如何解决在使用ASP.Net的信号器中执行`error`之前必须调用`SqlDependency.Start（）？

我正在尝试使用信号器进行实时网页更新，这是我第一次尝试。我的ajax命令正在调用我的服务器代码。但它不会从服务器端返回任何内容

这是剧本

<script>

    $(function () {

        //Proxy created on the fly
        var job = $.connection.myHub;

        //Declare a function on the hub so the server can invoke it
        job.client.displaystatus = function () {
            getData();
        };

        //Start the connection
        $.connection.hub.start();
        getData();

    });

    function getData() {
        //alert('ok');
        var $tb1 = $('#tb1');

        $.ajax({
            url: 'Default.aspx/GetData',
            contentType: "application/json; charset-utf-8",
            dataType: "json",
            type: "POST",
            success: function (data) {
                alert('Success:' + data);
            },
            error:function(){
                alert('Failed');
            }
        });
    }
</script>

MyHub.cs文件

[WebMethod]
public static IEnumerable<Product> GetData()
{
    string cs = ConfigurationManager.ConnectionStrings["TestDB"].ConnectionString;

    using (SqlConnection con = new SqlConnection(cs))
    {
        //Make sure the command object does not already have

        using (SqlCommand cmd = new SqlCommand("SELECT [Id],[Name],[PricDecimal],[QuantDecimal]FROM [TestDatabase].[dbo].[Product]"))
        {
            cmd.Connection = con;
            SqlDependency dependency = new SqlDependency(cmd);
            dependency.OnChange += new OnChangeEventHandler(dependency_OnChange);

            if (con.State == ConnectionState.Closed)
                con.Open();


            using (var reader = cmd.ExecuteReader())
                return reader.Cast<IDataRecord>()
                    .Select(x => new Product()
                    {
                        id = x.GetInt32(0),
                        Name = x.GetString(1),
                        PricDecimal = x.GetDecimal(2),
                        QuantDecimal = x.GetDecimal(3)
                    }).ToList();
        }
    }

}


private static void dependency_OnChange(object sender,SqlNotificationEventArgs e)
{
    MyHub.show();
}

public class Product
{
    public int id { get; set; }
    public string Name { get; set; }
    public decimal PricDecimal { get; set; }
    public decimal QuantDecimal { get; set; }
}

public class MyHub : Hub
{    

    public static void show()
    {
        IHubContext context = GlobalHost.ConnectionManager.GetHubContext<MyHub>();
        context.Clients.All.displayStatus();
    }
}

[assembly: OwinStartup(typeof(Startup))]

public class Startup
{
    public void Configuration(IAppBuilder app)
    {
    // For more information on how to configure your application, visit http://go.microsoft.com/fwlink/?LinkID=316888
        app.MapSignalR();
    //string cs = ConfigurationManager.ConnectionStrings["TestDB"].ConnectionString;
    //System.Data.SqlClient.SqlDependency.Start(cs);
    }
}

错误消息

例外类型 : “系统无效操作异常”

信息 : 使用SqlDependency而不提供选项值时，必须在执行添加到SqlDependency实例的命令之前调用SqlDependency.Start（）

---

我认为为时已晚，但您需要在新建SqlDependency之前编写这一行：

SqlDependency.Start("connection string");

---

没有人知道信号器？我不敢相信，这是堆栈溢出