SqlDependency仅在订阅时触发

我正在尝试SqlDependancy在SignalR项目中使用,但我似乎无法让OnChanged事件多次触发.它最初在订阅事件上触发,但在更改底层数据库后它永远不会再触发.我省略了SignalR和控制器代码,因为问题似乎在于存储库类.SqlDependancy.Start()在我Global.asax班上宣布.

从SQL服务器观察,我可以看到在我的应用程序启动时创建通知队列,并在关闭时终止.

    public IEnumerable NotifyAllClients()
    {
        List visitors = new List();
        using (var connection = new SqlConnection(new VisitorLogEntities().Database.Connection.ConnectionString))
        {
            using (var command = new SqlCommand(@"SELECT * FROM dbo.Visitors", connection))
          //using (var command = new SqlCommand(@"SELECT [Id],[AgreeToTerms],[Base64Image],[CheckInDate],[CheckOutTime],[Company],[CountryOfOrigin],[email],[FirstName],[LastName],[IsInBuilding],[MeetingSubject],[MeetingTime],[PatriotHost],[phone],[title] FROM dbo.Visitors", connection))
            {

                var dependency = new SqlDependency(command);
                dependency.OnChange += Database_OnChange;

                if (connection.State == System.Data.ConnectionState.Closed)
                    connection.Open();

                var reader = command.ExecuteReader();

                while (reader.Read())
                {
                 ////compile visitor objects
                 ////visitors.add(new Visitor());
                }
            }
            return visitors.OrderByDescending(x => x.CheckInDate);
        }
    }

    private void Database_OnChange(object sender, SqlNotificationEventArgs e)
    {
        //var dependency = (SqlDependency)sender;
        //dependency.OnChange -= Database_OnChange;

        ////this fires once, with the Type of 'Subscribe', but then never fires on CRUD changes
        if (e.Type == SqlNotificationType.Change)
        {
            VisitorHub.SendVisitors();
        }
        //NotifyAllClients();
    }

编辑:上面注释掉的代码行表示正常工作所需的更改.

从msdn http://msdn.microsoft.com/en-US/library/a52dhwx7(v=vs.80).aspx查看此示例.下载VS2005_General_en-us.pdf.页面24636,"在Windows应用程序中使用SqlDependency"是原始链接所导致的部分.请特别注意观察者应用程序中的步骤12和13.在步骤12中,您将看到删除onChange事件,然后调用步骤13再次设置它.

此外,我认为由于你的sql语句本身你看到了不良行为.sql语句必须遵循一些规则.有关详细信息,请参阅https://technet.microsoft.com/en-us/library/ms181122(v=sql.105).aspx.特别是写通知查询部分."该语句可能不使用星号(*)或table_name.*语法来指定列."