Unit Testing my Email Service

How do I write a unit test for a method that use an external service like System.Net.Mail?

Do I just check a return value to see if there were any errors sending the email and assume if there were none that it was sent successfully?

From stackoverflow

• Create a mock object for the mail service.

  In short this will mean that you write a class that can replace the behavior of sending a mail. This way you can create different situations for your test very easy: a succeeded mail, a failed one, etc.

  A nice and short introduction: http://www.programmersheaven.com/user/pheaven/blog/217-An-Introduction-to-Mock-Objects/

  Another one, with code samples: http://blogs.clearscreen.com/ragc/archive/2004/08/31/395.aspx

• It depends on the level of testing you want. If you are more concerned with testing the method, and the emailing is just a small part, then you might consider mocking the email sending service out. Gerrie's link is good.

  However, if you actually want to test the mail sending, here are some ideas.

  • Use SmtpDeliveryLocation to output the email to a folder instead of a mail server. You can then check the folder.
  • Send the email, then query the remote POP server to see if it arrived.
  • Send and assume it just worked if ther are no exceptions. I can't recall if SmtpClient.Send is blocking or non blocking.

  Because these tests typically mean talking to external things, they'd be called integration tests rather than unit tests.

  But in general, I think you'd be more concerned with ensuring you got the formatting/email addresses correct, than whether or not Microsoft implemented SmtpClient correctly. So unit testing and mocks make more sense there.

  Gerrie Schenck : Agreed, it depends on what you want to test: the mail server or the business logic which is responsible in generating a mail.