问题
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?
回答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.
回答2:
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.
回答3:
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();
}
}
回答4:
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.
回答5:
Look my friend:
dep.OnChange -= this.dep_OnChange;
you un-fired your event; which is not true; just delete this line;
回答6:
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.
来源：https://stackoverflow.com/questions/9430896/sqldependency-onchange-event-fires-only-one-single-time