SQLDependency_OnChange-Event仅触发一次 [英] SQLDependency_OnChange-Event fires only one single Time

问题描述

我正在使用SQLDependency来通知我数据库中是否有更改.程序启动后，它可以正常工作.当我进行第一次更改时，将触发该事件.哇，太好了.但是，如果我进行了第二次更改，则该事件不会再次触发.我搜索了我认为的所有网络，但未找到有关此问题"的任何信息.仅发现OnChange-Event在循环中触发的问题.谁能帮我吗?

I'm working with SQLDependency to notify me if there is a change in the Database. After Program Start-Up it works just fine. When I make a first change the Event fires. Wohoo... that's great. But if I made a second change the event doesn't fire again. I've searched all the web I think but haven't found anything about THIS Problem. Only found problems where the OnChange-Event fires in a Loop. Can anyone help me?

这里有一段代码:

private void GetStates()
    {
        if (!DoesUserHavePermission())
            return;

        SqlDependency.Stop(con);
        SqlDependency.Start(con);

        using (SqlConnection cn = new SqlConnection(con))
        {
            using (SqlCommand cmd = cn.CreateCommand())
            {
                cmd.CommandType = CommandType.Text;
                cmd.CommandText = "SELECT Bla, Bla2, ..FROM dbo.[BLA3]"

                cmd.Notification = null;
                cmd.Dispose();

                SqlDependency dep = new SqlDependency(cmd);
                dep.OnChange += new OnChangeEventHandler(dep_OnChange);

                cn.Open();

                using (SqlDataReader dr = cmd.ExecuteReader())
                {
                    state.Clear(); //In this Case "state" is a List<string>
                    while (dr.Read())
                    {
                        state.Add(dr.GetString(0) + "|" + dr.GetInt32(3));
                    }
                    dr.Dispose();
                    dr.Close();
                }                    
            }
        }
    }

我的OnChange-Event看起来像这样:

my OnChange-Event looks like this:

private void dep_OnChange(object sender, SqlNotificationEventArgs e)
    {
        SqlDependency dep = sender as SqlDependency;
        dep.OnChange -= this.dep_OnChange;

        using (SqlConnection cn = new SqlConnection(con))
        {
            using (SqlCommand cmd = cn.CreateCommand())
            {
                cmd.CommandType = CommandType.Text;
                cmd.CommandText = "SELECT Bla, Bla2, ..FROM dbo.[BLA3]";

                cmd.Notification = null;

                if (e.Type == SqlNotificationType.Change)
                {
                    if (cn.State != ConnectionState.Open)
                    {
                        cn.Open();
                    }

                    using (SqlDataReader dr = cmd.ExecuteReader())
                    {
                        state.Clear(); // Clear and Refill the stringlist "state"
                        while (dr.Read())
                        {
                            state.Add(dr.GetString(0) + "|" + dr.GetInt32(3));
                        }
                    }
                }
                cn.Close();
            }
        }
        this.GetStates(); //to go ahead and wait for a new change
    }

问题出在哪里?

推荐答案

我也遇到了这个问题.您需要创建一个新的SqlDependency实体(在OnChange事件中取消现有的实体的订阅后)，然后运行新的ExecuteReader命令.我从这篇文章中得到了这个主意:

I was running into this issue as well. You need to create a new SqlDependency entity (after unsubscribing the existing one from the OnChange event) and then run a new ExecuteReader command. I got the idea from this post:

http://www.codeproject.com/Articles/12335/使用-SqlDependency-for-data-change-events

这通常是有道理的，因为一旦收到更改通知，您通常将希望重新查询数据.

This usually makes sense, as once you have been notified of a change you will normally want to re-query the data.

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