How to use CountDownTimer on Xamarin.Android

CountDown.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Android.App;
using Android.Content;
using Android.OS;
using Android.Runtime;
using Android.Views;
using Android.Widget;

namespace MyTimer
{
	public delegate void TickEvent (long millisUntilFinished);
	public delegate void FinishEvent ();

	public class CountDown : CountDownTimer
	{
		public event TickEvent Tick;
		public event FinishEvent Finish;

		public CountDown(long totaltime, long interval) 
			: base(totaltime,interval)
		{
		}

		public override void OnTick (long millisUntilFinished)
		{
			if (Tick != null)
				Tick (millisUntilFinished);
		}

		public override void OnFinish ()
		{
			if (Finish != null)
				Finish ();
		}
	}
}