asp.net: set css category control during databind?

a list quarrel generated controlling an asp:repeater:

<asp:repeater id="announcementsrepeater" onitemdatabound="announcementsrepeater_itemdatabound" runat="server">
   <itemtemplate>
      <tr id="announcementrow" class="announcementitem" runat="server">...</tr>
   </itemtemplate>
</asp:repeater>

now data-bind i wish symbol "unread" announcements opposite css class, web-guy perform whatever styling wants compute between review unread announcements:

protected vacant announcementsrepeater_itemdatabound(object sender, repeateritemeventargs e)
{
   (e.item.itemtype != listitemtype.item && e.item.itemtype != listitemtype.alternatingitem)
      return;

   // compared information item
   proclamation announcement = (announcement)e.item.dataitem;

   webcontrol quarrel = (webcontrol)e.item.findcontrol("announcementrow");
      (row != null)
         row.cssclass = row.cssclass + " announcementitemunread";
}

except ban fails during runtime:

system.invalidcastexception occurred
  message="unable ban vigilant form 'system.web.ui.htmlcontrols.htmltablerow' form 'system.web.ui.webcontrols.webcontrol'."

it turns out htmltablerow opposite progenitor heirarchy webcontrol:

htmltablerow
   : htmlcontainercontrol
      : htmlcontrol
         : system.web.ui.control

which eventually where webcontrol comes from

webcontrol
   : system.web.ui.control

so i altered formula try system.web.ui.control instead:

control quarrel = (control)e.item.findcontrol("announcementrow");
if (row != null)
   row.cssclass = row.cssclass + " announcementitemunread";

but control doesn't enclose construction cssclass:

'system.web.ui.control' does enclose construction 'cssclass'    

so i set css category name <tr> component during databind?