asp.net - The textbox value assinged at server side is not getting rendered in ASPX page -

we have aspx page built using telerik controls, on dropdown selected index change event @ server side, assigning value textbox, when page viewed textbox value not shown.

code used:

protected sub dropdown_selectedindexchanged(byval o object, byval e telerik.webcontrols.radcomboboxselectedindexchangedeventargs) handles dropdown.selectedindexchanged      txtcost.text = finalcost.tostring()     page.clientscript.registerstartupscript(me.gettype(), "alert", "openmodelpopup();", true)  end sub 

add autopostback="true" dropdownlist tag in .aspx file.