Combo in cascata con JQuery e ASP.NET

C#: comune.aspx.csCopia

using System;
using System.Collections.Generic;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.OleDb;
using System.Configuration;
using System.Text;

public partial class _Comuni : System.Web.UI.Page
{
  protected void Page_Load(object sender, EventArgs e)
  {
    Response.Clear();

    string connectionString = ConfigurationManager.ConnectionStrings["cnnAccess"].ConnectionString.Replace("~", Server.MapPath("/"));
    string referer = "";
    try
    {
      referer = Request.UrlReferrer.Host;
    }
    catch { }

    string search = "";
    string field = "";
    if (string.IsNullOrEmpty(Request["q"]) == false) search = Request["q"].Trim();
    if (string.IsNullOrEmpty(Request["f"]) == false) field = Request["f"].Trim().ToLower();

    string queryWhere = "";
    string queryKey = "";

    switch (field)
    {
      case "codprov":
      case "codprovincia":
        queryWhere = "C.[CodProvincia] = ?";
        queryKey = search;
        break;
      case "prov":
      case "provincia":
        queryWhere = "P.[Provincia] like ?";
        queryKey = string.Format("%{0}%", search);
        break;
      case "codreg":
      case "codregione":
        queryWhere = "P.[CodRegione] = ?";
        queryKey = search;
        break;
      case "reg":
      case "regione":
        queryWhere = "R.[Regione] like ?";
        queryKey = string.Format("%{0}%", search);
        break;
      case "cap":
        queryWhere = "C.[CAP] = ?";
        queryKey = search;
        break;
      case "tel":
      case "telefono":
        queryWhere = "C.[PrefissoTelefonico] = ?";
        queryKey = search;
        break;
      case "erariale":
        queryWhere = "C.[CodErariale] = ?";
        queryKey = search;
        break;
      case "istat":
        queryWhere = "C.[CodISTAT] = ?";
        queryKey = search;
        break;
      default:
        queryWhere = "C.[Comune] like ?";
        queryKey = string.Format("%{0}%", search);
        if (search.Length < 3) return;
        break;
    }

    StringBuilder sb = new StringBuilder(1000);
    sb.Append("[");

    if (!referer.Equals(Request.Url.Host, StringComparison.InvariantCultureIgnoreCase))
    {
      Add(sb
        , string.Format("Sono permesse connessioni solo dal dominio sgart.it ({0})", Request.Url.Host)
        , ".."
        , ".."
        , "..."
        , "..."
        , "....."
        , "...."
        , "...."
        , "......"
        );
    }
    else
    {
      string sql = string.Format(@" SELECT C.[ID], C.[Comune], C.[CodProvincia], P.[CodRegione], C.[CAP], 
       C.[PrefissoTelefonico], C.[CodErariale], C.[CodISTAT], P.[Provincia], R.[Regione]
        FROM TabRegioni R
          INNER JOIN (TabProvince P
            INNER JOIN TabComuni C ON P.CodProvincia = C.CodProvincia) ON R.CodRegione = P.CodRegione
         WHERE {0} 
       ORDER BY [Comune] ", queryWhere);

      using (OleDbConnection cnn = new OleDbConnection(connectionString))
      {
        using (OleDbCommand cmd = cnn.CreateCommand())
        {
          cmd.CommandText = sql;
          cmd.CommandType = CommandType.Text;
          cmd.Parameters.Add("q", OleDbType.VarChar, 50).Value = queryKey;
          cnn.Open();
          using (OleDbDataReader dr = cmd.ExecuteReader(CommandBehavior.CloseConnection))
          {
            bool comma = false;
            while (dr.Read() == true)
            {
              if (comma)
              {
                sb.Append(",");
              }
              comma = true;
              Add(sb
                , dr["Comune"].ToString()
                , dr["CodProvincia"].ToString()
                , dr["Provincia"].ToString()
                , dr["CodRegione"].ToString()
                , dr["Regione"].ToString()
                , dr["CAP"].ToString()
                , dr["PrefissoTelefonico"].ToString()
                , dr["CodErariale"].ToString()
                , dr["CodISTAT"].ToString()
                );
            }
          }
        }
      }
    }
    sb.Append("]");
    Response.Write(sb.ToString());
    Response.End();
  }

  protected void Add(StringBuilder sb, string comune, string codProvincia, string provincia
    , string codRegione, string regione
    , string cap, string prefissoTelefonico, string codErariale, string codISTAT)
  {
    sb.Append(@"{");
    sb.AppendFormat("\"Comune\":\"{0}\",", comune);
    sb.AppendFormat("\"CodProvincia\":\"{0}\",", codProvincia);
    sb.AppendFormat("\"Provincia\":\"{0}\",", provincia);
    sb.AppendFormat("\"CodRegione\":\"{0}\",", codRegione);
    sb.AppendFormat("\"Regione\":\"{0}\",", regione);
    sb.AppendFormat("\"CAP\":\"{0}\",", cap);
    sb.AppendFormat("\"PrefissoTelefonico\":\"{0}\",", prefissoTelefonico);
    sb.AppendFormat("\"CodErariale\":\"{0}\",", codErariale);
    sb.AppendFormat("\"CodISTAT\":\"{0}\"", codISTAT);
    sb.Append(@"}");
  }
}