c# - Visual Studio not Recognising IDisposable Type -

i have following email class contains 1 method send:

using system.net; using system.net.mail;  static class email {     const string emailaddress = "joebloggs@domain.com";     const string subject = "error detected in application";      public static void send(string body)     {         using (var client = new smtpclient("mail.mailserevrname.com", 25)             { credentials = new networkcredential(), enablessl = false })         {             client.send(emailaddress,                         emailaddress,                         subject,                         body);         }     } } 

i have used code in multiple applications , have never had problem (no errors, email gets sent as/when expected).

however, in current project, when copy class in, seeing error:

'smtpclient': type used in using statement must implicitly convertible 'system.idisposable'

i have never seen error being produced above code. can see the documentation smtpclient implement idisposable:

public class smtpclient : idisposable

does know why error appearing , not in other applications? or know starting point investigate this?

as @j.steen's comment pointed out, in .net framework 3.5, smtpclient not implement idisposable.

as result, solution create class inherits smtpclient , implements idisposable , use in using.

edit:

class mailer: smtpclient, idisposable {     //... } 

is way implement idisposable , inherit smtpclient