<asp:DropDownList ID="DDLProvince" runat="server" CssClass="select" 
               AutoPostBack="True" Height="22px" 
               onselectedindexchanged="DDLProvince_SelectedIndexChanged" Width="150px">
           </asp:DropDownList>
           <asp:DropDownList ID="DDLCity" runat="server" CssClass="select" 
               CausesValidation="True" Height="19px" Width="150px">
           </asp:DropDownList>

protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                RegionalInformation ri = new RegionalInformation();

                
                string[][] allProvinces = ri.GetProvinces();

                for (int i = 0; i < allProvinces.Length; i++)
                {
                    ListItem lip = new ListItem(allProvinces[i][1], allProvinces[i][0]);
                    DDLProvince.Items.Add(lip);
                    
                }

            }
        }

        protected void DDLProvince_SelectedIndexChanged(object sender, EventArgs e)
        {
            RegionalInformation ri = new RegionalInformation();
            string a = Convert.ToString(DDLProvince.SelectedIndex + 1);

            string[][] citiesOfProvince = ri.GetCitiesOfProvince(a);
            for (int j = 0; j < citiesOfProvince.Length; j++)
            {
                ListItem li = new ListItem(citiesOfProvince[j][1], citiesOfProvince[j][0]);
                DDLCity.Items.Add(li);
            }
        }