private static void AdViewWillLeaveApplicationCallback(IntPtr bannerClient)
        {
            BannerClient client = IntPtrToBannerClient(bannerClient);

            if (client.OnAdLeftApplication != null)
            {
                client.OnAdLeftApplication(client, EventArgs.Empty);
            }
        }
        private static void AdViewDidDismissScreenCallback(IntPtr bannerClient)
        {
            BannerClient client = IntPtrToBannerClient(bannerClient);

            if (client.OnAdClosed != null)
            {
                client.OnAdClosed(client, EventArgs.Empty);
            }
        }
        private static void AdViewWillPresentScreenCallback(IntPtr bannerClient)
        {
            BannerClient client = IntPtrToBannerClient(bannerClient);

            if (client.OnAdOpened != null)
            {
                client.OnAdOpened(client, EventArgs.Empty);
            }
        }
        private static void AdViewDidReceiveAdCallback(IntPtr bannerClient)
        {
            BannerClient client = IntPtrToBannerClient(bannerClient);

            if (client.OnAdLoaded != null)
            {
                client.OnAdLoaded(client, EventArgs.Empty);
            }
        }
        private static void AdViewDidFailToReceiveAdWithErrorCallback(
            IntPtr bannerClient, string error)
        {
            BannerClient client = IntPtrToBannerClient(bannerClient);

            if (client.OnAdFailedToLoad != null)
            {
                AdFailureEventArgs args = new AdFailureEventArgs()
                {
                    Message = error
                };
                client.OnAdFailedToLoad(client, args);
            }
        }