How to correctly handle System.Net.Mail.SmtpException?

Question

I have a simple smtpClient:

var smtp = new SmtpClient { Host = host, ...};
smtp.Send(message);

I can have a different host: smtp.gmail.com , smtp.yandex.ru etc.

When smtp.Send(message); executed I have different exception (depends on the host) due to the same problem - 2-factor verification is off.

For gmail its System.Net.Mail.SmtpException: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. System.Net.Mail.SmtpException: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required.

For yahoo and yandex its System.Net.Mail.SmtpException depth 0: The operation has timed out. (0x80131500) System.Net.Mail.SmtpException depth 0: The operation has timed out. (0x80131500)

I don't now about others mail providers exception but how to correctly throw exception ("you need to enable 2-factor verification") just once? Is it possible? Or how to minimize code duplication?

Answer 1

I'm not sure how you are choosing which host to use (an if or switch statement?) but you could consider adding two new client classes which inherit from SmtpClient, eg for a YahooClient:

class YahooClient : SmtpClient {

     private const string Host = "smtp.yahoo.com";

     Send(MailMessage message) { 

          /// Call base send and handle exception            
          try {
             base.Send(message)
          }
          catch(ex as SmtpException) {
              // Handle accordingly
          }
     }
}

Furthermore, you could introduce implement a suitable interface and use IoC (or strategy pattern etc.) to inject the correct client based on your configuration, eg

class YahooClient : SmtpClient, IMySmtpClient {

}

interface IMySmtpClient {
    void Send(MailMessage message);
}

class ConsumingMailSender(IMySmtpClient client) {

       // Create message and send
       var message = new MailMessage etc....

       client.Send(message);
}

This is perhaps overkill but could avoid violating SRP and having to perform conditional logic in the method you are currently using to send the email in.