修复 C# Web API 中的循环引用错误

问：

我有一个用 C# 编写的相当简单的 Web API，使用 EF。这个想法几乎是一个待办事项应用程序，其具体关系是：一个用户可以有许多待办事项列表，每个待办事项列表可以有许多待办事项。

以下是我的数据模型：

public class User
{
    public Guid Id { get; set; }
    public string? Name { get; set; }
    public string? Email { get; set; }
    public string? Password { get; set; } // Be sure to properly hash and salt passwords
    public string? ProfilePhotoUrl { get; set; } // Store the URL to the user's profile photo
    public List<ToDoList>? ToDoLists { get; set; } // A user can have multiple to-do lists
}

public class ToDoList
{
    public Guid Id { get; set; }
    public string? Title { get; set; }
    public List<ToDo>? ToDos { get; set; } // Each to-do list contains multiple to-dos

    // Foreign Key to User
    public Guid UserId { get; set; }
    public User? User { get; set; }
}

public class ToDo
{
    public Guid Id { get; set; }
    public string? Name { get; set; }
    public string? Description { get; set; }
    public bool? InProgress { get; set; }
    public bool? IsComplete { get; set; }
    public DateTime Created { get; set; } = DateTime.Now;

    public ToDoList? ToDoList { get; set; } 
    public Guid? ToDoListId{ get; set; }
}

我有一个控制器，允许用户注册、登录等。我想做的一件事是检索用户可能拥有的每个待办事项列表。我在服务中有代码：

    public Task<User> GetToDoListsForUsers(Guid userId)
    {
        return _context.Users
            .Include(u => u.ToDoLists)
            .FirstOrDefaultAsync(u => u.Id == userId);
    }

然后由控制器调用：

    [HttpGet("{userId}/todolists")]
    public async Task<ActionResult<IEnumerable<ToDoList>>> GetToDoListsForUser(Guid userId)
    {
        var user = await _userService.GetToDoListsForUsers(userId);

        if (user == null)
        {
            return NotFound("User not found");
        }

        var toDoLists = user.ToDoLists;

        if (toDoLists.Count == 0)
        {
            return NotFound("user has no lists");
        }

        return Ok(toDoLists);
    }

但是，当我运行它时，我收到以下错误：

      An unhandled exception has occurred while executing the request.
      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: $.User.ToDoLists.User.ToDoLists.User.ToDoLists.User.ToDoLists.User.ToDoLists.User.ToDoLists.User.ToDoLists.User.ToDoLists.User.ToDoLists.User.ToDoLists.Id.

我相信这是因为 User 具有对 ToDoList 的引用，而 ToDoList 具有对 User 的引用，但我需要这两种方式的引用，因为它们是外键。

我还在网上遇到了这个解决方案，以添加以下几行：

        var jsonSettings = new JsonSerializerOptions
        {
            ReferenceHandler = ReferenceHandler.Preserve,
        };

        var serializedUser = JsonSerializer.Serialize(user, jsonSettings);
        var deserializedUser = JsonSerializer.Deserialize<User>(serializedUser, jsonSettings);

        var toDoLists = deserializedUser.ToDoLists;

到控制器方法，但这也不起作用。

在解决这个问题方面有什么帮助吗？