DropDownList SelectedIndexChanged AutoPostBack C# ASP.NET

default.aspx
<%@ Page Language="C#" AutoEventWireup="false" Codebehind="default.aspx.cs" Inherits="SmartQueryXML._default" ValidateRequest="false" %>
 
<asp:DropDownList ID="ddStates" runat="server" OnSelectedIndexChanged="ddStates_SelectedIndexChanged" AutoPostBack="True" Width="150px"></asp:DropDownList>
<asp:DropDownList ID="ddCounties" runat="server" Width="150px" OnSelectedIndexChanged="ddCounties_SelectedIndexChanged" AutoPostBack="True"></asp:DropDownList>
 
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default.aspx.cs
    protected void ddStates_SelectedIndexChanged(object sender, EventArgs e) {
      string ErrorMessage = "";
      string state = ddStates.SelectedItem.Value;
 
      // This doesn't work
      DataSet ds = new DataSet();
      ds = Helper.GetCounties(state, "%%", ref ErrorMessage);
 
      ddCounties.Items.Clear();
      ddCounties.Items.Add(new ListItem("Choose a County", ""));
 
      foreach(DataRow dr in ds.Tables[0].Rows) {
        ddCounties.Items.Add(new ListItem(dr["Name"].ToString(), state));
      }
 
      // This works
      //DataSet ds = new DataSet("Jurisdiction");
      //FileStream fs = new FileStream(Server.MapPath("Jurisdiction.xml"), FileMode.Open, FileAccess.Read);
      //StreamReader sr = new StreamReader(fs);
      //ds.ReadXml(sr);
      //fs.Close();
      //DataView dv = new DataView(ds.Tables[0]);
 
      //dv.Table = ds.Tables[0];
      //dv.RowFilter = "State = '" + state + "'";
      //ddCounties.DataSource = dv;
      //ddCounties.DataMember = "jurisdiction";
      //ddCounties.DataTextField = "county";
      //ddCounties.DataValueField = "county";
      //ddCounties.DataBind();
 
    }
 
    protected void ddCounties_SelectedIndexChanged(object sender, EventArgs e) {
      // Reset inputs when user selects a county
      //  Clean slate.
      string county = ddCounties.SelectedItem.Value;
    }