Well datasessions are like work areas, you should never take the number (ID) for granted

As Dragan suggested, you should store the datasessionID into a variable or _screen.property before switching, and later restore to this value:

local liDS
liDS = set('DataSession')
set datasession to another
do someStuff
set datasession to m.liDS

>Hi Thierry,
>
>The problem occurs in the development environment.
>
>My application initially opens 3 DATASESSIONS, which are automatically identified by VFP with IDs 2, 3 and 4.
>
>From there, I always refer to the DATASESSIONS directly in the code, for example:
>SET DATASESSION TO 2 or SET DATASESSION TO 3, etc.
>
>Since I'm starting to use THOR, I'm experiencing a conflict in the DATASESSIONS IDs.
>
>In the development environment, when THOR is active, it creates a DATASESSION with ID = 3 and from there, when I call my application for debugging from within the same development environment, my DATASESSIONs are automatically identified by the VFP with the IDs 2, 4 and 5.
>
>So, my references of DATASESSIONS IDs are changed.