How to Serialize CookieContainer in wp7 applications?

Question

I tried to Serialize cookie to save it and Deserialize at next time I start my application.But the result of Deserialize is empty.What's wrong?

void SaveCookie() {
    var appStorage = IsolatedStorageFile.GetUserStoreForApplication();
    if (this.checkBox_save_passowrd.IsChecked == true)
    {
        CookieContainer cc = SEC_Services.Httprequest.cookie;
        string fileName = "usercookie.xml";
        using (var file = appStorage.OpenFile(fileName, System.IO.FileMode.OpenOrCreate, System.IO.FileAccess.Write))
        {
            using (var writer = new StreamWriter(file))
            {
                System.Xml.Serialization.XmlSerializer xs = new System.Xml.Serialization.XmlSerializer(typeof(CookieContainer));
                xs.Serialize(writer, cc);
                writer.Close();
            }
        }
    }
    else {
        if (appStorage.FileExists("usercookie.xml"))
        {
            appStorage.DeleteFile("usercookie.xml");
        }
    }
}

void ReadCookie() {
    var appStorage = IsolatedStorageFile.GetUserStoreForApplication();
    if (appStorage.FileExists("usercookie.xml"))
    {
        using (System.IO.StreamReader reader = new StreamReader(appStorage.OpenFile("usercookie.xml", FileMode.Open)))
        {
            System.Xml.Serialization.XmlSerializer xs = new System.Xml.Serialization.XmlSerializer(typeof(CookieContainer));
            CookieContainer obj = (CookieContainer)xs.Deserialize(reader);
            reader.Close();
            SEC_Services.Httprequest.cookie = obj;
            if (obj.Count != 0) {
                NavigationService.Navigate(new Uri("/PanoramaPage.xaml", UriKind.Relative));
            }
        }
    }
}

I also found this simple C#: Writing a CookieContainer to Disk and Loading Back In For Use shows that CookieContainer could be Serialize.But there is no SoapFormatter in wp7 library