從 sql server 讀取數據時檢測到自引用循環

Question

我正在嘗試使用以下代碼每秒從數據庫中讀取數據：

public class DBChangeService : DelegatingHandler, IHostedService
{
    private Timer _timer;
    public SqlDependencyEx _sqlDependency;
    private readonly IHubContext<SignalServer> _hubcontext;
    private readonly IServiceScopeFactory _scopeFactory;
    string connectionString = "";
    public IConfiguration Configuration { get; }

    public DBChangeService(IConfiguration configuration, IHubContext<SignalServer> hubcontext, IServiceScopeFactory scopeFactory)
    {
        _scopeFactory = scopeFactory;
        _hubcontext = hubcontext;
        Configuration = configuration;
        connectionString = Configuration.GetConnectionString("DefaultConnection");
    }

    public Task StartAsync(CancellationToken cancellationToken)
    {
        _timer = new Timer(Heartbeat, null, 0, 1000);
        return Task.CompletedTask;
    }


    public void Heartbeat(object state)
    {
         _hubcontext.Clients.All.SendAsync("SBSystemBrodcasting", SBSystemApps());

    }

    public async Task<List<SomeModel>> SBSystemApps()
    {
        var data = new List<SomeModel>();

        using (var scope = _scopeFactory.CreateScope())
        {
            using (SqlConnection connection = new SqlConnection(connectionString))
            {
                string commandText = @"SELECT 
                        a.name as AppName,
                        a.state as AppState,
                        a.created_at as AppCreatedAt,
                        a.updated_at as AppUpdatedAt,
                        a.foundation as AppFoundation,

                        s.name as SpaceName,
                        o.name as OrgName
                    FROM
                        apps as a
                    INNER JOIN
                        spaces as s ON a.space_guid = s.space_guid
                    INNER JOIN
                        organizations as o ON s.org_guid = o.org_guid
                    where s.name = 'system' and o.name = 'system' and a.foundation = 2";

                try
                {
                    SqlCommand cmd = new SqlCommand(commandText, connection);

                    await connection.OpenAsync();
                    using (DbDataReader reader = await cmd.ExecuteReaderAsync())
                    {
                        while (await reader.ReadAsync())
                        {
                            var sqlresult = new SomeModel
                            {
                                AppName = reader["AppName"].ToString(),
                                AppState = reader["AppState"].ToString(),
                                AppCreatedAt = Convert.ToDateTime(reader["AppCreatedAt"]),
                                AppUpdatedAt = Convert.ToDateTime(reader["AppUpdatedAt"]),
                                AppFoundation = Convert.ToInt32(reader["AppFoundation"]),
                                SpaceName = reader["SpaceName"].ToString(),
                                OrgName = reader["OrgName"].ToString(),
                            };

                            data.Add(sqlresult);
                        }

                    }
                }
                finally
                {
                    connection.Close();
                }

                return data;
            }
        }
    }


    public Task StopAsync(CancellationToken cancellationToken)
    {
        //Timer does not have a stop. 
        _timer?.Change(Timeout.Infinite, 0);
        return Task.CompletedTask;
    }
}

收到以下錯誤：

Newtonsoft.Json.JsonSerializationException：檢測到類型為“System.Runtime.CompilerServices.AsyncTaskMethodBuilder 1+AsyncStateMachineBox 1[System.Collections.Generic.List`1[TestApp.Models.SomeModel],TestApp.Services”的屬性“task”的自引用循環.DBChangeService+d__11]'。

我已經嘗試了幾乎所有可以在 StackOverflow 上找到的可能解決方案，但都沒有奏效

在啟動時：

services.AddMvc().AddJsonOptions(options =>
            {
                options.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver();
                options.SerializerSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore;
            });

在我班級的標題上，我嘗試使用[JsonObject(IsReference = true)]但沒有任何想法對我[JsonObject(IsReference = true)] ？

模型：

[JsonObject(IsReference = true)]
    public partial class SomeModel
    {
        public string AppName { get; set; }
        public string AppState { get; set; }
        public DateTime AppCreatedAt { get; set; }
        public DateTime AppUpdatedAt { get; set; }
        public int AppFoundation { get; set; }
        public string OrgName { get; set; }
        public string SpaceName { get; set; }
    }

Answer 1

_hubcontext.Clients.All.SendAsync("SBSystemBrodcasting", SBSystemApps());

問題是您正在使用Task調用SendAsync 。 這不是 API 的設計用途。 您應該傳遞要使用的實際有效負載。

有多種方法可以解決這個問題。 一種是使用：

_hubcontext.Clients.All.SendAsync("SBSystemBrodcasting", SBSystemApps().GetAwaiter().GetResult());

盡管我建議您閱讀本指南，了解為什么它不是一個好主意。

另一種方法是更改SBSystemApps使其同步（即返回List<T>而不是Task<List<T>> 。