求助：C# Background Task仅执行一次，无限循环未触发

我实现了一个C# Background Task，用于周期性检查数据是否同步至Google Calendar，但该任务仅在应用启动时运行一次，循环内设置的断点始终未被触发，寻求技术排查方案。

问题代码 #

protected async override Task ExecuteAsync(CancellationToken stoppingToken)
{
    while (!stoppingToken.IsCancellationRequested)
    {
        string DbConnectionString = _configuration.GetValue<string>("Site:ConnectionString");

        using (IDbConnection conn = OpenConnection(DbConnectionString))
        {
            List<GoogleAccess> googleCredentials = conn.Query<Domain.GoogleAccess>("Select g.Id, g.CalendarId, g.RefreshToken, convert(nvarchar(36), g.CompanyUserId) as CompanyUserId from [dbo].[googleAccess] g", null, commandType: CommandType.Text).ToList();

            foreach (var credential in googleCredentials.Where(e => e.RefreshToken is not null).ToList())
            {
                var accessToken = RefreshToken(credential.RefreshToken);

                var allReservations = await TurnUp.Admin.Infrastructure.RestAPI.Reservation.GetAll(new Models.RestAPI.Reservation.PostModel.GetAllReservationPostModel() { });

                var covertedList = new List<EventModel>();

                foreach (var reservation in allReservations.result.items.Where(e => e.isCancelled == false).ToList())
                {
                    covertedList.Add(new EventModel
                    {
                        Id = reservation.id.ToString(),
                        Description = reservation.description,
                        Start = new EventDateTime { DateTime = DateTime.Parse(reservation.startTime.ToString()).ToString("yyyy-MM-dd'T'HH:mm:ss.fffK"), TimeZone = "Europe/Zurich" },
                        End = new EventDateTime { DateTime = DateTime.Parse(reservation.endTime.ToString()).ToString("yyyy-MM-dd'T'HH:mm:ss.fffK"), TimeZone = "Europe/Zurich" },
                        Summary = reservation.description
                    });
                }

                var allGoogleEvents = new List<EventModel>();
                RestClient restClient = new RestClient(new Uri("https://www.googleapis.com/calendar/v3/calendars/primary/events"));
                RestRequest restRequest = new RestRequest();

                restRequest.AddQueryParameter("key", $"{ApiKeys}");
                restRequest.AddParameter("maxResults", 2500);
                restRequest.AddParameter("OrderBy", "startTime");
                restRequest.AddParameter("showDeleted", "false");
                restRequest.AddParameter("timeMin", DateTime.UtcNow.AddMonths(-1).ToString("yyyy-MM-dd'T'HH:mm:ss.fffK"));
                restRequest.AddParameter("timeMax", DateTime.UtcNow.AddMonths(6).ToString("yyyy-MM-dd'T'HH:mm:ss.fffK"));
                restRequest.AddHeader("Authorization", "Bearer " + accessToken);
                restRequest.AddHeader("Accept", "application/json");

                try
                {
                    var response = restClient.Get(restRequest);

                    if (response.StatusCode == System.Net.HttpStatusCode.OK)
                    {
                        JObject calendarEvents = JObject.Parse(response.Content);
                        var test = calendarEvents["nextSyncToken"].ToString();
                        allGoogleEvents = calendarEvents["items"].ToObject<List<EventModel>>();
                    }
                }
                catch (Exception ex)
                {
                    throw new Exception("Coulden't connect to google agenda.");
                }

                var newList = covertedList.Except(allGoogleEvents).Concat(allGoogleEvents.Except(covertedList));

                foreach (var ev in newList)
                {
                    var restClientForAddingEvents = new RestClient();
                    var restRequestForAddingEvents = new RestRequest();
                    restClientForAddingEvents.BaseUrl = new System.Uri($"https://www.googleapis.com/calendar/v3/calendars/{credential.CalendarId}/events");

                    EventModel newEvent = new EventModel();

                    newEvent.Start.DateTime = DateTime.Parse(ev.Start.DateTime).ToString("yyyy-MM-dd'T'HH:mm:ss.fffK");
                    newEvent.End.DateTime = DateTime.Parse(ev.End.DateTime).ToString("yyyy-MM-dd'T'HH:mm:ss.fffK");
                    newEvent.Description = ev.Description;
                    newEvent.Id = Guid.NewGuid().ToString().Replace("-", "");

                    var model = JsonConvert.SerializeObject(newEvent, new JsonSerializerSettings
                    {
                        ContractResolver = new CamelCasePropertyNamesContractResolver()
                    });

                    restRequest.AddQueryParameter("key", $"{ApiKeys}");
                    restRequest.AddHeader("Authorization", "Bearer " + accessToken);
                    restRequest.AddHeader("Accept", "application/json");
                    restRequest.AddHeader("Content-Type", "application/json; charset=utf-8");
                    restRequest.AddParameter("application/json", model, ParameterType.RequestBody);

                    try
                    {
                        var restResponse = restClient.Post(restRequest);

                        if (restResponse.StatusCode == System.Net.HttpStatusCode.OK)
                        {

                        }
                    }
                    catch (Exception ex)
                    {
                        throw new Exception("Coulden't Create an event in google agenda.");
                    }
                }
            }
        }
        await Task.Delay(5000, stoppingToken);
    }
}

排查与修复步骤 #

• 验证后台任务注册：确认在Program.cs/Startup.cs中通过services.AddHostedService<YourSyncService>()正确注册服务，避免使用一次性实例注册方式，否则任务执行一次后会被回收。
• 捕获全局未处理异常：后台任务的未捕获异常会直接终止任务且默认无日志输出，需在外层添加异常捕获并记录日志：

  protected async override Task ExecuteAsync(CancellationToken stoppingToken)
  {
      _logger.LogInformation("同步任务启动");
      try
      {
          while (!stoppingToken.IsCancellationRequested)
          {
              // 原循环逻辑
              _logger.LogInformation("完成一次同步周期");
              await Task.Delay(5000, stoppingToken);
          }
      }
      catch (Exception ex)
      {
          _logger.LogError(ex, "同步任务发生未处理异常，已终止");
      }
  }
  

  同时确保主机配置了日志输出（如控制台日志），方便排查异常。
• 替换同步API调用为异步版本：代码中restClient.Get()和restClient.Post()是同步方法，会阻塞异步任务线程，导致调度器误判任务完成，需替换为异步重载：

  var response = await restClient.GetAsync(restRequest, stoppingToken);
  // ...
  var restResponse = await restClientForAddingEvents.PostAsync(restRequestForAddingEvents, stoppingToken);
  

• 修复请求对象复用错误：添加事件时误用了外层的restRequest（获取Google事件的请求对象），导致参数混乱引发异常，需改为使用新创建的restRequestForAddingEvents：

  // 替换原restRequest相关代码为：
  restRequestForAddingEvents.AddQueryParameter("key", $"{ApiKeys}");
  restRequestForAddingEvents.AddHeader("Authorization", "Bearer " + accessToken);
  restRequestForAddingEvents.AddHeader("Accept", "application/json");
  restRequestForAddingEvents.AddHeader("Content-Type", "application/json; charset=utf-8");
  restRequestForAddingEvents.AddParameter("application/json", model, ParameterType.RequestBody);
  

• 修正对象比较逻辑：Except方法依赖EventModel的Equals和GetHashCode实现，若未重写则会按引用比较，导致所有对象都被判定为不同，引发循环执行时间过长。需重写这两个方法：

  public class EventModel
  {
      // 原有属性
      public override bool Equals(object obj)
      {
          if (obj is not EventModel other) return false;
          return Id == other.Id && Start.DateTime == other.Start.DateTime && End.DateTime == other.End.DateTime;
      }
  
      public override int GetHashCode()
      {
          return HashCode.Combine(Id, Start.DateTime, End.DateTime);
      }
  }
  

• 检查数据库连接与查询：确认OpenConnection方法正确创建并释放连接，若数据库操作阻塞或失败，会导致任务卡在循环内某一步，需添加日志监控数据库操作的执行状态。

内容的提问来源于stack exchange，提问作者Wannes Limbourg