Combo in cascata con JQuery e WebService in ASP.NET

C#: /App_Code/WS.asmx.csCopia

using System;
using System.Collections.Generic;
using System.Web;
using System.Web.Services;
using System.Configuration;
using System.Data;
using System.Data.OleDb;

/// <summary>
/// Summary description for ws
/// </summary>
[WebService(Namespace = "http://www.sgart.it/utility/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
public class WS : System.Web.Services.WebService
{

  #region SELECT

  [WebMethod(Description = "Return a dataset with Regioni")]
  public DataSet SelectRegioni()
  {
    DataSet ds = new DataSet("Regioni");
    try
    {
      if (!IsValidOriginRequest) return null;

      using (OleDbConnection cnn = GetConnection())
      {
        using (OleDbCommand cmd = cnn.CreateCommand())
        {
          cmd.CommandType = CommandType.Text;
          cmd.CommandText = @"SELECT [CodRegione], [Regione]
              FROM [TabRegioni]
              ORDER BY [Regione]";
          using (OleDbDataAdapter da = new OleDbDataAdapter(cmd))
          {
            da.Fill(ds);
          }
        }
      }
    }
    catch (Exception ex)
    {
      Context.Response.StatusCode = 500;
      Context.Response.StatusDescription = "Regioni, " +ex.Message;
    }
    return ds;
  }

  [WebMethod(Description = "Return a dataset with Province")]
  public DataSet SelectProvince(string codRegione)
  {
    DataSet ds = new DataSet("Province");
    try
    {
      if (!IsValidOriginRequest) return null;
      using (OleDbConnection cnn = GetConnection())
      {
        using (OleDbCommand cmd = cnn.CreateCommand())
        {
          cmd.CommandType = CommandType.Text;
          cmd.CommandText = @"SELECT [CodProvincia], [Provincia]
            FROM [TabProvince] 
            WHERE [CodRegione] = ? 
            ORDER BY [Provincia]";
          OleDbParameter param = cmd.Parameters.Add("codRegione", OleDbType.VarChar, 4);
          param.Value = codRegione;
          using (OleDbDataAdapter da = new OleDbDataAdapter(cmd))
          {
            da.Fill(ds);
          }
        }
      }
    }
    catch (Exception ex)
    {
      Context.Response.StatusCode = 500;
      Context.Response.StatusDescription = "Province, " +ex.Message;
    }

    return ds;
  }

  [WebMethod(Description = "Return a dataset with Comuni")]
  public DataSet SelectComuni(string codProvincia)
  {
    DataSet ds = new DataSet("Comuni");
    try
    {
      if (!IsValidOriginRequest) return null;
      using (OleDbConnection cnn = GetConnection())
      {
        using (OleDbCommand cmd = cnn.CreateCommand())
        {
          cmd.CommandType = CommandType.Text;
          cmd.CommandText = @"SELECT [ID], [Comune], [CAP], [PrefissoTelefonico]
          , [CodErariale], [CodISTAT]
          FROM [TabComuni]
          WHERE [CodProvincia] = ?
          ORDER BY [Comune]";
          OleDbParameter param = cmd.Parameters.Add("CodProvincia", OleDbType.VarChar, 4);
          param.Value = codProvincia;
          using (OleDbDataAdapter da = new OleDbDataAdapter(cmd))
          {
            da.Fill(ds);
          }
        }
      }
    }
    catch (Exception ex)
    {
      Context.Response.StatusCode = 500;
      Context.Response.StatusDescription = "Comuni, " + ex.Message;
    }
    return ds;
  }

  #endregion

  #region Private method

  private OleDbConnection GetConnection()
  {
    string connectionString = ConfigurationManager.ConnectionStrings["cnnAccess"].ConnectionString.Replace("~", Server.MapPath("/"));
    return new OleDbConnection(connectionString);
  }

  public bool IsValidOriginRequest
  {
    get
    {
      string referer = "";
      try
      {
        referer = Context.Request.UrlReferrer.Host;
      }
      catch { }
      return Context.Request.IsLocal == true || referer.Equals(Context.Request.Url.Host, StringComparison.InvariantCultureIgnoreCase);
    }
  }
  #endregion
}