Why can SmtpClient.SendAsync only be called once?

Question

I\'m trying to write a notification service (for completely legit non-spam purposes) in .NET using SmtpClient. Initially I just looped through each message and sent it, however

Answer 1

As noticed by everyone else here, you can only send one email at a time, but the way to send another once the first has been sent is to handle the .SendCompleted event of the SmtpClient class, and then move on to the next email and send that.

If you want to send many emails simultaneously, then as the others have said, use multiple SmtpClient objects.

Answer 2

You might be able to use the following:

ThreadPool.QueueUserWorkItem(state => client.Send(msg));

This should allow your messages to be queued and sent as threads become available.

Answer 3

There is a reason to reuse the SmtpClient, it limits the # of connections to the SMTP server. I cannot instantiate a new class SmtpClient class for each thread the reports are building on or the SMTP server will balk with too many connections error. This is the solution I came up with when I couldn't find an answer here.

I ended up using an AutoResetEvent for keeping everything in sync. That way, I can keep calling my SendAsync in each thread, but wait for it to process the email and use the SendComplete event to reset it so the next one can continue.

I setup the Auto Reset Event.

        AutoResetEvent _autoResetEvent = new AutoResetEvent(true);

I setup the shared SMTP Client when my class is instantiated.

        _smtpServer = new SmtpClient(_mailServer);
        _smtpServer.Port = Convert.ToInt32(_mailPort);
        _smtpServer.UseDefaultCredentials = false;
        _smtpServer.Credentials = new System.Net.NetworkCredential(_mailUser, _mailPassword);
        _smtpServer.EnableSsl = true;
        _smtpServer.SendCompleted += SmtpServer_SendCompleted;

Then when I call the send async, I wait for the event to clear, then send the next one.

        _autoResetEvent.WaitOne();
        _smtpServer.SendAsync(mail, mail);
        mailWaiting++;

I use the SMTPClient SendComplete event to reset the AutoResetEvent so the next email will send.

private static void SmtpServer_SendCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e)
        {
            MailMessage thisMesage = (MailMessage) e.UserState;
            if (e.Error != null)
            {
                if (e.Error.InnerException != null)
                {
                    writeMessage("ERROR: Sending Mail: " + thisMesage.Subject + " Msg: "
                                 + e.Error.Message + e.Error.InnerException.Message);
                }

                else
                {
                    writeMessage("ERROR: Sending Mail: " + thisMesage.Subject + " Msg: " + e.Error.Message);
                }
            }
            else
            {
                writeMessage("Success:" + thisMesage.Subject + " sent.");
            }
        if (_messagesPerConnection > 20)
        {  /*Limit # of messages per connection, 
            After send then reset the SmtpClient before next thread release*/
            _smtpServer = new SmtpClient(_mailServer);
            _smtpServer.SendCompleted += SmtpServer_SendCompleted;
            _smtpServer.Port = Convert.ToInt32(_mailPort);
            _smtpServer.UseDefaultCredentials = false;
            _smtpServer.Credentials = new NetworkCredential(_mailUser, _mailPassword);
            _smtpServer.EnableSsl = true;
            _messagesPerConnection = 0;
        }
            _autoResetEvent.Set();//Here is the event reset
            mailWaiting--;
        }

Answer 4

According to the documentation:

After calling SendAsync, you must wait for the e-mail transmission to complete before attempting to send another e-mail message using Send or SendAsync.

So to send multiple mails at the same time you need multiple SmtpClient instances.

Answer 5

I think you misunderstand the XXXAsync class of methods. The purpose of these asynchronous calls is to allow the program to continue running, without the need of the method to finish processing and return first. You can then proceed with the result later by subscribe to something like XXXReceived event of the object.

To send more than one mail simultaneously, you may consider using more Threads.

Answer 6

You may only send one at a time per SMTP client. If you wish to make more than one send call, create more than one SMTP client.

HTH,

Colby Africa