Hs

If you leave set resource on, then

If set('resource') != "ON"
   * a VFP app is already open.
   * So end here
Endif

OR

If you use FOPEN() to open a file on app startup, you can keep it open through the whole VFP session; another instance won't be able to open the file.

These methods do require an extra file handle but only on the local machine and the code is easy to follow and maintain...

Regards

JR