c# - 反序列化时 Newtonsoft Json.NET JsonConverter 属性保留引用问题

Question

在一个项目的模型中，我使用 JsonConverter 属性来帮助这些模型的(反)序列化。

转换器目前看起来像这样:

public class CustomJsonConverter : Newtonsoft.Json.JsonConverter
{
    bool _canWrite = true;
    public override bool CanWrite
    {
        get { return _canWrite; }
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        serializer.PreserveReferencesHandling = PreserveReferencesHandling.Objects;
        serializer.DefaultValueHandling = DefaultValueHandling.Ignore;
        serializer.NullValueHandling = NullValueHandling.Ignore;

        _canWrite = false;
        var jObject = JObject.FromObject(value, serializer);
        _canWrite = true;

        jObject.WriteTo(writer);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        serializer.PreserveReferencesHandling = PreserveReferencesHandling.Objects;
        if (reader.TokenType == JsonToken.StartObject)
        {
            existingValue = existingValue ?? serializer.ContractResolver.ResolveContract(objectType).DefaultCreator();
            serializer.Populate(reader, existingValue);
            return existingValue;
        }
        else if (reader.TokenType == JsonToken.Null)
        {
            return null;
        }
        else
        {
            throw new JsonSerializationException();
        }
    }

    public override bool CanConvert(Type objectType)
    {
        return typeof(IModelBase).IsAssignableFrom(objectType);
    }
}

模型有一个如下所示的基类:

[JsonConverter(typeof(CustomJsonConverter))]
public abstract class ModelBase : IModelBase
{
    public string ID { get; set; }
    public DateTime CreatedAt { get; set; }
    public DateTime ModifiedAt { get; set; }
}

ModelBase 类的派生类具有类型也从 ModelBase 派生的属性。例如:

public class CustomerModel : ModelBase
{
    public string Name { get; set; }
    public UserModel CreatedBy { get; set; }
    public UserModel ModifiedBy { get; set; }
}

public class UserModel : ModelBase
{
    public string Name { get; set; }
    public UserModel CreatedBy { get; set; }
    public UserModel ModifiedBy { get; set; }
}

我在 ASP.NET Web API 2 应用程序(服务器端)和 C# 应用程序(客户端)中使用这些模型。对于大多数 API 调用，都会返回一个模型数组。序列化模型时，一切都按预期进行。然而，在反序列化时，只有第一次出现的每个引用被填充信息。

例如:

[
    {
        "$id": "1",
        "Name": "Customer1",
        "CreatedBy": {
            "$id": "2",
            "ID": "1",
            "Name": "User1"
        },
        "ModifiedBy": {
            "$id": "3",
            "ID": "3",
            "Name": "User3"
        },
        "ID": "1",
        "CreatedAt": "2019-02-06T00:00:04",
        "ModifiedAt": "2019-02-06T00:20:12"
    },
    {
        "$id": "4",
        "Name": "Customer2",
        "CreatedBy": {
            "$ref": "2"
        },
        "ModifiedBy": {
            "$ref": "2"
        },
        "ID": "2",
        "CreatedAt": "2019-02-06T00:10:00",
        "ModifiedAt": "2019-02-06T00:10:00"
    }
]

当尝试反序列化 Web API 返回的这个 JSON 对象时，第一个 CustomerModel 对象的 CreatedBy 和 ModifiedBy 属性将是正确的.但是，对于第二个 CustomerModel 对象，这些属性将是没有任何属性集的新 UserModel 实例。

要反序列化 JSON 字符串，我使用以下代码:

using (var sr = new StreamReader(streamFromWebAPICall))
{                
    using (var jtr = new JsonTextReader(sr))
    {
        var js = new JsonSerializer();
        return js.Deserialize(jtr, objectType);
    }
}

如何正确设置所有反序列化对象的属性？

编辑:

问题似乎出在 serializer.Populate(reader, existingValue) 中，其中引用未被记住。

Answer 1

您的基本问题是您提供的是 custom JsonConverter对于您还想为其启用 PreserveReferencesHandling 的类型.但是无论何时应用自定义转换器，它都必须手动处理一切，包括解析和生成"$ref" 和"$id" 属性。您的转换器不会执行此操作，因此您的反序列化代码不会正确反序列化您的对象图。

accepted answer给 Custom object serialization vs PreserveReferencesHandling 包括一个模板转换器，显示如何处理这些属性。但是，由于您的转换器似乎只是在(反)序列化之前切换一些序列化程序设置，您可以简单地调用递归(反)序列化对象，在持续时间内禁用转换器，如下所示:

public class CustomJsonConverter : Newtonsoft.Json.JsonConverter
{
    [ThreadStatic]
    static bool disabled;

    // Disables the converter in a thread-safe manner.
    bool Disabled { get { return disabled; } set { disabled = value; } }

    public override bool CanWrite { get { return !Disabled; } }

    public override bool CanRead { get { return !Disabled; } }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        using (new PushValue<bool>(true, () => Disabled, val => Disabled = val))
        using (new PushValue<PreserveReferencesHandling>(PreserveReferencesHandling.Objects, () => serializer.PreserveReferencesHandling, val => serializer.PreserveReferencesHandling = val))
        using (new PushValue<DefaultValueHandling>(DefaultValueHandling.Ignore, () => serializer.DefaultValueHandling, val => serializer.DefaultValueHandling = val))
        using (new PushValue<NullValueHandling>(NullValueHandling.Ignore, () => serializer.NullValueHandling, val => serializer.NullValueHandling = val))
        {
            serializer.Serialize(writer, value);
        }
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        if (reader.TokenType == JsonToken.Null)
        {
            return null;
        }
        using (new PushValue<bool>(true, () => Disabled, val => Disabled = val))
        using (new PushValue<PreserveReferencesHandling>(PreserveReferencesHandling.Objects, () => serializer.PreserveReferencesHandling, val => serializer.PreserveReferencesHandling = val))
        using (new PushValue<DefaultValueHandling>(DefaultValueHandling.Ignore, () => serializer.DefaultValueHandling, val => serializer.DefaultValueHandling = val))
        using (new PushValue<NullValueHandling>(NullValueHandling.Ignore, () => serializer.NullValueHandling, val => serializer.NullValueHandling = val))
        {
            return serializer.Deserialize(reader, objectType);
        }
    }

    public override bool CanConvert(Type objectType)
    {
        throw new NotImplementedException();
    }
}

public struct PushValue<T> : IDisposable
{
    Action<T> setValue;
    T oldValue;

    public PushValue(T value, Func<T> getValue, Action<T> setValue)
    {
        if (getValue == null || setValue == null)
            throw new ArgumentNullException();
        this.setValue = setValue;
        this.oldValue = getValue();
        setValue(value);
    }

    // By using a disposable struct we avoid the overhead of allocating and freeing an instance of a finalizable class.
    public void Dispose()
    {
        if (setValue != null)
            setValue(oldValue);
    }
}

请注意，禁用转换器的逻辑需要线程安全，因为 Json.NET 将跨线程共享契约和转换器。

演示 fiddle #1 here .

作为替代方案，您可以完全取消转换器并应用 [JsonObject(IsReference = true)]直接到ModelBase:

[JsonObject(IsReference = true)]
public abstract class ModelBase : IModelBase
{
    public string ID { get; set; }
    public DateTime CreatedAt { get; set; }
    public DateTime ModifiedAt { get; set; }
}

然后使用在设置中指定的 DefaultValueHandling.Ignore 和 NullValueHandling.Ignore 进行序列化和反序列化，如下所示:

static object Deserialize(Stream streamFromWebAPICall, Type objectType)
{
    using (var sr = new StreamReader(streamFromWebAPICall))
    {
        using (var jtr = new JsonTextReader(sr))
        {
            var settings = new JsonSerializerSettings
            {
                DefaultValueHandling = DefaultValueHandling.Ignore,
                NullValueHandling = NullValueHandling.Ignore,
            };
            var js = JsonSerializer.CreateDefault(settings);
            return js.Deserialize(jtr, objectType);
        }
    }
}

演示 fiddle #2 here .

注意你也可以设置[JsonObject(ItemNullValueHandling = NullValueHandling.Ignore)]自Json.NET 11.0.1 ，但在 JsonObjectAttribute 上似乎没有 ItemDefaultValueHandling 设置，因此将其添加到序列化程序设置中(或对可选的值类型值使用可空值，例如 CreatedAt ) 是必需的。