asp.net(C#)如何在page_load事件中从sql取到某列值给dropdownlist

protected void Page_Load(object sender, EventArgs e)
{
if(!Page.IsPostBack)
{
string connstring = "server=localhost;database=student;uid=sa;pwd=";
SqlConnection conn = new SqlConnection(connstring);
SqlDataAdapter da = new SqlDataAdapter("select * from student",conn);
DataSet ds = new DataSet();
da.Fill(ds);
conn.Close();
DropDownList1.DataSource = ds;
DropDownList1.DataTextField = "name";
DropDownList1.DataBind();
}
}

string SqlConn=System.Configuration.ConfigurationSettings.AppSettings["ConnString"];
SqlConnection Conn=new SqlConnection(SqlConn);
Conn.Open();
string mysql="select * from db_hetongguanli_y_g";
SqlCommand cm=new SqlCommand (mysql,Conn);
SqlDataReader dr=cm.ExecuteR