private async void StartCountdownTimer(int startingVal = 0, CancellationToken token)
    {
        var countdownView = FindViewById<TextView>(Resource.Id.large_text_label);

        if (startingVal <= 0 || countdownView == null)
        {
            // Timer is invalid.
            return;
        }

        while (startingVal > 0 && !token.IsCancellationRequested)
        {
            RunOnUiThread(() => countdownView.Text = startingVal.ToString());
            await Task.Delay(1000, token);
        }

        // Finished
    }

این لینک هم ببینید