$(function() {
            $(".auto-city").autocomplete({
                source: function(request, response) {
                    $.ajax({
                        url: "/Handler/ajaxAutocomplete.ashx",
                        data: { "type" : "citta", "q": request.term },
                        dataType: "json",
                        type: "POST",
                        contentType: "application/json; charset=utf-8",
                        dataFilter: function(data) { return data; },
                        success: function(data) {
                            response($.map(data.d, function(item) {
                                return {
                                    value: item.Comune
                                }
                            }))
                        },
                        error: function(XMLHttpRequest, textStatus, errorThrown) {
                            
                        }
                    });
                },
                minLength: 2
            });
        });

 
<%@ WebHandler Language="C#" Class="ajaxAutocomplete" %>

using System;
using System.IO;
using System.Web;
using System.Web.Script.Serialization;
using System.Collections.Generic;
using System.Runtime.Serialization.Json;
using System.Text;
using System.Data;
using System.Linq;
using System.Linq.Expressions;

public class ajaxAutocomplete : IHttpHandler
{

    public void ProcessRequest(HttpContext context)
    {
        context.Response.ContentType = "text/plain";
        context.Response.Expires = -1;

        string result = string.Empty;
        string type = context.Request["type"];
        string q = string.Empty;

        q = context.Request.Form["term"].Trim();
        JavaScriptSerializer ser = new JavaScriptSerializer();
        switch (type)
        {
            case "citta":
                List<tipoComune> comuni = CittaProvinceNazioni.CittaProvincia(string.Empty);

                result = ser.Serialize(comuni.Where(item => item.Comune.ToLower().StartsWith(q.ToLower())));
                break;
                
            default:
                break;
        }

        context.Response.ContentType = "text/plain";
        context.Response.Write(result);
    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }

    private void LogRequest(string Log)
    {
        //StreamWriter sw = new StreamWriter(HttpContext.Current.Server.MapPath("/Log") + "\\Log.txt", true);
        //sw.WriteLine(DateTime.Now.ToString() + " - " + Log);
        //sw.Flush();
        //sw.Close();
    }
}