problem with extending XMLHttpRequest in IE7and IE8

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ASP.NET

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Hi,
I wanted to extend XMLHttpRequest so that i could call another function in Onreadystatechange

here is code which is working in all browsers but IE7 and IE8

(function(open) {

    XMLHttpRequest.prototype.open = function(method, url, async, user, pass) {

        this.addEventListener("readystatechange", function() {
          if(this.readyState == "4"){myfunction();}
        }, false);

        open.call(this, method, url, async, user, pass);
    };

})(XMLHttpRequest.prototype.open);

i changed it to this one to make it work in IE7 and IE8 but it doesn't work

(function (open) {

    XMLHttpRequest.prototype.open = function (method, url, async, user, pass) {

        this.attachEvent("onreadystatechange", function () {
            if (this.readyState == "4") { Myfunction(); }
        });

        open.call(this, method, url, async, user, pass);
    };

})(XMLHttpRequest.prototype.open);

but it's interesting that XmlHttpRequest doesn't support neither attachEvent nor addEventListener.

Thanks