just u use  at selection screen on field event. it will
restrict the wrong values.
u can write the condition in that event

can u give any example?

Hi Busha,

i want to check the field kunnr from kna1, then

parameter : pa_kunnr type kunnr. (This is input field)

select single kunnr into pa_kunnr
       from kna1
       where kunnr eq pa_kunr.

if sy-subrc ne o.
message i000(zexp). ( zexp is message class,
                         i message type
                       000 message number)
endif.

The above code is right which was written by Raju, but this 
code need to wtite within At-Selection screen event, bec. 
we can validate input data within this event only.