RefHandlingDemo.cs

using System;
using System.Collections.Generic;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace RefHandlingDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            var janeEmployee = new Employee
            {
                Name = "Jane Doe"
            };

            var johnEmployee = new Employee
            {
                Name = "John Smith"
            };

            janeEmployee.Reports = new List<Employee> { johnEmployee };
            johnEmployee.Manager = janeEmployee;

            var options = new JsonSerializerOptions
            {
                ReferenceHandler = ReferenceHandler.Preserve
            };

            string serialized = JsonSerializer.Serialize(janeEmployee, options);
            Console.WriteLine(serialized);

            Employee janeDeserialized = JsonSerializer.Deserialize<Employee>(serialized, options);
            Console.WriteLine(janeDeserialized.Reports[0].Manager == janeDeserialized);
        }
    }

    class Employee
    {
        public string Name { get; set; }
        
        public Employee Manager { get; set; }

        public List<Employee> Reports { get; set; }
    }
}