AdMob implement for display Google Admob in Unity project. NOTE: MonoSingleton is open source file on Github. and IAd is my interface for ad provider

AdMob.cs

using UnityEngine;
using System.Collections;
using GoogleMobileAds.Api;
using System;
using GoogleMobileAds;

public class AdMob : MonoSingleton<AdMob>, IAd
{

	[Header("Config ad id")]
	[SerializeField]
	private string
		iOSInterstitial;
	[SerializeField]
	private string
		androidInterstitial;
	[Header("Add list of test device which use for test purpose and only display test ad.")]
	[SerializeField]
	private string[]
		testDevices;
	[Header("Handle ad fail to load.")]
	[SerializeField]
	private int
		retryIfFail = 0;
	private int retry = 0;

	private InterstitialAd interstitial;

	public void prepareFullscreenAd ()
	{
		if (retry > 0) {
			// Retrying to load then we do nothing.
			return;
		}
		requestNewInterstitialAd ();
	}

	public void showFullsreenAd ()
	{

		if (interstitial != null && interstitial.IsLoaded ()) {
			interstitial.Show ();
		}
	}

	private void requestNewInterstitialAd ()
	{
		#if UNITY_EDITOR
		string adUnitId = "unused";
		#elif UNITY_ANDROID
            string adUnitId = androidInterstitial;
		#elif UNITY_IPHONE
            string adUnitId = iOSInterstitial;
		#else
            string adUnitId = "unexpected_platform";
		#endif

		// Create an interstitial.
		interstitial = new InterstitialAd (adUnitId);
		// Register for ad events.
		interstitial.AdLoaded += HandleInterstitialLoaded;
		interstitial.AdFailedToLoad += HandleInterstitialFailedToLoad;
		// Load an interstitial ad.
		interstitial.LoadAd (createAdRequest ());
	}

	// Returns an ad request with custom ad targeting.
	private AdRequest createAdRequest ()
	{
		AdRequest.Builder builder = new AdRequest.Builder ();
		if (testDevices != null && testDevices.Length > 0) {
			for (int i = 0; i < testDevices.Length; i++) {
				builder.AddTestDevice (testDevices [i]);
			}
		}
		return builder.Build ();
	}


#region Interstitial callback handlers
	public void HandleInterstitialLoaded (object sender, EventArgs args)
	{
		print ("HandleInterstitialLoaded event received.");
		retry = 0;
	}
	public void HandleInterstitialFailedToLoad (object sender, AdFailedToLoadEventArgs args)
	{
		Debug.Log ("HandleInterstitialFailedToLoad event received with message: " + args.Message);
		if (retry < retryIfFail) {
			requestNewInterstitialAd ();
			retry++;
		} else {
			retry = 0;
		}
		
		
	}
 #endregion
}