如何使用 SmtpClient.SendAsync 发送带有附件的电子邮件？

Question

我正在通过 ASP.NET MVC 使用服务组件。 我想以异步方式发送电子邮件，让用户无需等待发送就可以做其他事情。

当我发送没有附件的消息时，它工作正常。 当我发送带有至少一个内存附件的消息时，它会失败。

所以，我想知道是否可以使用带有内存附件的异步方法。

这里是发送方式

    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);
        }
    }

这是我当前的错误

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 ---

---

解决方案

    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

我已经尝试过您的功能，它甚至适用于带有内存附件的电子邮件。但这里有一些说明：

• 您尝试发送什么类型的附件？可执行文件？
• 发件人和收件人是否在同一个电子邮件服务器上？
• 您应该“捕捉”异常，而不是仅仅吞下它，这样您才能获得有关您的问题的更多信息。

• 异常说明了什么？

• 使用 Send 而不是 SendAsync 是否有效？您正在使用 'using' 子句并在发送电子邮件之前关闭 Stream。

这是关于这个主题的好文章：

Sending Mail in .NET 2.0

Answer 2

这里不要使用“使用”。您在调用 SendAsync 后立即销毁内存流，例如可能在 SMTP 读取它之前（因为它是异步的）。在回调中销毁您的流。

Answer 3

对原始问题中提供的解决方案的扩展也可以正确清理可能也需要处理的附件。

    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);
    }