C ＃ use smtpclient The sendasync method fails to send mail, and always returns canceled

problem ：

call SmtpClient.SendAsync, stay SendCompleted The callback function of always gets e.Cancelled by true.
And then I tested it , Same code , Just put SmtpClient.SendAsync Change to SmtpClient.Send Method , Mail sent successfully .

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reason ：

Send... Before sending email SmtpClient The instance of is released . because SendAsync Is an asynchronous operation , After calling this method, you just push the email to SMTP The server , The sending operation has not been completed yet , When SmtpClient When the instance is released , It cancels any outstanding asynchronous operations , So this email was also cancelled . therefore , The right thing to do is to put SmtpClient The instance of is placed in SendCompleted And then release ,MailMessage Examples are the same , Otherwise it will be reported

System.Net.Mail.SmtpException: Failed to send email . —> System.ObjectDisposedException: The released object cannot be accessed .

Error of .

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Reference resources ：

SmtpClient.SendAsync Calls are Automatically Cancelled

Dispose SmtpClient in SendComplete?