Xamarin.Android/iOS で IsolatedStorage を使う ref: http://qiita.com/amay077/items/672aaf058357a4ecf8a5

IsolatedStorageiOSTestViewController.cs

using System;
using System.Drawing;

using MonoTouch.Foundation;
using MonoTouch.UIKit;
using System.IO.IsolatedStorage;
using System.IO;

namespace IsolatedStorageiOSTest
{
    public partial class IsolatedStorageiOSTestViewController : UIViewController
    {
        public IsolatedStorageiOSTestViewController() : base ("IsolatedStorageiOSTestViewController", null)
        {
        }

        public override void DidReceiveMemoryWarning()
        {
            // Releases the view if it doesn't have a superview.
            base.DidReceiveMemoryWarning();

            // Release any cached data, images, etc that aren't in use.
        }

        public override void ViewDidLoad()
        {
            base.ViewDidLoad();

            var file = IsolatedStorageFile.GetUserStoreForApplication();

            // 分離ストレージにtest.txtというファイルを作成しストリームを開く
            using (IsolatedStorageFileStream strm = file.CreateFile("test.txt"))
                using (StreamWriter writer = new StreamWriter(strm))
            {
                // データを書き込む
                writer.Write("Hello!");
                writer.Write("Storage.");
            }
        }

        public override bool ShouldAutorotateToInterfaceOrientation(UIInterfaceOrientation toInterfaceOrientation)
        {
            // Return true for supported orientations
            return (toInterfaceOrientation != UIInterfaceOrientation.PortraitUpsideDown);
        }
    }
}

MainActivity.cs

using System;

using Android.App;
using Android.Content;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Android.OS;
using System.IO.IsolatedStorage;
using System.IO;

namespace IsolatedStorageTest
{
    [Activity (Label = "IsolatedStorageTest", MainLauncher = true)]
    public class Activity1 : Activity
    {
        int count = 1;

        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.Main);

            var file = IsolatedStorageFile.GetUserStoreForApplication();

            // 分離ストレージにtest.txtというファイルを作成しストリームを開く
            using (IsolatedStorageFileStream strm = file.CreateFile("test.txt"))
            using (StreamWriter writer = new StreamWriter(strm))
            {
                // データを書き込む
                writer.Write("Hello!");
                writer.Write("Storage.");
            }
        }
    }
}