Function GetHtmlPageSource(ByVal url As String, Optional ByVal username As String = Nothing, Optional ByVal password As String = Nothing) As String
        Dim st As System.IO.Stream
        Dim sr As System.IO.StreamReader

        Try
            ' invia una Web request
            Dim req As System.Net.WebRequest = System.Net.WebRequest.Create(url)

            ' se sono specificati username/password usa le credenziali
            If Not username Is Nothing AndAlso Not password Is Nothing Then
                req.Credentials = New System.Net.NetworkCredential(username, password)
            End If

            ' ricava la risposta e legge lo stream coi risultati
            Dim resp As System.Net.WebResponse = req.GetResponse
            st = resp.GetResponseStream
            sr = New System.IO.StreamReader(st)

            Return sr.ReadToEnd
        Catch ex As Exception
            Return ""
        Finally
            sr.Close()
            st.Close()
        End Try
    End Function