Σε VB6 είχα φτιάξει την παρακάτω function

Function SendMail(EM_TO, Em_CC, EM_BCC, EM_Subject, EM_Body, EM_Attachment As String, Display As Boolean)
    Dim objOA As Outlook.Application
    Dim objMI As Outlook.MailItem
    Dim obgAtt As Outlook.Attachments
    Set objOA = New Outlook.Application
    Set objMI = objOA.CreateItem(olMailItem)
    If EM_TO <> "" Then objMI.To = EM_TO
    If Em_CC <> "" Then objMI.CC = Em_CC
    If EM_BCC <> "" Then objMI.BCC = EM_BCC
    If EM_Subject <> "" Then objMI.Subject = EM_Subject
    If EM_Body <> "" Then objMI.Body = EM_Body
    If EM_Attachment <> "" Then objMI.Attachments.Add EM_Attachment, 1, , EM_Attachment

    If Display Then
        objMI.Display
    Else
        objMI.Send
    End If
    Set objOA = Nothing
    Set objMI = Nothing
End Function

Κλήση της function
Private Sub Button1_Click()
 On Error GoTo MSGERR
 
 Dim MailSubject As String, MailBody As String, Response As Long, showMail As Boolean
 Dim SavedDirectory as string

SavedDirectory="C:\MyFolder\mytext.txt"

 MailSubject = "θεμα"

 MailBody = "Κειμενο"

 Response = MsgBox("Εμφάνιση ....", vbInformation + vbYesNo + vbDefaultButton2, AppDescription)
  If Response = vbYes Then
    showMail = True
  Else
    showMail = False
  End If
 SendMail MailAddress, "", "", MailSubject, MailBody, SavedDirectory & "\" & exFile, showMail
 End If
 Exit Sub
MSGERR:
  MsgBox err.Number & " " & err.Description, vbInformation + vbOKOnly, AppDescription
End Sub

Στα reference πρέπει να δηλωθεί το Microsoft Outlook object library

Νομίζω ότι είναι εύκολο να το τροποποιήσεις σε C#

Ακόμα και σε VB6 υπήρχαν πιο απλοί τρόποι να στείλει κανείς ένα email, π.χ. το CDO. Ευτυχώς, στο .NET 2.0 τα πράγματα είναι πολύ εύκολα και γίνονται μέσω της κλάσης System.Net.Mail.MailMessage, χωρίς να απαιτείται το Outlook ή κάποιος άλλος MAPI client. Το documentation της κλάσης περιέχει ένα παράδειγμα για το πως να στείλει κανείς μήνυμα με attachment.

public static void CreateMessageWithAttachment(string server)
{
    // Specify the file to be attached and sent.
    // This example assumes that a file named Data.xls exists in the
    // current working directory.
    string file = "data.xls";
    // Create a message and set up the recipients.
    MailMessage message = new MailMessage("<A href="mailto:[email protected]">[email protected]</A>","<A href="mailto:[email protected]">[email protected]</A>","Quarterly data report.","See the attached spreadsheet.");

    // Create  the file attachment for this e-mail message.
    Attachment data = new Attachment(file, MediaTypeNames.Application.Octet);
    // Add the file attachment to this e-mail message.
    message.Attachments.Add(data);
    //Send the message.
    SmtpClient client = new SmtpClient(server);
    // Add credentials if the SMTP server requires them.
    client.Credentials = CredentialCache.DefaultNetworkCredentials;
    client.Send(message);
    data.Dispose();
}