27

我正在使用 SQLDependency 来通知我数据库是否有变化。程序启动后，它工作得很好。当我进行第一次更改时，事件会触发。呜呜……太好了。但是，如果我进行第二次更改，则事件不会再次触发。我搜索了所有我认为的网络，但没有找到任何关于这个问题的信息。仅发现 OnChange-Event 在循环中触发的问题。谁能帮我？

这里有一段小代码：

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：

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？

4

6 回答 6

13

我也遇到了这个问题。您需要创建一个新的 SqlDependency 实体（在从 OnChange 事件取消订阅现有实体之后），然后运行一个新的 ExecuteReader 命令。我从这篇文章中得到了这个想法：

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

这通常是有道理的，因为一旦您收到更改通知，您通常会想要重新查询数据。

于 2013-03-20T20:52:21.547 回答
4

数据库第一次发生更改后，您必须再次执行该命令并重新订阅该事件。

以下代码对我有用。

class Program
{
    static string connectionString = "Server=.;Database=test_sql_dependency;Integrated Security=True;";

    static void Main(string[] args)
    {
        // 1. create database

        // 2. enable service broker by executing this sql command on the database.
        // alter database test_sql_dependency set enable_broker

        // 3. start sql dependency, for some sql server connection string or with queue if you want.

        //var queueName = "myFirstQueue";
        //SqlDependency.Start(connectionString, queueName);
        SqlDependency.Start(connectionString);

        // complete the rest of the steps in seperate method to be able to call it again when you need to 
        // re-subscribe to the event again, becuase by default it will be executed only one time

        RegisterSqlDependency();

        Console.WriteLine("Listening to database changes...");
        Console.ReadLine();
    }

    static void RegisterSqlDependency()
    {
        using (SqlConnection connection = new SqlConnection(connectionString))
        {
            if (connection.State != System.Data.ConnectionState.Open)
            {
                connection.Open();
            }

            // 4. create a sql command 
            // you can't say select *, and also you have to specefy the db owner (dbo.)
            SqlCommand command = new SqlCommand("select Id, Name from dbo.Employee", connection);

            // 5. create dependency and associtate it to the sql command
            SqlDependency dependency = new SqlDependency(command);

            // 6. subscribe to sql dependency event
            dependency.OnChange += new OnChangeEventHandler(OnDependencyChange);

            // 7. execute the command
            using (SqlDataReader reader = command.ExecuteReader())
            {

            }
        }
    }

    static void OnDependencyChange(object sender, SqlNotificationEventArgs e)
    {
        var InsertOrUpdateOrDelte = e.Info;



        //-----------------------------Finally-------------------------
        // after you knew that there is a change happened 
        // you have to unsubscribe the event and execute the command again and then re-subscribe to the event

        // 1. unsubscribe the event
        SqlDependency dependency = sender as SqlDependency;
        dependency.OnChange -= OnDependencyChange;

        // 2. re-subscribe to the event and execute the command again
        RegisterSqlDependency();

    }
}
于 2018-10-27T07:28:20.900 回答
3

在您private void dep_OnChange(object sender, SqlNotificationEventArgs e)取消订阅dep_OnChange事件后的方法中，您应该private void GetStates()再次调用，以再次初始化dep.OnChange事件。

于 2015-04-10T17:18:27.417 回答
1

不确定这是否是您的问题，但您在创建命令后立即处理它：

using (SqlCommand cmd = cn.CreateCommand()) 
{
  ...
  cmd.Dispose();

它看起来像一个错误。

于 2012-02-24T15:34:06.347 回答
1

在 GetStates() 中：

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

只有在第一次注册 sql 依赖项时才应执行这些行。

当您从 OnChange 事件调用方法时限制它们。

于 2015-03-03T06:05:49.393 回答
0

看我的朋友：

dep.OnChange -= this.dep_OnChange;

你取消了你的活动；这不是真的；只需删除这一行；

于 2013-11-10T13:33:42.987 回答