JSON.. NET序列化可空结构体和TypeNameHandling.Auto

以下程序:

using System;
using Newtonsoft.Json;
namespace JsonPlayground
{
    public class Program
    {
        private struct Message
        {
            public string Value { get; }
            [JsonConstructor]
            public Message(string value)
            {
                if (value == null) throw new ArgumentNullException(nameof(value));
                Value = value;
            }
        }
        private class ObjectWithOptionalMessage
        {
            public Message? Message { get; }
            public ObjectWithOptionalMessage(Message? message)
            {
                Message = message;
            }
        }
        public static void Main(string[] args)
        {
            var objWithMessage = new ObjectWithOptionalMessage(new Message("Hello!"));
            var serializerSettings = new JsonSerializerSettings
            {
                TypeNameHandling = TypeNameHandling.Auto,
                Formatting = Formatting.Indented
            };
            Console.WriteLine(JsonConvert.SerializeObject(objWithMessage, serializerSettings));
        }
    }
}

给出如下输出:

{
  "Message": {
    "$type": "System.Nullable`1[[JsonPlayground.Program+Message, JsonPlayground]], System.Private.CoreLib",
    "Value": "Hello!"
  }
}

我想不出为什么$type应该包含在一个明显声明良好的属性中。如果我声明Message属性不可为空，则不包括$type属性。

我做错了什么?

    private class ObjectWithOptionalMessage
    {
        [JsonProperty(TypeNameHandling = TypeNameHandling.None)]
        public Message? Message { get; }
        public ObjectWithOptionalMessage(Message? message)
        {
            Message = message;
        }
    }