How to Serialize Polymorphic Objects in JSON.NET Without the '$type' Field?

JSON.NET polymorphic object serialization (no "$type" field required)

Deserializing polymorphic objects in JSON.NET is easy using TypeNameHandling.Auto, but what if you need to remove the "$type" field?

Method 1: Rename "$type" to a custom name

Unfortunately, JSON.NET's built-in options cannot rename "$type".

Method 2: Add subtype information to the base class

Instead of using a container class to hold subtype information, add a property called "Type" to the base class (SubTypeClassBase):

<code class="language-csharp">[JsonConverter(typeof(SubTypeClassConverter))]
public class SubTypeClassBase
{
    [JsonConverter(typeof(StringEnumConverter))]
    public SubType Type { get { return typeToSubType[GetType()]; } }
}</code>

This ensures that subtype information is always included when serializing objects of type SubTypeClassBase.

Custom deserialization

To properly deserialize JSON, create a custom JsonConverter (SubTypeClassConverter) that reads the "Type" property, identifies the actual type, and deserializes accordingly:

<code class="language-csharp">public class SubTypeClassConverter : JsonConverter
{
    // 重写CanConvert以支持SubTypeClassBase对象
    public override bool CanConvert(Type objectType) { return objectType == typeof(SubTypeClassBase); }

    // 重写ReadJson以处理反序列化
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        // 将JSON加载为JToken并获取“Type”标记
        var token = JToken.Load(reader);
        var typeToken = token["Type"];
        if (typeToken == null) throw new InvalidOperationException("无效对象");

        // 根据“Type”值确定实际类型
        var actualType = SubTypeClassBase.GetType(typeToken.ToObject<SubType>(serializer));

        // 根据需要创建实际类型的实例
        if (existingValue == null || existingValue.GetType() != actualType)
        {
            var contract = serializer.ContractResolver.ResolveContract(actualType);
            existingValue = contract.DefaultCreator();
        }

        // 使用JSON数据填充实际类型
        using (var subReader = token.CreateReader())
        {
            serializer.Populate(subReader, existingValue);
        }

        return existingValue;
    }

    // 重写WriteJson以防止写入“Type”属性
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) { throw new NotImplementedException(); }
}</code>

With this approach, JSON.NET will automatically include the "Type" attribute when serializing the SubTypeClassBase object and use the SubTypeClassConverter to correctly deserialize JSON based on the specified type.

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