现在有一个业务场景，有一个雇员类，里面有一个管理者的属性，也是雇员类这种类型，普通雇员的管理者是经理，经理的管理者是CEO，CEO的管理要么是空，要么是自己管理自己，空是正常的，但如果CEO的管理者是自己，用.NET9实现，如下：

using System.Text.Json;
// 堆代码 duidaima.com
var ceo = new Employee { NO = 1, Name = "CEO" };
ceo.Manager = ceo;  // 这里报错
Console.WriteLine(JsonSerializer.Serialize(ceo));
Console.WriteLine("---------------");

var manager = new Employee { NO = 2, Name = "Manager" };
manager.Manager = ceo;
Console.WriteLine(JsonSerializer.Serialize(manager));
Console.WriteLine("---------------");

var employee = new Employee { NO = 3, Name = "Employee" };
employee.Manager = manager;
Console.WriteLine(JsonSerializer.Serialize(employee));

class Employee
{
    public int NO { get; set; }
    public string? Name { get; set; }
    public Employee? Manager { get; set; }
}

运行，报错结果如下，出现了循环引用的异常：

当使用源生成器进行 JSON 序列化时，生成的上下文在序列化或反序列化循环引用时会抛出异常。现在，可以通过在 JsonSourceGenerationOptionsAttribute 中指定 ReferenceHandler 来自定义此行为。以下是使用 JsonKnownReferenceHandler.Preserve 的示例：

using System.Text.Json;
using System.Text.Json.Serialization;

var ceo = new Employee { NO = 1, Name = "CEO" };
ceo.Manager = ceo;
Console.WriteLine(JsonSerializer.Serialize(ceo, ContextWithPreserveReference.Default.Employee));
Console.WriteLine("---------------");

var manager = new Employee { NO = 2, Name = "Manager" };
manager.Manager = ceo;
 Console.WriteLine(JsonSerializer.Serialize(manager, ContextWithPreserveReference.Default.Employee));
Console.WriteLine("---------------");

var employee = new Employee { NO = 3, Name = "Employee" };
employee.Manager = manager;
Console.WriteLine(JsonSerializer.Serialize(employee, ContextWithPreserveReference.Default.Employee));
Console.ReadLine();

[JsonSourceGenerationOptions(ReferenceHandler = JsonKnownReferenceHandler.Preserve)]
[JsonSerializable(typeof(Employee))]
internal partial class ContextWithPreserveReference : JsonSerializerContext
{
}
class Employee
{
    public int NO { get; set; }
    public string? Name { get; set; }
    public Employee? Manager { get; set; }
}

运行结果如下：