Ajax - How to Bind DropDown List

string connectionstring= "Data Source=.; Integrated Security=true; Database=realasn1;";
sqlconnection con;
SqlDataAdapter da;
Dataset ds = new Dataset();
protected void Page_Load(object sender, EventArgs e)
{
con = new (connectionstring);
con.open();
da = new SqlDataAdapter("select id,name from country", con);
ds.Clear();
da.Fill(ds);
if (!IsPostBack)
{
bindcountry(ds);
}
con.close();
}

public void bindcountry(Dataset dbset)
{
//bind office country
ddl_1.DataSource = dbset;
ddl_1.DataTextField = dbset.Tables[0].Columns[1].ToString();
ddl_1.DataValueField = dbset.Tables[0].Columns[0].ToString();
ddl_1.DataBind();
ListItem li = new ListItem();
li.Text = "--Select Country--";
li.Value = "0";
ddl_1.Items.Insert(0, li);
}

protected void ddl_1_SelectedIndexChanged(object sender, EventArgs e)
{
System.Threading.Thread.Sleep(500);//for update panel
if (int.Parse(ddl_1.SelectedValue) > 0)
{
con = new (connectionstring);
con.open();
da = new SqlDataAdapter("select id,name from city where countryid=" + int.Parse(ddl_1.SelectedValue) , con);
ds.Clear();
da.Fill(ds);
bindcity(ds));
}
}

public void bindcity(DataSet dbset)
{
ddl_2.DataSource = dbset;
ddl_2.DataTextField = dbset.Tables[0].Columns[1].ToString();
ddl_2.DataValueField = dbset.Tables[0].Columns[0].ToString();
ddl_2.DataBind();
ListItem li = new ListItem();
li.Text = "--Select City--";
li.Value = "0";
ddl_2.Items.Insert(0, li);
}