>When I put the NVL function in the method addfoxcode
>MODIFY CLASS _WebServices OF "C:\Program Files (x86)\Microsoft Visual FoxPro 9\Ffc\_ws3utils.vcx"
>I save the class but when I go back to it, the change I've just made is gone !


It looks like you are experiencing UAC Virtualization and write operation is redirected to VirtualStore. (That's why we do not install VFP to default location in Program Files)

To apply your changes to _ws3utils.vcx run VFP as administrator (right-click its icon and choose Run as Administrator) so it has the privileges necessary to write to Program Files.

hth
Andrzej