What are best practices for using SmtpClient, SendAsync and Dispose under .NET 4.0

Question

I'm a bit perplexed on how to manage SmtpClient now that it is disposable, especially if I make calls using SendAsync. Presumably I should not call Dispose until SendAsync completes. But should I ever call it (e.g., using "using"). The scenario is a WCF service which mails out email periodically when calls are made. Most of the computation is fast, but the sending of email can take a second or so, so Async would be preferable.

Should I create a new SmtpClient each time I send mail? Should I create one for the entire WCF? Help!

Update In case it makes a difference, each email is always customized to the user. The WCF is hosted on Azure and Gmail is used as the mailer.

Answer 1

Note: .NET 4.5 SmtpClient implements async awaitable method SendMailAsync. For lower versions, use SendAsync as described below.

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You should always dispose of IDisposable instances at the earliest possibility. In the case of async calls, this is on the callback after the message is sent.

var message = new MailMessage("from", "to", "subject", "body"))
var client = new SmtpClient("host");
client.SendCompleted += (s, e) => {
                           client.Dispose();
                           message.Dispose();
                        };
client.SendAsync(message, null);

It's a bit annoying the SendAsync doesn't accept a callback.