New Post: object does not contains method or property

function WebClient() {

    DownloadString: function (path, callback){
        var wc = new System.Net.WebClient();
            
        var result = wc.DownloadString(path);
        if (typeof(callback) != "undefined")
        {
                callback(result);
        }
        return result;
    };

}

var w = new WebClient();
w.DownloadString("http://www.google.de/");