attacher un element en vb.net via outlook

salut tous le monde.

je peux pas attacher des document txt ou doc ou xls quant je veux envoyé des email en vb.net via outlook

le programme m'a donnée ce message d'erreur :

http://tryimg.com/u/d3d25dedde.jpg

c'est mon code :

Code:

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Try
            Dim olApp As Outlook.Application
            olApp = CreateObject("Outlook.Application")
            Dim olNs As Outlook.NameSpace
            olNs = olApp.GetNamespace("MAPI")
            olNs.Logon()
            Dim olItem As Outlook.ContactItem
            olItem = olApp.CreateItem(Outlook.OlItemType.olContactItem)
            With olItem
                .FullName = "red_one"
                .CompanyName = "my company"
                .Email1Address = "red_one_hola@hotmail.com"
                .JobTitle = "Developer"
            End With
            olItem.Save()

            Dim olAppt As Outlook.AppointmentItem
            olAppt = olApp.CreateItem(Outlook.OlItemType.olAppointmentItem)

            olAppt.Start = Now()

            Dim olMail As Outlook.MailItem
            olMail = olApp.CreateItem(Outlook.OlItemType.olMailItem)

            olMail.To = olItem.Email1Address
            olMail.Subject = "test .........."
            olMail.Body = "test pour attacher un element "
       
           'Le problème dans ce code : 
               Dim att As String =Application.StartupPath + "\test.txt"
            Dim at = New Attachment(att)
            olMail.Attachments.Add(at, "txt", "", "test")
            olMail.Send()

            MsgBox(" Message a éte envoyé ", vbMsgBoxSetForeground)
            olNs.Logoff()
            olNs = Nothing
            olMail = Nothing
            olAppt = Nothing
            olItem = Nothing
            olApp = Nothing

        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try

merci :)