SQLDependency_OnChange-Event fires only one single Time

Question

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?

Here a little code piece:

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();
                }                    
            }
        }
    }

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
    }

Where is the problem?

Answer 1

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/Using-SqlDependency-for-data-change-events

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

Answer 2

After changes happened to the database at the first time, you have to execute the command again and re-subscribe to the event.

The following code is working for me.

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();

    }
}

Answer 3

In your private void dep_OnChange(object sender, SqlNotificationEventArgs e) method after you unsubscribe the dep_OnChange event you should call the private void GetStates() again, to initialize the dep.OnChange event again.

Answer 4

In GetStates() :

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

these lines should be executed only when registering the sql dependency for first time.

Restrict them when you call the method from OnChange event.

Answer 5

Not sure if that is your problem but you dispose the command right after you have created it:

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

It looks like a bug.

Answer 6

Look my friend:

dep.OnChange -= this.dep_OnChange;

you un-fired your event; which is not true; just delete this line;