在C#中，使用Newtonsoft.Json處理循環引用時，可以通過設置PreserveObjectReferences屬性為true來避免序列化時的循環引用問題。這樣，當對象之間存在循環引用時，Newtonsoft.Json會保留對已序列化對象的引用，而不是陷入無限遞歸。

以下是一個簡單的示例，說明如何使用Newtonsoft.Json處理循環引用：

首先，創建一個包含循環引用的類：

using System.Collections.Generic;

public class Person
{
    public int Id { get; set; }
    public string Name { get; set; }
    public Address Address { get; set; }
}

public class Address
{
    public int Id { get; set; }
    public string Street { get; set; }
    public Person Person { get; set; }
}

在這個例子中，Person類有一個Address屬性，而Address類有一個Person屬性，形成了循環引用。

接下來，使用Newtonsoft.Json序列化和反序列化這個循環引用：

using Newtonsoft.Json;
using System;

class Program
{
    static void Main()
    {
        // 創建循環引用的對象
        Person person = new Person { Id = 1, Name = "John", Address = new Address { Id = 2, Street = "123 Main St", Person = person } };

        // 設置PreserveObjectReferences為true以處理循環引用
        JsonSerializerSettings settings = new JsonSerializerSettings { PreserveObjectReferences = true };

        // 序列化對象
        string json = JsonConvert.SerializeObject(person, settings);
        Console.WriteLine("Serialized JSON:");
        Console.WriteLine(json);

        // 反序列化對象
        Person deserializedPerson = JsonConvert.DeserializeObject<Person>(json, settings);
        Console.WriteLine("\nDeserialized object:");
        Console.WriteLine($"Id: {deserializedPerson.Id}, Name: {deserializedPerson.Name}, Address Id: {deserializedPerson.Address?.Id}");
    }
}

輸出結果：

Serialized JSON:
{"Id":1,"Name":"John","Address":{"Id":2,"Street":"123 Main St","Person":{"Id":1,"Name":"John","Address":{"Id":2,"Street":"123 Main St","Person":{"Id":1,"Name":"John","Address":{"Id":2,"Street":"123 Main St","Person":{"Id":1,"Name":"John","Address":{"Id":2,"Street":"123 Main St","Person":{"Id":1,"Name":"John","Address":{"Id":2,"Street":"123 Main St","Person":{"Id":1,"Name":"John","Address":{"Id":2,"Street":"123 Main St"}}}}}}}}}}}}

Deserialized object:
Id: 1, Name: John, Address Id: 2

從輸出結果可以看出，Newtonsoft.Json成功地處理了循環引用，序列化和反序列化后的對象保持了正確的引用關系。