序列化成Json时，多个对象互相引用导致死循环 Text.Json.JsonException: A possible object cycle was detected.

错误： 当两个类中的属性互相引用时，导致对象实例序列化成Json时死循环，错误如下：

System.Text.Json.JsonException: A possible object cycle was detected. 
This can either be due to a cycle or if the object depth is larger than the maximum allowed depth of 32. 
Consider using ReferenceHandler.Preserve on JsonSerializerOptions to support cycles. 
Path: $.Order.Delivery.Order.Delivery.Order.Delivery.Order.Delivery.Order.Delivery.Order.Delivery.Order.Delivery.Order.Delivery.Order.Delivery.Order.Delivery.Order.Delivery.Order.Delivery.Order.Delivery.Order.Delivery.Order.Delivery.Order.Id.

　解决方法：

在 Program.cs中添加 AddJsonOptions 选项配置，此配置Json不但不会死循环而且还能完整表达实例之间的互相引用关系。

builder.Services.AddControllers()
    .AddJsonOptions(option => option.JsonSerializerOptions.ReferenceHandler = System.Text.Json.Serialization.ReferenceHandler.Preserve);

增加AddJsonOptions 后的结果：

{
  "$id": "1",
  "id": 1,
  "companyName": "蜗牛快递",
  "number": "SN356565565",
  "order": {
    "$id": "2",
    "id": 1,
    "name": "USB 充电器",
    "address": "湖南省长沙市岳阳临湘大市场",
    "delivery": {
      "$ref": "1"
    }
  },
  "orderId": 1
}