void DoReview()
    {
        var spp = new StoreProductParameters(appId);
        var productViewController = new SKStoreProductViewController();
        // must set the Finished handler before displaying the view controller
        productViewController.Finished += (sender2,err) => {
            // Apple's docs says to use this method to close the view controller
            this.navigationController.DismissViewController(true,null);
            MySettings.AskedForReview = true;
        };
        productViewController.LoadProduct(spp,(ok,err) => { // ASYNC !!!
            if (ok)
            {
                this.navigationController.PresentViewController(productViewController,true,null);
            }
            else
            {
                Console.WriteLine(" Failed ");
                if (err != null)
                    Console.WriteLine(" with error " + err);
            }
        });
    }