如何使用sqldependency绑定事件并在C＃中退订

Question

i have problem to bind event with custom parameter, its not unsubscribe while subscribe next time. basically when 1st time subscribe event, its hold the 1st event and make new event when 2nd time subscribe. mean there are 2 events and 3rd time there will be 3 events and so on. but i need 1 event only each time while subscribe and unsubcribe last event.

1. dependency_Update function

public void dependency_Update(Guid clientid)
{
    SqlDependency.Stop(connString);
    using (SqlCommand command = new SqlCommand(SQL.newemailmessagescountbyclient_sql(), connection))
    {
       connection.Open();
       command.Parameters.Add(new SqlParameter("@clientid", clientid));
       command.Notification = null;
       SqlDependency.Start(connString);
       SqlDependency dependency = new SqlDependency(command);
       dependency.OnChange += new OnChangeEventHandler((s, e) => dependency_OnChange(s, e, dependency, clientid));
       dependency.OnChange -= new OnChangeEventHandler((s, e) => dependency_OnChange(s, e, dependency, clientid));
       var reader = command.ExecuteReader();
       connection.Close();

     }
 }

2. dependency_OnChange event

public void dependency_OnChange(object sender, SqlNotificationEventArgs e, SqlDependency dependency, Guid clientid)
{
   if (e.Type == SqlNotificationType.Change)
   {
     //subscribe function again
     dependency_Update(clientid);
   }
}

can any one have solution for that, please help me, i appreciate your valuable time, thanks

Answer 1

This block

dependency.OnChange += new OnChangeEventHandler((s, e) => dependency_OnChange(s, e, dependency, clientid));
dependency.OnChange -= new OnChangeEventHandler((s, e) => dependency_OnChange(s, e, dependency, clientid));

Doesn't do anything useful. First, its intention is strange - you are trying to subscribe and immediately unsubscribe. Second, it doesn't work as expected, because, despite looking the same - your two anonymous function handlers are different. In effect - you never really unsubscribe your handler from OnChange event. If you want to use anonymoys function, do it like that:

SqlDependency dependency = new SqlDependency(command);
OnChangeEventHandler handler = null;
handler = (s, e) => dependency_OnChange(s, e, clientid, handler);
dependency.OnChange += handler;

public void dependency_OnChange(object sender, SqlNotificationEventArgs e, Guid clientid, OnChangeEventHandler handler) {
    var dependency = (SqlDependency)sender;
    if (e.Type == SqlNotificationType.Change) {
        dependency.OnChange -= handler;
        //subscribe function again
        dependency_Update(clientid);
    }
}

Or like this:

OnChangeEventHandler handler = null;
handler = (s, e) => {
      dependency.OnChange -= handler;
      dependency_OnChange(s, e, clientid);                    
};
dependency.OnChange += handler;