使用[DataContract]标记的IEnumerable<T>无法进行反序列化

Question

我对Json.net比较陌生，尝试了下面这个简单的例子进行序列化和反序列化一个对象，结果出现了以下错误：

using Microsoft.VisualStudio.TestTools.UnitTesting;
using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.Text;
using System.Collections;

namespace Timehunter.Base.ServicesTests
{
    /// <summary>
    /// Summary description for JsonError
    /// </summary>
    [TestClass]
    public class JsonError
    {
  [TestMethod]
        public void TestMethod1()
        {
            JsonSerializerSettings serializerSettings = new JsonSerializerSettings()
            {
                DateFormatHandling = DateFormatHandling.IsoDateFormat,
                DateParseHandling = Newtonsoft.Json.DateParseHandling.DateTimeOffset
            };

            Act.Activities acts = new Act.Activities();
            acts.Add(new Act.Activity() { Id = 1, Name = "test1" });
            acts.Add(new Act.Activity() { Id = 2, Name = "test2" });
            string json = Newtonsoft.Json.JsonConvert.SerializeObject(acts, serializerSettings);

            Timehunter.Base.Act.Activities target = Newtonsoft.Json.JsonConvert.DeserializeObject<Timehunter.Base.Act.Activities>(json, serializerSettings);
            Assert.AreEqual("test1", target.List[0].Name, "Name of first activity");
        }
    }
}
namespace Timehunter.Base
{
    [DataContract]
    public class Activity
    {
        private int _id;
        private string _name;

        [DataMember]
        public int Id
        {
            get { return this._id; }
            set { this._id = value; }
        }
        [DataMember]
        public string Name
        {
            get { return this._name; }
            set { this._name = value; }
        }

        public Activity()
        {
            this._id = new int();
            this._name = string.Empty;
        }
    }
    [DataContract]
    public class Activities : IEnumerable<Activity>
    {
        private List<Activity> _list;
        [DataMember]
        public List<Activity> List
        {
            get { return this._list; }
            set { this._list = value; }
        }
        public Activities()
        {
            this._list = new List<Activity>();
        }

        public void Add(Activity item)
        { this._list.Add(item); }

        public bool Remove(Activity item)
        { return this._list.Remove(item); }

        public int Count()
        { return this._list.Count; }

        public IEnumerator<Activity> GetEnumerator()
        {
            return this._list.GetEnumerator();
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }
}

然后我遇到了以下错误：

Test Name:  TestMethod1
Test FullName:  Timehunter.Base.ServicesTests.JsonError.TestMethod1
Test Source:    C:\Users\hawi.HAWCONS\Documents\Visual Studio 2015\Projects\Timehunter.Data\Timehunter.Base.ServicesTests\JsonError.cs : line 67
Test Outcome:   Failed
Test Duration:  0:00:00,2038359

Result StackTrace:  
at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateNewList(JsonReader reader, JsonArrayContract contract, Boolean& createdFromNonDefaultCreator)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateList(JsonReader reader, Type objectType, JsonContract contract, JsonProperty member, Object existingValue, String id)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateValueInternal(JsonReader reader, Type objectType, JsonContract contract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerMember, Object existingValue)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
   at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)
   at Newtonsoft.Json.JsonConvert.DeserializeObject(String value, Type type, JsonSerializerSettings settings)
   at Newtonsoft.Json.JsonConvert.DeserializeObject[T](String value, JsonSerializerSettings settings)
   at Timehunter.Base.ServicesTests.JsonError.TestMethod1() in C:\Users\hawi.HAWCONS\Documents\Visual Studio 2015\Projects\Timehunter.Data\Timehunter.Base.ServicesTests\JsonError.cs:line 79
Result Message: 
Test method Timehunter.Base.ServicesTests.JsonError.TestMethod1 threw exception: 
Newtonsoft.Json.JsonSerializationException: Cannot create and populate list type Timehunter.Base.Act.Activities. Path '', line 1, position 1.

我做错了什么？

- hawi

Answer 1

更新 2

由于向后兼容性的原因，此更改将在 11.0.2 中被还原。请参考原回答以获取解决方案。

更新

已报告为 问题 ＃1598：数据合同属性不会导致 IEnumerable 的 JSon 对象序列化 并在提交 {{link2：e9e2d00}} 中修复。它应该在 10.0.3 之后的下一个版本中发布，该版本可能是 Json.NET 版本 11。

原回答

我注意到您已经使用 [DataContract] 和 [DataMember] 标记了您的 Activities 类：

[DataContract]
public class Activities : IEnumerable<Activity>
{
    private List<Activity> _list;
    [DataMember]
    public List<Activity> List
    {
        get { return this._list; }
        set { this._list = value; }
    }
    // ...
}

应用[DataContact]将导致DataContractJsonSerializer将IEnumerable<T>序列化为具有属性的JSON对象，而不是JSON数组。由于Json.NET 支持数据合同属性当应用于非可枚举对象时，您可能会认为它也会在可枚举对象和集合上尊重它们。

然而，似乎这并没有实现。如果我使用DataContractJsonSerializer序列化您的类，我会看到

{"List":[{"Id":1,"Name":"test1"},{"Id":2,"Name":"test2"}]}

但如果我使用Json.NET进行序列化，我发现[DataContract]被忽略了：

[{"Id":1,"Name":"test1"},{"Id":2,"Name":"test2"}]

然后在反序列化期间，它会抛出异常，因为它不知道如何向您的 IEnumerable<Activity> 类添加成员。(如果您的类实现了 ICollection<Activity> 或者 有一个带有 IEnumerable<Activity> 参数的构造函数，它就能够添加成员。) 这样会起作用吗？文档页面 Serialization Attributes 表示：DataContractAttribute 可以用作 JsonObjectAttribute 的替代品。DataContractAttribute 将默认成员序列化为选择加入。这意味着 Json.NET 应该按照您的预期工作。如果您愿意，可以报告问题-至少应该澄清文档。 解决方法：如果您想强制Json.NET将集合序列化为对象，您需要使用[JsonObject]。

[DataContract]
[JsonObject(MemberSerialization = MemberSerialization.OptIn)]
public class Activities : IEnumerable<Activity>
{
    private List<Activity> _list;

    [DataMember]
    [JsonProperty]
    public List<Activity> List
    {
        get { return this._list; }
        set { this._list = value; }
    }

    // Remainder unchanged.
}

如果您有许多应用了[DataContract]的可枚举类，或者无法为您的模型添加Json.NET的依赖项，您可以创建一个自定义的ContractResolver，该解析器会检查可枚举类上是否存在[DataContract]并将其序列化为对象：

public class DataContractForCollectionsResolver : DefaultContractResolver
{
    // As of 7.0.1, Json.NET suggests using a static instance for "stateless" contract resolvers, for performance reasons.
    // http://www.newtonsoft.com/json/help/html/ContractResolver.htm
    // http://www.newtonsoft.com/json/help/html/M_Newtonsoft_Json_Serialization_DefaultContractResolver__ctor_1.htm
    // "Use the parameterless constructor and cache instances of the contract resolver within your application for optimal performance."
    static DataContractForCollectionsResolver instance;

    static DataContractForCollectionsResolver() { instance = new DataContractForCollectionsResolver(); }

    public static DataContractForCollectionsResolver Instance { get { return instance; } }

    protected DataContractForCollectionsResolver() : base() { }

    protected override JsonContract CreateContract(Type objectType)
    {
        var t = (Nullable.GetUnderlyingType(objectType) ?? objectType);
        if (!t.IsPrimitive 
            && t != typeof(string)
            && !t.IsArray
            && typeof(IEnumerable).IsAssignableFrom(t) 
            && !t.GetCustomAttributes(typeof(JsonContainerAttribute),true).Any()) 
        { 
            if (t.GetCustomAttributes(typeof(DataContractAttribute),true).Any()) 
                return base.CreateObjectContract(objectType);
        }
        return base.CreateContract(objectType);
    }
}

然后使用以下设置：

var serializerSettings = new JsonSerializerSettings()
{
    DateFormatHandling = DateFormatHandling.IsoDateFormat,
    DateParseHandling = Newtonsoft.Json.DateParseHandling.DateTimeOffset,
    ContractResolver = DataContractForCollectionsResolver.Instance
};

- dbc

---

谢谢，但我不得不稍微修改CreateContract方法才能让它对我起作用：'code' ... && !t.GetCustomAttributes(typeof(JsonContainerAttribute),true).Any()) { if (t.GetCustomAttributes(typeof(DataContractAttribute),true).Any()) return base.CreateObjectContract(objectType); ... - hawi

@hawi - 按照您的建议，答案已更新。看起来 Json.NET 将 DataContract 视为继承，尽管数据合同序列化程序并不是这样处理的。（http://json.codeplex.com/discussions/357850） - dbc