Memory leak when using windows service with timer and sql server - memory-leaks

Question

I have windows service that periodically checks the database every 10 seconds, retrieves the mails not yet sent and performs the task for pending mails.
The problem is that the memory increases step by step. Please help if any adjustment needed.
My code visual basic:
Private Sub TimerMail_Elapsed(sender As Object, e As Timers.ElapsedEventArgs) Handles TimerMail.Elapsed
Dim SqlConnection As New SqlConnection("Data Source=.;Initial Catalog=xxx;Integrated Security=True")
Dim cmd As New System.Data.SqlClient.SqlCommand
cmd.CommandType = System.Data.CommandType.Text
cmd.CommandText = "SELECT DISTINCT UserName, objet, contenu, email FROM message WHERE envoimail='false'"
SqlConnection.Open()
cmd.Connection = SqlConnection
Dim monReader As SqlDataReader = cmd.ExecuteReader()
While monReader.Read
Try
EnvoiMail(monReader("objet"), monReader("contenu"), monReader("email"), mailport, serveur, username, password, True)
Dim SqlConnection2 As New SqlConnection("Data Source=.;Initial Catalog=CGP;Integrated Security=True")
Dim cmd2 As New System.Data.SqlClient.SqlCommand
cmd2.CommandType = System.Data.CommandType.Text
cmd2.CommandText = "UPDATE destinataire SET envoimail='true' WHERE UserName='" & monReader("UserName") & "'"
SqlConnection2.Open()
cmd2.Connection = SqlConnection2
cmd2.ExecuteNonQuery()
Catch ex As Exception
logger.Error(ex.Message)
End Try
Threading.Thread.Sleep(60000)
End While
End Sub

Answer 1

What you have is not a resource leak, but rather delayed finalization of resources, due to non-deterministic nature of garbage collection in .Net. There are quite a few instances of SqlConnection objects created in your code -- in the reader loop you create a new connection for every row in your SELECT query results. You have to make sure you close SqlConnection. Every instance of SqlConnection should look like this:
Using connection As New SqlConnection(connectionString)
connection.Open()
' Do work here; connection closed on following line.
End Using
The Using statement will ensure connection is closed when execution goes outside of the Using block. See more details in MSDN.

Answer 2

Inside my solution I used COM3 port to connect the windows service to modem device and using GSMComm library.
after several tests, i constat that my problem is not located on sending mail function, but in senidng sms.
The service is stable and every think is going well, but after the service is running (COM3 port open by the service programme) and device unplagued suddenly, the service still running but the memory increased potentially from some Mo to more than Go.
comm = New GsmCommMain(3, 10000, 30000)
If comm.IsConnected Then
If comm.IsOpen Then
'------------
Else
Me.Stop()
End If
Else
Me.Stop()
End If