Csharp 修复C#Web API中的循环引用错误

我有一个用C#编写的相当简单的Web API，使用EF.这个 idea 在很大程度上是一个待办事项应用程序，具体的关系是:一个用户可以有多个待办事项列表，每个待办事项列表可以有多个待办事项.

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每个用户可能拥有的待办事项列表.我在一个服务中有代码:

    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，当我运行此命令时，我得到以下错误:

      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.

我相信这是因为用户引用了ToDoList，而ToDoList又引用了User，但我需要双向引用这个引用，因为它们是外键.

我还在网上看到了这个解决方案，添加了以下几行:

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

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

        var toDoLists = deserializedUser.ToDoLists;

添加到控制器方法，但这也不起作用.

有什么可以帮助你解决这个问题的吗？