ASP.Net JSON Generic Handler with CallBack example

You will need to use a Generic Handler ASHX file and not ASPX file.

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

using System;
using System.Web;
using System.Data;
using System.Configuration;
using System.Data.SqlClient;
using System.Collections.Generic;
using System.Web.Script.Serialization;

public class JSONHandlerCS : IHttpHandler
{
    public void ProcessRequest(HttpContext context)
    {
        string callback = context.Request["callback"];
        int customerId = 0;
        int.TryParse(context.Request["customerId"], out customerId);
        string json = this.GetCustomersJSON(customerId);
        if (!string.IsNullOrEmpty(callback))
        {
            json = string.Format("{0}({1});", callback, json);
        }
        
        context.Response.ContentType = "text/json";
        context.Response.Write(json);
    }

    private string GetCustomersJSON(int customerId)
    {
        List<object> customers = new List<object>();
        using (SqlConnection conn = new SqlConnection())
        {
            conn.ConnectionString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
            using (SqlCommand cmd = new SqlCommand())
            {
                cmd.CommandText = "SELECT * FROM Customers WHERE CustomerId = @CustomerId OR @CustomerId = 0";
                cmd.Parameters.AddWithValue("@CustomerId", customerId);
                cmd.Connection = conn;
                conn.Open();
                using (SqlDataReader sdr = cmd.ExecuteReader())
                {
                    while (sdr.Read())
                    {
                        customers.Add(new
                        {
                            CustomerId = sdr["CustomerId"],
                            Name = sdr["Name"],
                            Country = sdr["Country"]
                        });
                    }
                }
                conn.Close();
            }
            return (new JavaScriptSerializer().Serialize(customers));
        }
    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }
}

Screenshot