What I do to fetch all item in dropdownlist from database at the time of page load event fired?

protected void Page_Load(object sender, EventArgs e)
       {
           if (!IsPostBack)
           {
               string myQuery = "select * from studentDetail";
               cmd = new SqlCommand(myQuery, conn);
               cmd.Connection = conn;
               SqlDataAdapter sda = new SqlDataAdapter();
               sda.SelectCommand = cmd;
               DataSet ds = new DataSet();
               sda.Fill(ds);
               GridView1.DataSource = ds;
               GridView1.DataBind();
               int hasRows = ds.Tables[0].Rows.Count;
               int c = 0;
               if(c< hasRows){
                   DropDownList1.Items.Add(ds.Tables[0].Rows[c]["rollnum"].ToString());
                   c++;
               }
               conn.Close();


           }
       }