ローカルストレージに対する共通ファイルアクセス方法の例

HelloPCLStorage.cs

using System;
using System.Threading.Tasks;
using PCLStorage;
using Xamarin.Forms;

namespace HelloPCLStorage
{
	public class App : Application
	{
		private Entry usernameEntry;

		public App()
		{
			var saveButton = new Button
			{
				Text = "保存"
			};

			usernameEntry = new Entry
			{
				Placeholder = "Username"
			};
			// The root page of your application
			var content = new ContentPage
			{
				Title = "HelloPCLStorage",
				Content = new StackLayout
				{
					VerticalOptions = LayoutOptions.Center,
					Children = {
						new Label {
							HorizontalTextAlignment = TextAlignment.Center,
							Text = "Welcome to Xamarin Forms!"
						},
						usernameEntry,
						saveButton
					}
				}
			};

			saveButton.Clicked += async (sender, e) =>
			{
				await SaveSettings();
			};

			MainPage = new NavigationPage(content);

		}

		private string settingsFilename = "settings.dat";

		public async Task LoadSettings()
		{
			// 保存先のフォルダインスタンスを生成
			IFolder rootFolder = FileSystem.Current.LocalStorage;
			ExistenceCheckResult result = await rootFolder.CheckExistsAsync(settingsFilename);
			switch (result)
			{
				case ExistenceCheckResult.FileExists:
					// 存在する時だけ取得反映
					IFile file = await rootFolder.GetFileAsync(settingsFilename);
					usernameEntry.Text = await file.ReadAllTextAsync();
					break;
				case ExistenceCheckResult.FolderExists:
				case ExistenceCheckResult.NotFound:
				default:
					break;
			}
		}

		public async Task SaveSettings()
		{
			// 保存先のフォルダインスタンスを生成
			IFolder rootFolder = FileSystem.Current.LocalStorage;
			// 保存先のファイルを開く（存在いている場合は上書き
			IFile file = await rootFolder.CreateFileAsync(settingsFilename, 
			                                              CreationCollisionOption.ReplaceExisting);
			// 単一項目のみなので、文字列をそのまま保存
			await file.WriteAllTextAsync(usernameEntry.Text);
		}

		protected override async void OnStart()
		{
			// Handle when your app starts
			await LoadSettings();
		}

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

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