Having your ASP.NET error handling routine, which sends you emails when an error occurs on your site, itself fail is annoying. Especially when you think you've made the code robust enough. Anyway the error handler for one site I work on was failing with "
ArgumentException: The specified string is not in the form required for a subject".
So what is exactly "the form required for a subject"? Googling for this error message returns a lot of junk and misinformed forum posts. It turns out that setting the Subject on a System.Net.Mail.Message internally calls MailBnfHelper.HasCROrLF (thank you Reflector) which does exactly what it says on the tin. Therefore one forum poster's solution of
subject.Replace("\r\n", " ") isn't going to work when your have either a carriage return or line feed in there.
So, obviously, the solution is:
message.Subject = subject.Replace('\r', ' ').Replace('\n', ' ');
Personally, I think that the MailMessage should to this for you or at least Microsoft should document what actually constitutes a "form required for a subject" in MSDN or, even better, in the actual error message itself!