private static void InterstitialDidDismissCallback(
            IntPtr interstitialClient)
        {
            InterstitialClient client = IntPtrToInterstitialClient(
                interstitialClient);

            if (client.OnInterstitialDismissed != null)
            {
                client.OnInterstitialDismissed(client, EventArgs.Empty);
            }
        }
        private static void InterstitialWillLeaveApplicationCallback(
            IntPtr interstitialClient)
        {
            InterstitialClient client = IntPtrToInterstitialClient(
                interstitialClient);

            if (client.OnInterstitialLeftApplication != null)
            {
                client.OnInterstitialLeftApplication(client, EventArgs.Empty);
            }
        }
        private static void InterstitialWillAppearCallback(
            IntPtr interstitialClient)
        {
            InterstitialClient client = IntPtrToInterstitialClient(
                interstitialClient);

            if (client.OnInterstitialShown != null)
            {
                client.OnInterstitialShown(client, EventArgs.Empty);
            }
        }
        private static void InterstitialWillPresentScreenCallback(
            IntPtr interstitialClient)
        {
            InterstitialClient client = IntPtrToInterstitialClient(
                interstitialClient);

            if (client.OnInterstitialOpened != null)
            {
                client.OnInterstitialOpened(client, EventArgs.Empty);
            }
        }
        private static void InterstitialDidFailToShowCallback(
            IntPtr interstitialClient, string error)
        {
            InterstitialClient client = IntPtrToInterstitialClient(
                interstitialClient);

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