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：

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

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1 2	builder.Services.AddControllers()     .AddJsonOptions(option => option.JsonSerializerOptions.ReferenceHandler = System.Text.Json.Serialization.ReferenceHandler.Preserve);

增加AddJsonOptions 后的结果：

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{   "$id": "1",   "id": 1,   "companyName": "蜗牛快递",   "number": "SN356565565",   "order": {     "$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 }