Cross-domain JSONP with jQuery call step-by-step guide

What we want to accomplish?

Simple way to communicate cross-domain with ASMX .NET 3.5 Web Service

How can we do it?

1. Implement a web service method like the following

   [ScriptService]
   public class JSONP_EndPoint : System.Web.Services.WebService
   {
       [WebMethod]
       [ScriptMethod(UseHttpGet = true,ResponseFormat = ResponseFormat.Json)]
       public string Sum(string x,string y)
       {
           return x + y;
       }
   }

2. Add New class library with a name ContentTypeHttpModule

The reason for this is no matter how you specify the content-type of your ajax call ASP.NET send the request with Content-Type text/xml; charset=utf-8 this is security feature explained here by ScottGu 

3. Add the following code to your Class 

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Web;

namespace ContentTypeHttpModule
{
    public class ContentTypeHttpModule : IHttpModule
    {
        private const string JSON_CONTENT_TYPE = "application/json; charset=utf-8";

        #region IHttpModule Members
        public void Dispose()
        {
        }

        public void Init(HttpApplication app)
        {
            app.BeginRequest += OnBeginRequest;
            app.ReleaseRequestState += OnReleaseRequestState;
        }
        #endregion

        public void OnBeginRequest(object sender, EventArgs e)
        {
            HttpApplication app = (HttpApplication)sender;
            HttpRequest resquest = app.Request;
            if (!resquest.Url.AbsolutePath.Contains("JSONP-EndPoint.asmx")) return;

            if (string.IsNullOrEmpty(app.Context.Request.ContentType))
            {
                app.Context.Request.ContentType = JSON_CONTENT_TYPE;
            }
        }

        public void OnReleaseRequestState(object sender, EventArgs e)
        {
            HttpApplication app = (HttpApplication)sender;
            HttpResponse response = app.Response;
            if (app.Context.Request.ContentType != JSON_CONTENT_TYPE) return;

            response.Filter = new JsonResponseFilter(response.Filter);
        }
    }

    public class JsonResponseFilter : Stream
    {
        private readonly Stream _responseStream;
        private long _position;

        public JsonResponseFilter(Stream responseStream)
        {
            _responseStream = responseStream;
        }

        public override bool CanRead { get { return true; } }

        public override bool CanSeek { get { return true; } }

        public override bool CanWrite { get { return true; } }

        public override long Length { get { return 0; } }

        public override long Position { get { return _position; } set { _position = value; } }

        public override void Write(byte[] buffer, int offset, int count)
        {
            string strBuffer = Encoding.UTF8.GetString(buffer, offset, count);
            strBuffer = AppendJsonpCallback(strBuffer, HttpContext.Current.Request);
            byte[] data = Encoding.UTF8.GetBytes(strBuffer);
            _responseStream.Write(data, 0, data.Length);
        }

        private string AppendJsonpCallback(string strBuffer, HttpRequest request)
        {
            return request.Params["callback"] +"(" + strBuffer + ");";
        }

        public override void Close()
        {
            _responseStream.Close();
        }

        public override void Flush()
        {
            _responseStream.Flush();
        }

        public override long Seek(long offset, SeekOrigin origin)
        {
            return _responseStream.Seek(offset, origin);
        }

        public override void SetLength(long length)
        {
            _responseStream.SetLength(length);
        }

        public override int Read(byte[] buffer, int offset, int count)
        {
            return _responseStream.Read(buffer, offset, count);
        }
    }
}

4. Register the HttpModule in the service project

4.1 Add referance to the HttpModule assembly to the service project

4.2 Add this code to web.config to register the module

<add name="ContentTypeHttpModule"
                    type="ContentTypeHttpModule.ContentTypeHttpModule, ContentTypeHttpModule" />

This goes under system.web / httpmodules section

5. Add a web project for testing the application

5.1 add the following libs

jquery-1.3.1.js

json2.js

5.2 add new script file caller.js

function test() {
    $.ajax({ url: "http://localhost:1690/JSONP-EndPoint.asmx/Sum",
    data: { x: JSON.stringify("Now i am getting jsop string"), y: JSON.stringify("2nd param") },
        dataType: "jsonp",
        success: function(json) {
            alert(json.d);
        },
        error: function() {
            alert("Hit error fn!");
        }
    });
}

5.3 Add referances to jquery-1.3.1.js and json2.js

5.4 Add Default.aspx page with input button that has onclick=”return test();”

6. Remarks

6.1 I use the JSON.stringify function to serialize the string data parameters.

6.2 .d is a security features on ASP.NET 3.5