如何检索CheckBoxList中的选择项?

procedure TDefault.Button3_Click(sender: System.Object; e: System.EventArgs);
var a:ListItem;
    s:string;
begin
    for a in Self.CheckBoxList1.Items do
      begin
         if a.Selected then s:=s+' '+a.Value;

      end;
      Label2.Text:=s;