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10th September 2010

Isolated Storage on Windows Phone 7

Filed under: C#,Databases,Windows Phone 7 — Tags: , , , — Alex Holt @ 9:30 pm

One of the most amazing omissions from the Windows Phone 7 OS is an SQL Server of some description that applications can use (I think there is a SQL Server Compact edition baked into the build, but it’s only exposed to the native apps). This leaves anyone familiar with doing all their storage via relational databases in a bit of a lurch.

Having a look around, there were a few work-arounds, namely third party libraries such as Perst & Sterling. These act as a layer of abstraction that allow you to use LINQ to access your objects and could be of benefit for some of the more complex applications. I found both of them to have their drawbacks.

Perst: From McObject is one alternative, but if you plan on doing a commercial application, you should bare in mind their licensing, of which, their FAQ says:

No, Perst is not free. McObject Perst is available under a dual license. Under the GPL, you may evaluate the source code free of charge and you may use Perst free of charge in an application for which the source code is also freely available. If you wish to use a Perst in an application but do not or cannot redistribute your application source code, you can use Perst under a commercial license.

Sterling: This seemed promising, but I had a number of issues getting the code to work (it was late, in fairness) I decided to give up.

In reality though, most applications that are going to be made don’t need any complicated database structure behind them. Database implementations are most likely going to be overkill. This is where I decided to do what probably the majority of WP7 developers have decided to do, and “Roll-Their-Own” database solution. Mine is based on a general accepted practice, using XML Serialisation to store my POCOs. While it might have limitations and performance issues later down the line, for now – it’s performing very¬†admirably and there seemed no point in premature optimisation just yet.

My Solution

The main class I created (MyDataContext.cs) contains the functions for both saving and loading the data, as well as the ‘schema’ for the entire database.

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using System;
using System.Collections.Generic;
using IsolatedExampleApp.Data.Models;
using System.IO.IsolatedStorage;
using System.IO;
using System.Xml.Serialization;
 
namespace IsolatedExampleApp.Data
{
  public class IsolatedDatabase
  {
    public IsolatedDatabase()
    {
      // For each model that is a list item, you need to add a initialising statement to the Constructor
      Albums = new List();
      Artists = new List();
    }
 
    // These properties effectively form the publicly viewable schema of the database.
    public List Albums { get; set; }
    public List Artists { get; set; }
    public Options Options { get; set; }
 
    public void Load()
    {
      using (IsolatedStorageFile store = IsolatedStorageFile.GetUserStoreForApplication())
      using (IsolatedStorageFileStream stream = new IsolatedStorageFileStream("IsolatedExampleApp.txt", FileMode.OpenOrCreate, FileAccess.Read, store))
      using (StreamReader reader = new StreamReader(stream))
      {
        XmlSerializer serializer = new XmlSerializer(typeof(IsolatedDatabase));
        var unserialized = reader.EndOfStream ? new IsolatedDatabase() : (IsolatedDatabase)serializer.Deserialize(reader);
 
        // Each schema is repopulated with information when the app is loaded.
        Albums = unserialized.Albums;
        Artists = unserialized.Artists;
        Options = unserialized.Options;
      }
    }
 
    public bool Save()
    {
      try
      {
        using (IsolatedStorageFile store = IsolatedStorageFile.GetUserStoreForApplication())
        using (IsolatedStorageFileStream stream = new IsolatedStorageFileStream("IsolatedExampleApp.txt", FileMode.Create, FileAccess.Write, store))
        {
          XmlSerializer serializer = new XmlSerializer(typeof(IsolatedDatabase));
          serializer.Serialize(stream, this);
        }
        return true;
      }
      catch (Exception ex)
      {
        return false;
      }
    }
  }
}

As you can see from the code above, as you develop more data stores, the code needs updating in multiple locations.

  • Properties need setting for each one (they are the publicly available access points for the data).
  • Any LISTS<> require their lists initialised by the Constructor method.
  • When the data is ‘Load()’ed, each property must have it’s values manually set.
  • All the Models still need defining elsewhere in your application (in my example above, they are in the IsolatedExampleApp.Data.Models namespace.

If you can overlook this repeated typing, then this seems to be a good starting point for persisting data in your Windows Phone 7 Application. To use it should then be fairly straight forward.

_db = new IsolatedDatabase();
_db.Load();
 
// Adding an album...
_db.Albums.Add(newAlbum);
_db.Save();
 
// Deleting an album...
_db.Albums.Remove(deleteAlbum);
_db.Save();
 
// Add or Update an album...
Album existingAlbum = _db.Albums.Where(x => x.ID == album.ID).FirstOrDefault();
if (existingAlbum != null)
{
  Delete(existingAlbum);
}
Add(album);
_db.Save();
 
// Or just getting a specific album...
IEnumerable albums = _db.Albums as IEnumerable;
return albums.Where(x => x.ID == id).FirstOrDefault();

Any feedback, thoughts, options and optimisations for the above code will be well received. Happy isolating!

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