SQLDependency OnChange Event fires only single Time

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C#

ASP.NET

SQL

MVC

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I have a problem, which I can not understand. I want to send notification to client as soon as data is insert in [dbo].[Notifications] table. for this I am using sqldependency for rise onchange event and to sent information client with SignalR. Problem is that when i make a first Change the Event fires, But if i made a second change the Event does not Fire any more

 public class NotoficationRepository : BaseClass
    {
        public int IsNotification(int UserID)
        {
            try
            {
                private string sqlString = string.Format("SELECT COUNT_BIG(*) AS [COUNT]
                 FROM [dbo].[Notifications] 
                 WHERE UserID = '{0}' AND 
                 IsNotificationShow = '{1}'", UserID, false);

                var count = 0;
                SqlDependency.Stop(ConnectionSring);
                SqlDependency.Start(ConnectionSring);
                using (var connection = new SqlConnection(ConnectionSring))
                {
                    connection.Open();
                    using (command = new SqlCommand(sqlString, connection))
                    {
                        command.Notification = null;

                        SqlDependency dependency = new SqlDependency(command);
                        dependency.OnChange += new OnChangeEventHandler(dependency_OnChange);

                        if (connection.State == ConnectionState.Closed)
                            connection.Open();
                        count = int.Parse(command.ExecuteScalar().ToString());
                    }
                }    

                return count;    
            }
            catch
            {
                throw;
            }
            finally
            {
                base.Dispose();
            }
        }
}

OnChangeEvent

void dependency_OnChange(object sender, SqlNotificationEventArgs e)
    {
        if (e.Info == SqlNotificationInfo.Insert)
        {
            MessagesHub.SendMessages();
        }         
    }

BaseClass

public class BaseClass : IDisposable
    {
        private bool disposed = false;

        protected NotificationEntities DB;

        protected string ConnectionSring;
        public BaseClass()
        {
            DB = new NotificationEntities();
            ConnectionSring = DB.Database.Connection.ConnectionString;
        }

        ~BaseClass()
        {
            Dispose(false);
        }

        public void Dispose()
        {            
            Dispose(true);
            GC.SuppressFinalize(this);            
        }

        protected virtual void Dispose(bool disposing)
        {
            if (disposed)
                return;

            if (disposing)
            {
                if (DB != null)
                {
                    DB.Dispose();
                    DB = null;
                }
            }

            disposed = true;
        }
}

Can you help me?? what is the problem?