>How about:
>

>IF TYPE(This.ControlSource)='C'
>    if empty(This.InputMask)
>       This.InputMask='!'+REPLICATE('X',LEN(EVALUATE(This.ControlSource))-1)
>    endif
>ENDIF
>

This.InputMask contains by default a value in the txtField class. I need to detect if at the form designer level, we have overwritten that value.