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

Question

我有一个类接口类型的类,如:

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

我还有IBar可以在运行时设置的接口的多个具体实现.其中一些具体类需要自定义JsonConverter进行序列化和反序列化.

利用该TypeNameHandling.Auto选项,非转换器需要IBar类可以完美地序列化和反序列化.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.

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

是否有解决方法或处理此类情况的正确方法？

Answer 1

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,但至少它是有效的.所以我的想法是JsonConverter每个类型实现IBar和一个转换器IBar本身.

那么让我们从模型开始:

public interface IBar { }

public class BarA : IBar  { }

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

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

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);
    }
}

您可能会注意到它是从BarBaseJsonConverter类中继承的,而不是JsonConverter.而且我们也不使用serializer参数WriteJson和ReadJson方法.serializer在自定义转换器中使用参数存在问题.你可以在这里阅读更多.我们需要创建新的实例,JsonSerializer基类是一个很好的候选者:

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只是一个要创建的类JsonSerializerSettings:

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,您仍然可以使用自定义转换器进行序列化和反序列化:

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