Read only field

hi,

i want to develop a report such that

roll no : ...... (i use roll no as parameters)

class no: ..... (same as parameters)

student object : ......

in run time i want to concatenate students roll no and class no ...

and then want to display it in student object. i want to keap student object field as read only..

so please suggest me how can i declare student object read only .(i am making normal report program not using module pool) and fill it with detail on the same screen.