How to apply a web service extension?

 using System;
using System.Web;
using System.Web.Services;
using System.Web.Services.Protocols;
using System.Data.SqlClient;
using System.Web.Services.Description;
using System.Xml;
[WebService(Name = "MyWebService", Description = "This web service is used to experi,ment with some of the Web service concepts.", Namespace = "http://www.mycompany.com")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
public class MyWebService : System.Web.Services.WebService
{
[CustomSoapExtension.TestSoapExtension()]
[WebMethod()]
public string HelloWorld()
{
return "Hello, World!";
}
}
...