<system.web><httpModules>
<add name="ContentTypeModule"  type="Project.ContentTypeModule, Project" />

// For not-JSON asmx, create another method or extend the ASMX, like this:
[WebMethod]
[ScriptMethod(UseHttpGet = true, ResponseFormat = ResponseFormat.Json)]
public TO HelloWorld()
{
    return base.HelloWorld();
}



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

namespace Project
{
    public class ContentTypeModule : 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("WebServiceJSONP.asmx"))
                return;
            else
                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);
        }
    }
}

<system.web><httpModules>
<add name="ContentTypeModule"  type="Project.ContentTypeModule, Project" />

// For not-JSON asmx:
[WebMethod]
[ScriptMethod(UseHttpGet = true, ResponseFormat = ResponseFormat.Json)]
public TO HelloWorld()
{
    return base.HelloWorld();
}



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

namespace Project
{
    public class ContentTypeModule : 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("WebServiceJSONP.asmx"))
                return;
            else
                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);
        }
    }
}

http://bloggingabout.net/blogs/adelkhalil/archive/2009/08/14/cross-domain-jsonp-with-jquery-call-step-by-step-guide.aspx

The filter only needs to know which requests need to be returned as JSONP, doesn't care how. Modify "OnBeginRequest" accordinly.
If original .asmx doesn't return JSON, extend it like example shows.
JSONP only works for GET requests, so don't forget to change [ScriptMethod] if needed.

Filter JSON->JSONP for .NET 3.5

ajax, Net, json

C#