自定义对象序列化与 PreserveReferencesHandling

Question

当使用自定义JsonConverter时，是否有任何标准方法可以在序列化时获取当前对象的"$id"字段值，并在反序列化时通过其"$id"值获取对象？

Answer 1

在custom JsonConverter 中，您可以使用JsonSerializer.ReferenceResolver 返回的IReferenceResolver 手动读写Json.NET 的"$id" 和"$ref" 属性。

以下转换器为此提供了模板：

public abstract class ReferenceHandlingCustomCreationConverter<T> : JsonConverter where T : class
{
    const string refProperty = "$ref";
    const string idProperty = "$id";

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

    protected virtual T Create(Type objectType, T existingValue, JsonSerializer serializer, JObject obj)
    {
        return existingValue ?? (T)serializer.ContractResolver.ResolveContract(objectType).DefaultCreator();
    }

    protected abstract void Populate(JObject obj, T value, JsonSerializer serializer);

    protected abstract void WriteProperties(JsonWriter writer, T value, JsonSerializer serializer, JsonObjectContract contract);

    public override sealed object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var contract = serializer.ContractResolver.ResolveContract(objectType);
        if (!(contract is JsonObjectContract))
        {
            throw new JsonSerializationException(string.Format("Invalid non-object contract type {0}", contract));
        }
        if (!(existingValue == null || existingValue is T))
        {
            throw new JsonSerializationException(string.Format("Converter cannot read JSON with the specified existing value. {0} is required.", typeof(T)));
        }

        if (reader.MoveToContent().TokenType == JsonToken.Null)
            return null;
        var obj = JObject.Load(reader);

        var refId = (string)obj[refProperty].RemoveFromLowestPossibleParent();
        var objId = (string)obj[idProperty].RemoveFromLowestPossibleParent();
        if (refId != null)
        {
            var reference = serializer.ReferenceResolver.ResolveReference(serializer, refId);
            if (reference != null)
                return reference;
        }

        var value = Create(objectType, (T)existingValue, serializer, obj);

        if (objId != null)
        {
            // Add the empty array into the reference table BEFORE poppulating it, to handle recursive references.
            serializer.ReferenceResolver.AddReference(serializer, objId, value);
        }

        Populate(obj, value, serializer);

        return value;
    }

    public override sealed void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        var contract = serializer.ContractResolver.ResolveContract(value.GetType());
        if (!(contract is JsonObjectContract))
        {
            throw new JsonSerializationException(string.Format("Invalid non-object contract type {0}", contract));
        }
        if (!(value is T))
        {
            throw new JsonSerializationException(string.Format("Converter cannot read JSON with the specified existing value. {0} is required.", typeof(T)));
        }

        writer.WriteStartObject();

        if (serializer.ReferenceResolver.IsReferenced(serializer, value))
        {
            writer.WritePropertyName(refProperty);
            writer.WriteValue(serializer.ReferenceResolver.GetReference(serializer, value));
        }
        else
        {
            writer.WritePropertyName(idProperty);
            writer.WriteValue(serializer.ReferenceResolver.GetReference(serializer, value));

            WriteProperties(writer, (T)value, serializer, (JsonObjectContract)contract);
        }

        writer.WriteEndObject();
    }
}

public static partial class JsonExtensions
{
    public static JsonReader MoveToContent(this JsonReader reader)
    {
        if (reader.TokenType == JsonToken.None)
            reader.Read();
        while (reader.TokenType == JsonToken.Comment && reader.Read())
            ;
        return reader;
    }

    public static JToken RemoveFromLowestPossibleParent(this JToken node)
    {
        if (node == null)
            return null;
        var contained = node.AncestorsAndSelf().Where(t => t.Parent is JContainer && t.Parent.Type != JTokenType.Property).FirstOrDefault();
        if (contained != null)
            contained.Remove();
        // Also detach the node from its immediate containing property -- Remove() does not do this even though it seems like it should
        if (node.Parent is JProperty)
            ((JProperty)node.Parent).Value = null;
        return node;
    }
}

实现转换器需要实现两个抽象方法：

protected abstract void Populate(JObject obj, T value, JsonSerializer serializer);

protected abstract void WriteProperties(JsonWriter writer, T value, JsonSerializer serializer, JsonObjectContract contract);

一个默认的通用实现可能如下所示：

public class DefaultReferenceHandlingCustomCreationConverter<T> : ReferenceHandlingCustomCreationConverter<T> where T : class
{
    protected override void Populate(JObject obj, T value, JsonSerializer serializer)
    {
        using (var reader = obj.CreateReader())
            serializer.Populate(reader, value);
    }

    protected override void WriteProperties(JsonWriter writer, T value, JsonSerializer serializer, JsonObjectContract contract)
    {
        foreach (var property in contract.Properties.Where(p => p.Writable && !p.Ignored))
        {
            // TODO: handle JsonProperty attributes including
            // property.Converter, property.IsReference, property.ItemConverter, property.ItemReferenceLoopHandling, 
            // property.ItemReferenceLoopHandling, property.ObjectCreationHandling, property.ReferenceLoopHandling, property.Required                            
            var itemValue = property.ValueProvider.GetValue(value);
            writer.WritePropertyName(property.PropertyName);
            serializer.Serialize(writer, itemValue);
        }
    }
}

使用它，那么你将序列化如下：

var settings = new JsonSerializerSettings
{
    Converters = { new DefaultReferenceHandlingCustomCreationConverter<RootObject>() },
    ReferenceLoopHandling = ReferenceLoopHandling.Serialize,
};
var json = JsonConvert.SerializeObject(parent, Formatting.Indented, settings);

注意事项：

• 转换器设计用于处理序列化为 JSON 对象的 c# 类。也可以创建一个转换器来手动写入和读取集合的"$ref"、"$id" 和"$values" 属性，例如如this answer 到 Cannot preserve reference to array or readonly list, or list created from a non-default constructor 所示。

• 转换器在反序列化过程中拆分创建和填充对象的任务，因此不适用于仅具有参数化构造函数的对象。这是正确解析递归自引用所必需的。

• 需要使用ReferenceLoopHandling.Serialize 进行序列化。

小提琴样本here.