在 Json.net 中使用自定义 JsonConverter 和 TypeNameHandling

问题描述

我有一个具有接口类型属性的类，例如:

I have a class with an interface-typed property like:

public class Foo
{
    public IBar Bar { get; set; }
}

我还有多个可以在运行时设置的IBar 接口的具体实现.其中一些具体类需要一个自定义的 JsonConverter 来进行序列化和反序列化.

I also have multiple concrete implementations of the IBar interface that can be set at runtime. Some of these concrete classes require a custom JsonConverter for serialization & deserialization.

利用 TypeNameHandling.Auto 选项可以完美地序列化和反序列化需要 IBar 类的非转换器.另一方面，自定义序列化的类没有 $type 名称输出，虽然它们按预期序列化，但它们不能反序列化为它们的具体类型.

Utilizing the TypeNameHandling.Auto option the non-convertor requiring IBar classes can be serialized and deserialized perfectly. The custom-serialized classes on the other hand have no $type name output and while they are serialized as expected, they cannot be deserialized to their concrete type.

我尝试在自定义 JsonConverter 中自己写出 $type 名称元数据；但是，在反序列化时，转换器将被完全绕过.

I attempted to write-out the $type name metadata myself within the custom JsonConverter; however, on deserialization the converter is then being bypassed entirely.

是否有解决方法或适当的方法来处理这种情况?

Is there a workaround or proper way of handling such a situation?

推荐答案

我解决了类似的问题，我找到了解决方案.它不是很优雅，我认为应该有更好的方法，但至少它有效.所以我的想法是每个实现 IBar 的类型都有 JsonConverter 和 IBar 本身的转换器.

I solved the similar problem and I found a solution. It's not very elegant and I think there should be a better way, but at least it works. So my idea was to have JsonConverter per each type that implements IBar and one converter for IBar itself.

让我们从模型开始:

public interface IBar { }

public class BarA : IBar  { }

public class Foo
{
    public IBar Bar { get; set; }
}

现在让我们为 IBar 创建转换器.仅在反序列化 JSON 时使用.它将尝试读取 $type 变量并调用转换器以实现类型:

Now let's create converter for IBar. It will be used only when deserializing JSON. It will try to read $type variable and call converter for implementing type:

public class BarConverter : JsonConverter
{
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        throw new NotSupportedException();
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var jObj = JObject.Load(reader);
        var type = jObj.Value<string>("$type");

        if (type == GetTypeString<BarA>())
        {
            return new BarAJsonConverter().ReadJson(reader, objectType, jObj, serializer);
        }
        // Other implementations if IBar

        throw new NotSupportedException();
    }

    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof (IBar);
    }

    public override bool CanWrite
    {
        get { return false; }
    }

    private string GetTypeString<T>()
    {
        var typeOfT = typeof (T);
        return string.Format("{0}, {1}", typeOfT.FullName, typeOfT.Assembly.GetName().Name);
    }
}

这是 BarA 类的转换器:

public class BarAJsonConverter : BarBaseJsonConverter
{
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        // '$type' property will be added because used serializer has TypeNameHandling = TypeNameHandling.Objects
        GetSerializer().Serialize(writer, value);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var existingJObj = existingValue as JObject;
        if (existingJObj != null)
        {
            return existingJObj.ToObject<BarA>(GetSerializer());
        }

        throw new NotImplementedException();
    }

    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(BarA);
    }
}

您可能会注意到它继承自 BarBaseJsonConverter 类，而不是 JsonConverter.而且我们在 WriteJson 和 ReadJson 方法中不使用 serializer 参数.在自定义转换器中使用 serializer 参数存在问题.你可以阅读更多这里.我们需要创建 JsonSerializer 的新实例，而基类是一个很好的候选对象:

You may notice that it's inherited from BarBaseJsonConverter class, not JsonConverter. And also we do not use serializer parameter in WriteJson and ReadJson methods. There is a problem with using serializer parameter inside custom converters. You can read more here. We need to create new instance of JsonSerializer and base class is a good candidate for that:

public abstract class BarBaseJsonConverter : JsonConverter
{
    public JsonSerializer GetSerializer()
    {
        var serializerSettings = JsonHelper.DefaultSerializerSettings;
        serializerSettings.TypeNameHandling = TypeNameHandling.Objects;

        var converters = serializerSettings.Converters != null
            ? serializerSettings.Converters.ToList()
            : new List<JsonConverter>();
        var thisConverter = converters.FirstOrDefault(x => x.GetType() == GetType());
        if (thisConverter != null)
        {
            converters.Remove(thisConverter);
        }
        serializerSettings.Converters = converters;

        return JsonSerializer.Create(serializerSettings);
    }
}

JsonHelper只是一个创建JsonSerializerSettings的类:

public static class JsonHelper
{
    public static JsonSerializerSettings DefaultSerializerSettings
    {
        get
        {
            return new JsonSerializerSettings
            {
                Converters = new JsonConverter[] { new BarConverter(), new BarAJsonConverter() }
            };
        }
    }
}

现在它可以工作了，您仍然可以使用自定义转换器进行序列化和反序列化:

Now it will work and you still can use your custom converters for both serialization and deserialization:

var obj = new Foo { Bar = new BarA() };
var json = JsonConvert.SerializeObject(obj, JsonHelper.DefaultSerializerSettings);
var dObj = JsonConvert.DeserializeObject<Foo>(json, JsonHelper.DefaultSerializerSettings);

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