How about:

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

>
>What would be the best approach to update this condition to take care of not executing this code if I have overwrited the value of the InputMask?