SmtpClient.SendAsync Calls are Automatically Cancelled

Question

Whenever I call smtpClient.SendAsync(...) from within my ASP.NET MVC application, the asynchronous requests are automatically cancelled, even though SendAsyncCancel() is never called.

Synchronous .Send(...) requests, on the other hand, go through just fine.

My EmailService service wrapper handles sending asynchronous email with SmtpClient from within my ASP.NET MVC 3 application. A service instance is injected into each MVC controller by StructureMap, which wraps a new SmtpClient instance in a using (...) { } statement.

Here is my EmailService.SendAsync wrapper method for SmtpClient:

public void SendAsync(EmailMessage message)
{
    try
    {
        using (var smtpClient = new SmtpClient(_cfg.Host, _cfg.Port)
        {
            EnableSsl = _cfg.EnableSsl,
            Credentials = _credentials
        })
        {
            smtpClient.SendCompleted += new SendCompletedEventHandler(Email_OnCompleted);

            var mailMessage = new MailMessage(message.From, message.To)
                                {
                                    Subject = message.Subject,
                                    Body = message.Body
                                };

            smtpClient.SendAsync(mailMessage, message);

            _logger.Info(string.Format("Sending async email to {0} with subject [{1}]", message.To, message.Subject));
        }
    }
    catch (Exception ex)
    {
        _logger.Error("Async email error: " + ex);
        throw;
    }

}

Here is my Email_OnCompleted delegate:

public void Email_OnCompleted(object sender, AsyncCompletedEventArgs e)
{
    var mail = (EmailMessage)e.UserState;

    if (e.Error != null)
    {
        _logger.Error(string.Format("Error sending email to {0} with subject [{1}]: {2}", mail.To, mail.Subject, e.Error));
    }
    else if (e.Cancelled)
    {
        _logger.Warn(string.Format("Cancelled email to {0} with subject [{1}].", mail.To, mail.Subject));
    }
    else
    {
        _logger.Info(string.Format("Sent email to {0} with subject [{1}].", mail.To, mail.Subject));
    }
}

Why are async emails being cancelled, but synchronous emails go through just fine? Could it be a dispose issue?

score 19 · Accepted Answer · edited May 23 '17 at 12:23

19

It can definitely be a dispose issue. When you dispose the client it cancels any outstanding async operations.

You should dispose the client in Email_OnCompleted.

An SO post on where to dispose: Dispose SmtpClient in SendComplete?

edited May 23 '17 at 12:23

Community

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answered Nov 29 '11 at 19:56

Anders Abel

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3

Fantastic! I ripped out the using (...) statement and async sending now works perfectly. Thanks, Anders! – Petrus Theron Nov 29 '11 at 20:17
Is it possible that the controller will be disposed of along with _logger before the async send completes, resulting in a null reference exception on the ILogger instance? – Petrus Theron Nov 29 '11 at 20:52
2

More accurately... "When you dispose the client it cancels any outstanding async operations." = When you dispose of the SmtpClient it cancels any outstanding async operations for the instance of the class. – Paul Zahra Dec 14 '15 at 15:42
really helpful. i was wondering about this error and found this – Mohsan Jan 26 '16 at 06:47

SmtpClient.SendAsync Calls are Automatically Cancelled

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