Rename FTP File

Hello,

I successfully programmed FTP up- and download but now I need to move a file on FTP to another directory.
But the FTP methods RNFR and RNTO seem not to be valid .net commands
I get the following message (in french) :
Un appel à System.Net.FtpWebRequest.Method a échoué avec le message :
Cette méthode n'est pas pris en charge
Nom du paramètre : value
In english : a call to System.Net.FtpWebRequest.Method has failed with the message :
This method is not valid
Name of parameter : value

Does anybody have a solution to moive a file on FTP ?
Thanks !