MyContentPage.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Xamarin.Forms;

namespace ModalNavigationVS
{
    class MyContentPage : ContentPage
    {
        public MyContentPage()
        {
            Button OkDialogButton = new Button
            {
                Text = "Show Alert"
            };

            OkDialogButton.Clicked += async (sender, e) =>
            {
                await DisplayAlert("Attention!", "You really to know about this.", "OK");
            };

            Button GetFeedbackButton = new Button
            {
                Text = "Get Feedback"
            };

            GetFeedbackButton.Clicked += async (sender, e) =>
            {
                Boolean answer = await DisplayAlert("Start", "Are you ready?", "Yes", "No");
            };

            StackLayout layout = new StackLayout
            {
                Children = { OkDialogButton, GetFeedbackButton }
            };

            this.Content = layout;
        }
    }
}