Duplicate class definitions

Hi there,

 My project references System.Web which contains System.Web.Mail.MailMessage. Then I defined the following in my project's code...

namespace System.Web.Mail
{
  public class MailMessage {}
}

...and I don't get an error when I compile! Why is this?

 It appears to use my version. Is there some order of precedence for declarations such that imported/compiled one's are used after one's defined in source? If so, where can I read about this?

 Cheers, WT.
...