How to send an email with attachments using SmtpClient.SendAsync?

Question

I am using a service component through ASP.NET MVC. I would like to send the email in a asynchronous way to let the user do other stuff without having to wait for the sending.

When I send a message without attachments it works fine. When I send a message with at least one in-memory attachment it fails.

So, I would like to know if it is possible to use an async method with in-memory attachments.

Here is the sending method

    public static void Send() {

        MailMessage message = new MailMessage("from@foo.com", "too@foo.com");
        using (MemoryStream stream = new MemoryStream(new byte[64000])) {
            Attachment attachment = new Attachment(stream, "my attachment");
            message.Attachments.Add(attachment);
            message.Body = "This is an async test.";

            SmtpClient smtp = new SmtpClient("localhost");
            smtp.Credentials = new NetworkCredential("foo", "bar");
            smtp.SendAsync(message, null);
        }
    }

Here is my current error

System.Net.Mail.SmtpException: Failure sending mail.
 ---> System.NotSupportedException: Stream does not support reading.
   at System.Net.Mime.MimeBasePart.EndSend(IAsyncResult asyncResult)
   at System.Net.Mail.Message.EndSend(IAsyncResult asyncResult)
   at System.Net.Mail.SmtpClient.SendMessageCallback(IAsyncResult result)
   --- End of inner exception stack trace ---

---

Solution

    public static void Send()
    {

            MailMessage message = new MailMessage("from@foo.com", "to@foo.com");
            MemoryStream stream = new MemoryStream(new byte[64000]);
            Attachment attachment = new Attachment(stream, "my attachment");
            message.Attachments.Add(attachment);
            message.Body = "This is an async test.";
            SmtpClient smtp = new SmtpClient("localhost");
            //smtp.Credentials = new NetworkCredential("login", "password");

            smtp.SendCompleted += delegate(object sender, System.ComponentModel.AsyncCompletedEventArgs e)
            {
                    if (e.Error != null)
                    {
                            System.Diagnostics.Trace.TraceError(e.Error.ToString());

                    }
                    MailMessage userMessage = e.UserState as MailMessage;
                    if (userMessage != null)
                    {
                            userMessage.Dispose();
                    }
            };

            smtp.SendAsync(message, message);
    }

Answer 1

An extension to the Solution supplied in the original question also correctly cleans up an attachments that may also require disposal.

    public event EventHandler EmailSendCancelled = delegate { };

    public event EventHandler EmailSendFailure = delegate { };

    public event EventHandler EmailSendSuccess = delegate { };
    ...

        MemoryStream mem = new MemoryStream();
        try
        {
            thisReport.ExportToPdf(mem);

            // Create a new attachment and put the PDF report into it.
            mem.Seek(0, System.IO.SeekOrigin.Begin);
            //Attachment att = new Attachment(mem, "MyOutputFileName.pdf", "application/pdf");
            Attachment messageAttachment = new Attachment(mem, thisReportName, "application/pdf");

            // Create a new message and attach the PDF report to it.
            MailMessage message = new MailMessage();
            message.Attachments.Add(messageAttachment);

            // Specify sender and recipient options for the e-mail message.
            message.From = new MailAddress(NOES.Properties.Settings.Default.FromEmailAddress, NOES.Properties.Settings.Default.FromEmailName);
            message.To.Add(new MailAddress(toEmailAddress, NOES.Properties.Settings.Default.ToEmailName));

            // Specify other e-mail options.
            //mail.Subject = thisReport.ExportOptions.Email.Subject;
            message.Subject = subject;
            message.Body = body;

            // Send the e-mail message via the specified SMTP server.
            SmtpClient smtp = new SmtpClient();
            smtp.SendCompleted += SmtpSendCompleted;
            smtp.SendAsync(message, message);
        }
        catch (Exception)
        {
            if (mem != null)
            {
                mem.Dispose();
                mem.Close();
            }
            throw;
        }
    }

    private void SmtpSendCompleted(object sender, AsyncCompletedEventArgs e)
    {
        var message = e.UserState as MailMessage;
        if (message != null)
        {
            foreach (var attachment in message.Attachments)
            {
                if (attachment != null)
                {
                    attachment.Dispose();
                }
            }
            message.Dispose();
        }
        if (e.Cancelled)
            EmailSendCancelled?.Invoke(this, EventArgs.Empty);
        else if (e.Error != null)
        {
            EmailSendFailure?.Invoke(this, EventArgs.Empty);
            throw e.Error;
        }
        else
            EmailSendSuccess?.Invoke(this, EventArgs.Empty);
    }

Answer 2

I have tried your function and it works even for email with in memory attachments. But here are some remarks:

• What type of attachments did you try to send ? Exe ?
• Are both sender and receiver on the same email server ?
• You should “catch” exception and not just swallow it, than you will get more info about your problem.

• What does the exception says ?

• Does it work whan you use Send instead of SendAsync ? You are using 'using' clause and closing Stream before email is sent.

Here is good text about this topic:

Sending Mail in .NET 2.0

Answer 3

Don't use "using" here. You are destroying the memory stream immediately after calling SendAsync, e.g. probably before SMTP gets to read it (since it's async). Destroy your stream in the callback.