Two ways to send email via SmtpClient asynchronously, different results

Question

Simple concept here. This is for a site being built using MVC 3 and Entity Framework 4. After a user registers on the site, an email is sent to their email address. I first implemented this using SmtpClient.Send() and it worked fine. Then I got the bright idea to try sending the email asynchronously. I'm experiencing issues with the two async approaches I've tried.

First implementation (from this unanswered post: https://stackoverflow.com/questions/7558582/how-to-dispose-using-smtpclient-send-and-asynccallback):

public bool Emailer(){
    .
    .
    .
    using (var smtpClient = new SmtpClient())
    {
        smtpClient.EnableSsl = true;
        smtpClient.Host = "smtp.gmail.com";
        smtpClient.Port = 587;
        smtpClient.UseDefaultCredentials = false;
        smtpClient.Credentials = new NetworkCredential("[email protected]", "mypassword");

        var sd = new SendEmailDelegate(smtpClient.Send);
        var cb = new AsyncCallback(SendEmailResponse);
        sd.BeginInvoke(message, cb, sd);

        return true;
    }
}

private delegate void SendEmailDelegate(System.Net.Mail.MailMessage m);

private static void SendEmailResponse(IAsyncResult ar)
{
    try
    {
        SendEmailDelegate sd = (SendEmailDelegate)(ar.AsyncState);
        sd.EndInvoke(ar); // "cannot access a disposed object" errors here
    }
    catch (Exception e)
    {
        _logger.WarnException("Error on EndInvoke.", e);
    }
}

This worked half the time. The other half I would get a "Cannot access a disposed object" error in the CallBack.

Next implementation (from a member with solid reputation: What are best practices for using SmtpClient, SendAsync and Dispose under .NET 4.0):

var smtpClient = new SmtpClient();
smtpClient.EnableSsl = true;
smtpClient.Host = "smtp.gmail.com";
smtpClient.Port = 587;
smtpClient.UseDefaultCredentials = false;
smtpClient.Credentials = new NetworkCredential("[email protected]", "mypassword");

smtpClient.SendCompleted += (s, e) =>
    {
        smtpClient.Dispose();
        message.Dispose();
    };
smtpClient.SendAsync(message, null);

With this implementation I dont get any errors, but there's a noticeably longer delay (~5 seconds) in debugging mode when smtpClient.SendAsync() executes, leading me to think that its not being sent asynchronously.

Questions:

1) what's wrong in the first method that's causing the "disposed object" errors?

2) does the second implementation have a problem that's causing the email to not send asynchronously? Is the 5-second delay meaningless?

Might be also important to note that although the site will not need to support sending a large number of emails (only user registration, opt-in newsletters, etc), we do anticipate a large number of users in the future, hence my decision to send emails asynchronously.

Thanks.

Answer 1

Your fist method will not work properly because of the USING-block. After the using-block ends, the SmtpClient object will de disposed. So you can't get access to it in your event handler.