Must manually close StreamWriter in Usings?

Question

At the 2nd iteration, the "file being used" error occurs at the "using (StreamWriter" line. Even though StreamWriter is supposed to auto-close itself after going out of the usings.

EDIT 1: Real Code

Note: mails is a List<MailMessage> (instantiated with From/To addresses)

foreach (var x in mails)
{

                x.Subject = "Updated Google Exchange Rates (" +
                            DateTime.Now.ToString(new CultureInfo("en-US")) +
                            ")";

                StringBuilder emailBody = new StringBuilder();
                emailBody.AppendLine("abc"); //<-- simplified 
                x.Body = emailBody.ToString();

                _txtName = x.Subject.Replace(...); //Replaces invalid file-name chars

                //Note that _txtName will always be unique due to x.Subject's dependency on DateTime
                using (StreamWriter sw = new StreamWriter("./Exchange Rate History/" + _txtName))
                {
                    sw.WriteLine(emailBody);
                    sw.Close();
                }

                Attachment attachment = new Attachment("./Exchange Rate History/" + _txtName);
                attachment.Name = _txtName;
                x.Attachments.Add(attachment);

                SmtpClient smtpClient = new SmtpClient("...")
                {
                    Credentials = new System.Net.NetworkCredential("", ""),
                    Port = 25
                };

                smtpClient.Send(x);
                smtpClient.Dispose();
}

I had to add "sw.Close();" At before the end of the "usings" for this loop to work. Why?

EDIT 2: Oh no! sw.Close() stopped working! "file being used" again.

Answer 1

Found the problem, and its not StreamWriter not closing itself after usings.

Chris Sinclair is right about DateTime not guaranteeing an unique file name. If your for-loop is short (therefore, fast), you can end up with duplicate names, which is what happened in my case.

For 5 emails, _txtName generated 5 identical file names, meaning I ended up with one file in the end since StreamWriter overwrites by default.

Also, I forgot to use x.Attachments.Dispose(); at the end of each loop. So when it re-iterated, x.Attachments.add() is still trying attach the same file (upload time) while StreamWriter begins to write to the same file cause _txtName generated duplicate names due to DateTime being slower than the for-loop.

TL;DR: The for-loop is too fast too furious.