I have one doubt in drop down list page index change event

i can explain for my doubt with example.see.my table  have multiple type of employee details like as Mangemment,Programmer,Designers,Accountants,ContractEmployees.Each type of department having many employee details.Now my drop down list having types of department names like that above types of department names..if i select any one type of department data wil be display in gridview format .gridview data  column as employee Name,Address,Phone_No,email_address and each every row with checkboxes.gridview showing data according to select the dept.No problem.In my problem as if select management type of employee two after then i select programmer type of employee four .so i need now totally six employee values.

 How to write code in dropdown list selected index changed event.

Note : Dropdown list palced in out side of gridview. i put First selected type of employee values in viewstate then i go to select second type of employee values,now i am not getting for previous selected values i mean viewstate values,here viewstate values is empty.whenever i select dropdown list value viewstate values are emty.how to get viewstate value while dropdown list selected index changed.

 

Plz tel me its very urgent.

 Advance wishes.

Regards

Thanigaimani