Question

我正尝试使用以下代码每秒从数据库读取数据：

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;
    }
}

出现以下错误：

Newtonsoft.Json.JsonSerializationException：自引用循环检测到类型为“任务”的属性 'System.Runtime.CompilerServices.AsyncTaskMethodBuilder 1+AsyncStateMachineBox 1 [System.Collections.Generic.List`1 [TestApp.Models.SomeModel]，TestApp.Services.DBChangeService + d__11]'。

我已经尝试了我可以在StackOverflow上找到的几乎所有可能的解决方案，但是都无法正常工作

在启动时：

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

在班级标题上，我尝试使用[JsonObject(IsReference = true)]，但是没有任何想法对我有用吗？

型号：

[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());

问题是您正在使用SendAsync来调用Task。这不是设计使用API的方式。您应该通过要使用的实际有效载荷。

有多种解决方法。一种是使用：

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

尽管我建议阅读this guidance，以了解为什么它不是一个好主意。

另一种方法是更改SBSystemApps使其同步（即返回List<T>而不是Task<List<T>>。

从SQL Server读取数据时检测到自引用循环

1 个答案: