Cannot access a closed Stream

Hey guys, i am geting this error: Cannot access a closed Stream, while trying to send an attach email.

here my code:

 

public MemoryStream Ms
    {
        get { return ms; }
        set { ms = value; }
    }


 public string SendMail()
    {
        string erorr = "";
        try
        {
            System.Net.Mail.MailMessage MailMass = new System.Net.Mail.MailMessage();

            MailMass.From = new System.Net.Mail.MailAddress("ben@intercity.jcwebhostza.com");
            MailMass.To.Add(new System.Net.Mail.MailAddress(To));
            MailMass.Subject = Subject;
            MailMass.Body = Body;
            MailMass.IsBodyHtml = true;

            MailMass.ReplyTo = new System.Net.Mail.MailAddress("ben@intercity.jcwebhostza.com");
            MailMass.Priority = System.Net.Mail.MailPriority.Normal;
            System.Net.Mail.SmtpClient smtpC = new System.Net.Mail.SmtpClient();
            System.Net.NetworkCredential basicAuthenticationInfo = new System.Net.NetworkCredential("ben@intercity.jcwebhostza.com", "ben01int");

            if (Ms != null && Pdf_name != "")
            {
                MemoryStream open_ms = Ms;
                open_ms.Position = 0;
                using (Attachment att = new Attachment(open_ms, Pdf_name, MediaTypeNames.Application.Pdf))
                {
                    {
                        MailMass.Attachments.Add(att);
                    }
                }
            }

            smtpC.Host = "mail.intercity.jcwebhostza.com";
            smtpC.Port = 25;
            smtpC.UseDefaultCredentials = false;
            smtpC.Credentials = basicAuthenticationInfo;

            smtpC.Send(MailMass);
        }
        catch (Exception E)
        {
            erorr = E.Message.ToString();
        }

        return erorr;

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please help