反序列化：接口，抽象类

    [DataContract]
    public abstract class ProductPromotionDetail
    { }

　　

   [JsonConverter(typeof(ConcreteConverter<ProductPromotionDetail_4>))]
        public ProductPromotionDetail Detail { get; set; }

　　

  public class ProductPromotionDetail_4 : ProductPromotionDetail
    {}

　　

  public class ConcreteConverter<T> : JsonConverter
    {
        public override bool CanConvert(Type objectType) => true;

        public override object ReadJson(JsonReader reader,
         Type objectType, object existingValue, JsonSerializer serializer)
        {
            return serializer.Deserialize<T>(reader);
        }

        public override void WriteJson(JsonWriter writer,
            object value, JsonSerializer serializer)
        {
            serializer.Serialize(writer, value);
        }
    }
	

　　

Refer：https://www.c-sharpcorner.com/UploadFile/20c06b/deserializing-interface-properties-with-json-net/