void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)

{

    DataView dv = (DataView)Session["Source"];

if (Session["sortorder"] == "Ascending") 

{       

Session["sortorder"] = "Descending";       

dv.Sort = Session["SavedSort"] + ", " +        DropDownList1.SelectedValue.ToString() + " ASC";   

}   

else   

{       

Session["sortorder"] = "Ascending";        dv.Sort = Session["SavedSort"] + ", " +        DropDownList1.SelectedValue.ToString() + " DESC";   

}   

GridView1.DataBind();

}