Can't stop service

Question

Dear Gurus!
I create service with next code:
public void AddEventChanges()
lbNames.Items.Clear();
SqlDependency.Stop(connectionString);
SqlDependency.Start(connectionString);
using (SqlConnection connection = new SqlConnection(connectionString))
using (SqlCommand cmd = new SqlCommand("Select id_event,name,description from dbo.events", connection))
cmd.Notification = null;
SqlDependency dependency = new SqlDependency(cmd);
dependency.OnChange += new OnChangeEventHandler(WaitChange);
connection.Open();
using (SqlDataReader reader = cmd.ExecuteReader())
while (reader.Read())
lbNames.Items.Add(reader.GetString(1));
connection.Close();
And try to stop it with next code:
[OperationContract]
public void StopTrackingChanges()
SqlDependency.Stop(connectionString);
But service don't stop 
Sometimes it stopped and sometimes it don't. What I am doing wrong?

Answer 1

I made a working example.
Took as a basis the example of the implementation of asynchronous
read from the database   from Wenlong Dong's Blog, adapted it for notification with the use
of service broker. Thank you all.