使用JsonConverterAttribute时，自定义继承JsonConverter失败

问题描述

我正在尝试反序列化派生类型，并且我想使用自定义属性Type来区分派生类型.

I am trying to deserialize derived type, and I want to use a custom property Type to distinguish between derived types.

[
  {
    "Type": "a",
    "Height": 100
  },
  {
    "Type": "b",
    "Name": "Joe"
  }
]

我遇到的解决方案是创建一个自定义JsonConverter.在ReadJson上，我读取Type属性，并通过ToObject<T>函数实例化该类型.一切正常，直到使用JsonConverterAttribute. ReadJson方法无限循环，因为该属性也适用于子类型.

The solution I came to was to create a custom JsonConverter. On ReadJson I read the Type property and instantiate that type through the ToObject<T> function. Everything works fine until I use a JsonConverterAttribute. The ReadJson method loops infinitely because the attribute is applied on subtypes too.

如何防止将此属性应用于子类型?

How do I prevent this attribute from being applied to the subtypes?

[JsonConverter(typeof(TypeSerializer))]
public abstract class Base
{
    private readonly string type;

    public Base(string type)
    {
        this.type = type;
    }

    public string Type { get { return type; } }
}

public class AType : Base
{
    private readonly int height;

    public AType(int height)
        : base("a")
    {
        this.height = height;
    }

    public int Height { get { return height; } }
}

public class BType : Base
{
    private readonly string name;

    public BType(string name)
        : base("b")
    {
        this.name = name;
    }

    public string Name { get { return name; } }
}

public class TypeSerializer : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(Base);
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        serializer.Serialize(writer, value);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var j = JObject.Load(reader);

        var type = j["Type"].ToObject<string>();

        if (type == "a")
            // Infinite Loop! StackOverflowException
            return j.ToObject<AType>();
        if (type == "b")
            return j.ToObject<BType>();

        throw new NotImplementedException(type);
    }
}

[TestFixture]
public class InheritanceSerializeTests
{
    [Test]
    public void Deserialize()
    {
        var json = @"{""Type"":""a"", ""Height"":100}";
        JObject.Parse(json).ToObject<Base>(); // Crash
    }
}

推荐答案

我目前正在处理的项目有一个非常相似的问题:我想制作一个自定义JsonConverter并通过属性将其映射到我的实体，但随后代码陷入了无限循环.

I had a very similar problem with a project that I am currently working on: I wanted to make a custom JsonConverter and map it to my entities via attributes, but then the code got trapped in an infinite loop.

在我的情况下，技巧是使用serializer.Populate而不是JObject.ToObject(即使我愿意也不能使用.ToObject；我使用的是3.5.8版，其中该功能不存在).下面是我的ReadJson方法作为示例:

What did the trick in my case was using serializer.Populate instead of JObject.ToObject (I couldn't use .ToObject even if I wanted to; I am using version 3.5.8, in which this function does not exist). Below is my ReadJson method as an example:

public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
    JContainer lJContainer = default(JContainer);

    if (reader.TokenType == JsonToken.StartObject)
    {
        lJContainer = JObject.Load(reader);
        existingValue = Convert.ChangeType(existingValue, objectType);
        existingValue = Activator.CreateInstance(objectType);

        serializer.Populate(lJContainer.CreateReader(), existingValue);
    }

    return existingValue;
}

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