public string BuildMeetingRequest(DateTime start, DateTime end, string attendees, string organizer, string subject, string description, string UID, string location)
    {
        System.Text.StringBuilder sw = new System.Text.StringBuilder();

        sw.AppendLine("BEGIN:VCALENDAR");
        sw.AppendLine("VERSION:2.0");
        sw.AppendLine("METHOD:REQUEST");
        sw.AppendLine("BEGIN:VEVENT");
        sw.AppendLine(attendees);
        sw.AppendLine("CLASS:PUBLIC");
        sw.AppendLine(string.Format("CREATED:{0:yyyyMMddTHHmmssZ}", DateTime.UtcNow));
        sw.AppendLine("DESCRIPTION:" + description);
        sw.AppendLine(string.Format("DTEND:{0:yyyyMMddTHHmmssZ}", end));
        sw.AppendLine(string.Format("DTSTAMP:{0:yyyyMMddTHHmmssZ}", DateTime.UtcNow));
        sw.AppendLine(string.Format("DTSTART:{0:yyyyMMddTHHmmssZ}", start));
        sw.AppendLine("ORGANIZER;CN=\"NAME\":mailto:" + organizer);
        sw.AppendLine("SEQUENCE:0");
        sw.AppendLine("UID:" + UID);
        sw.AppendLine("LOCATION:" + location);
        sw.AppendLine("SUMMARY;LANGUAGE=en-us:" + subject);
        sw.AppendLine("BEGIN:VALARM");
        sw.AppendLine("TRIGGER:-PT720M");
        sw.AppendLine("ACTION:DISPLAY");
        sw.AppendLine("DESCRIPTION:Reminder");
        sw.AppendLine("END:VALARM");
        sw.AppendLine("END:VEVENT");
        sw.AppendLine("END:VCALENDAR");

        return sw.ToString();
    }

meetingInfo = BuildMeetingRequest(start, end, attendees, organizer, subject, description, UID, location);

        System.Net.Mime.ContentType mimeType = new System.Net.Mime.ContentType("text/calendar; method=REQUEST");
        AlternateView ICSview = AlternateView.CreateAlternateViewFromString(meetingInfo,mimeType);
        MailMessage message = new MailMessage();

        message.To.Add(to);
        message.From = new MailAddress(from);
        message.AlternateViews.Add(ICSview);

        SmtpClient client = new SmtpClient();
        client.Send(message);