Newtonsoft.Json.Serialization;namespace ConsoleApplication1{class Program {publicenum Gender
{Male, Female
}
publicclass Person
{publicstring Name { get; set; }
[JsonConverter(typeof(StringEnumConverter))]public Gender Gender { get; set; }publicint Age { get; set; }
public Person(string name, Gender gender, int age)
{Name = name; Gender = gender; Age = age;
}
}
publicclass HideAgeContractResolver : DefaultContractResolver
{//REF: http://james.newtonking.com/projects/json/help/index.html?topic=html/ContractResolver.htmprotectedoverride JsonProperty CreateProperty(MemberInfo member,
MemberSerialization memberSerialization)
{ JsonProperty p = base.CreateProperty(member, memberSerialization);if (p.PropertyName == "Age")
{//依性別決定是否要序列化p.ShouldSerialize = instance =>
{Person person = (Person)instance;
return person.Gender == Gender.Male;};
}
return p;}
}
publicclass LimitPropsContractResolver : DefaultContractResolver
{string[] props = null;
public LimitPropsContractResolver(string[] props)
{//指定要序列化屬性的清單this.props = props;}
//REF: http://james.newtonking.com/archive/2009/10/23/efficient-json-with-json-net-reducing-serialized-json-size.aspxprotectedoverride IList<JsonProperty> CreateProperties(Type type,
MemberSerialization memberSerialization)
{IList<JsonProperty> list =
base.CreateProperties(type, memberSerialization);//只保留清單有列出的屬性return list.Where(p => props.Contains(p.PropertyName)).ToList();}
}
staticvoid Main(string[] args)
{ List<Person> list = new List<Person>();list.Add(new Person("George", Gender.Male, 18));
list.Add(new Person("Mary", Gender.Female, 40));
//正常輸出Console.WriteLine(JsonConvert.SerializeObject(
list, Formatting.Indented));
var settings = new JsonSerializerSettings();//加上ContractResolver，正向表列哪些屬性要序列化settings.ContractResolver =
new LimitPropsContractResolver("Name,Age".Split(','));
Console.WriteLine(JsonConvert.SerializeObject(
list, Formatting.Indented, settings));
//加上ContractResolver，依物件的屬性值動態決定要不要序列化 settings.ContractResolver = new HideAgeContractResolver();Console.WriteLine(JsonConvert.SerializeObject(
list, Formatting.Indented, settings));
Console.ReadLine();
}
}
}
程式執行結果如下，共有三段輸出，第一段為正常版；第二段套用LimitPropsContractResolver("Name,Age".Split(','))，故JSON中只見Name及Age，Gender被隱藏；第三段套用了HideAgeContractResolver()，如結果所示，Mary的JSON內容不包含年齡，George則包含。
[
{
"Name": "George",
"Gender": "Male",
"Age": 18
},
{
"Name": "Mary",
"Gender": "Female",
"Age": 40
}
]
[
{
"Name": "George",
"Age": 18
},
{
"Name": "Mary",
"Age": 40
}
]
[
{
"Name": "George",
"Gender": "Male",
"Age": 18
},
{
"Name": "Mary",
"Gender": "Female"
}
]
演練完畢，內心激動澎湃，對Json.NET的景仰如淊淊江水，綿綿不絕~
如果奧斯卡有最佳元件獎，我提名它!