c# - Must manually close StreamWriter in Usings? -

at 2nd iteration, "file being used" error occurs @ "using (streamwriter" line. though streamwriter supposed auto-close after going out of usings.

edit 1: real code

note: mails list<mailmessage> (instantiated 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 _txtname unique due 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 add "sw.close();" @ before end of "usings" loop work. why?

edit 2: oh no! sw.close() stopped working! "file being used" again.

found problem, , not streamwriter not closing after usings.

chris sinclair right datetime not guaranteeing unique file name. if for-loop short (therefore, fast), can end duplicate names, happened in case.

for 5 emails, _txtname generated 5 identical file names, meaning ended 1 file in end since streamwriter overwrites default.

also, forgot use x.attachments.dispose(); @ end of each loop. when re-iterated, x.attachments.add() still trying attach same file (upload time) while streamwriter begins write same file cause _txtname generated duplicate names due datetime being slower for-loop.

tl;dr: for-loop fast furious.