>Thank you very much Fabio,
>
>This is very strange, i also reinstalled the vfp. I did what you said, and still the same problem. I added this code, at the end of all the lines in the "activate" property of the form:
>
>* Break when an expression becomes true
>IF VARTYPE(principal)#"O"
> WAIT "Reference lost" window
>ELSE
> WAIT "Reference Ok" Window
> WAIT VARTYPE(principal) Window
>ENDIF
>
>And shows: "Reference Ok"
>
>
>And i added a button on the form with the same code on its "click" property, and when i click this button, shows: "Reference Lost" :(
>
>Why? I don't know :( But the reference is lost.

You have to set a Debug breakpoint, not a code breakpoint.
With this, VFP stop the execution when m.principal change.