C# - Xamarin - インタフェースと抽象クラス - クラスに約束を守らせる(拡張、アップキャスト、ダウンキャスト、as演算子、null)

Head First C#
原著

開発環境

• OS: Windows 10 Pro
• IDE(統合開発環境): Visual Studio Community 2017

Head First C# ―頭とからだで覚えるC#の基本 (Andrew Stellman (著)、Jennifer Greene (著)、佐藤 嘉一 (監修, 監修)、木下 哲也 (翻訳)、オライリージャパン)の6章(インタフェースと抽象クラス - クラスに約束を守らせる)、エクササイズ(p. 241)を取り組んでみる。

コード

MainPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:App2"
             x:Class="App2.MainPage">

    <StackLayout>
        <Button x:Name="button1"
                Text="button1"
                Clicked="button1_Clicked"/>
    </StackLayout>

</ContentPage>

MainPage.xaml.cs

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

namespace App2
{
 public partial class MainPage : ContentPage
 {
  public MainPage()
  {
   InitializeComponent();
  }

        private void button1_Clicked(object sender, EventArgs e)
        {
            ScaryScary fingersTheClown = new ScaryScary("大きな靴", 32);
            FunnyFunny someFunnyClown = fingersTheClown;
            IScaryClown someOtherScarClown = someFunnyClown as ScaryScary;
            if (someOtherScarClown != null)
            {
                someOtherScarClown.Honk();
            }
        }
    }
    public interface IClown
    {
        string FunnyThingIHave { get; }
        void Honk();
    }
    public interface IScaryClown: IClown
    {
        string ScaryThingHave { get; }
        void ScareLittleChildren();
    }
    public class FunnyFunny : ContentPage, IClown
    {
        private string funnyThingIHave;

        public FunnyFunny(string funnyThingIHave)
        {
            this.funnyThingIHave = funnyThingIHave;
        }

        public string FunnyThingIHave => funnyThingIHave;

        public void Honk()
        {
            DisplayAlert("", "プップー！私は" + funnyThingIHave, "cancel");
        }
    }
    public class ScaryScary : FunnyFunny, IScaryClown
    {
        private int numberOfScaryThings;
        public ScaryScary(string funnyThingIHave, int numberOfScaryThings):
            base(funnyThingIHave)
        {
            this.numberOfScaryThings = numberOfScaryThings;
        }
        public string ScaryThingHave => numberOfScaryThings + "匹のクモを持っています";

        public void ScareLittleChildren()
        {
            DisplayAlert("", "ブー！残念！", "canel");
        }
    }
}

App.xaml

<?xml version="1.0" encoding="utf-8" ?>
<Application xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="App1.App">
 <Application.Resources>
    
 </Application.Resources>
</Application>

App.xaml.cs

using System;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

[assembly: XamlCompilation(XamlCompilationOptions.Compile)]
namespace App1
{
    public partial class App : Application
    {
        public App()
        {
            InitializeComponent();

            // MainPage = new MainPage();
            MainPage = new NavigationPage(
                new MainPage() { Title = "" });
        }

        protected override void OnStart()
        {
            // Handle when your app starts
        }

        protected override void OnSleep()
        {
            // Handle when your app sleeps
        }

        protected override void OnResume()
        {
            // Handle when your app resumes
        }
    }
}