ขอความรู้เรื่องการเขียน C# Class เพื่อจะเรียก จังหวัด อำเภอ ตำบล

<tr>
   <td>จังหวัด</td><td>:</td>
   <td><asp:DropDownList ID="province" runat="server" 
                onselectedindexchanged="province_SelectedIndexChanged" ></asp:DropDownList>
   </td>
</tr>

 protected void province_SelectedIndexChanged(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(constr);
        //---------อำเภอ---------------
        string com_am = "select AMPHUR_CODE,AMPHUR_NAME from amphur with(Nolock) " +
            "where PROVINCE_ID ='" + province.SelectedValue + "' ";
        //Response.Write(com_am);
        con.Open();
        SqlDataAdapter da_am = new SqlDataAdapter(com_am, con);
        DataSet ds_am = new DataSet();
        da_am.Fill(ds_am);
        con.Close();
        d_distict.Items.Clear();
        for (int i = 0; i < ds_am.Tables[0].Rows.Count; i++)
        {
            String am_name = ds_am.Tables[0].Rows[i]["AMPHUR_NAME"].ToString();
            String am_code = ds_am.Tables[0].Rows[i]["AMPHUR_CODE"].ToString();
            ListItem am = new ListItem(am_name,am_code );
            d_distict.Items.Add(am);
        }
        d_distict.Focus();

    }