dropdown lost the OptionGroup after postback

private void MakeDropDown()
{

ListItem item1 = new ListItem("Only 1 Star", "1");
item1.Attributes["OptionGroup"] = "By Star Ratting";
ListItem item2 = new ListItem("Only 2 Star", "2");
item2.Attributes["OptionGroup"] = "By Star Ratting";
ListItem item3 = new ListItem("Only 3 Star", "3");
item3.Attributes["OptionGroup"] = "By Star Ratting";
ListItem item4 = new ListItem("Only 4 Star", "4");
item4.Attributes["OptionGroup"] = "By Star Ratting";
ListItem item5 = new ListItem("Only 5 Star", "5");
item5.Attributes["OptionGroup"] = "By Star Ratting";
ListItem item6 = new ListItem("View All", "6");
item6.Attributes["OptionGroup"] = "By Star Ratting";


ListItem item7 = new ListItem("Reviews With material Facts", "7");
item7.Attributes["OptionGroup"] = "By Material Fact";
ListItem item8 = new ListItem("Reviews Without material Facts", "8");
item8.Attributes["OptionGroup"] = "By Material Fact";
ListItem item9 = new ListItem("Only Text Reviews", "9");
item9.Attributes["OptionGroup"] = "By Review Type";
ListItem item10 = new ListItem("Only Video Reviews", "10");
item10.Attributes["OptionGroup"] = "By Review Type";


DdlSortingViaStar.Items.Add(item1);
DdlSortingViaStar.Items.Add(item2);
DdlSortingViaStar.Items.Add(item3);
DdlSortingViaStar.Items.Add(item4);
DdlSortingViaStar.Items.Add(item5);
DdlSortingViaStar.Items.Add(item6);
DdlSortingViaStar.Items.Add(item7);
DdlSortingViaStar.Items.Add(item8);
DdlSortingViaStar.Items.Add(item9);
DdlSortingViaStar.Items.Add(item10);



}



on page Load:
if (!IsPostBack)
{

MakeDropDown();
}

I also Try This
MakeDropDown();

Direct On Page Load

if i bind fuction on SelectedIndexChanged event,then selected data will be lost and dropdown list data is Just Doubble, So for this i try this:

string yy = DdlSortingViaStar.SelectedValue;
DdlSortingViaStar.Items.Clear();
MakeDropDown();
DdlSortingViaStar.Text = yy;

And it work Fine,
Thank you very much,For suggestion.....