Read write data for Windows Store/Phone C#

I'm going to answer my own question. It took me a month, but I finally have a solution that works.

The solution is to create a Windows Phone Plugin, but you also need to create a dummy plugin for Unity to use. Here's a guide on how to do this: http://docs.unity3d.com/Manual/wp8-plugins-guide-csharp.html

However, I found it difficult to create the plugin because I did not have a professional license of Visual Studio. I ended up using Visual Studio Express 2012 to create the Unity plugin and Visual Studio 2012 for Windows Phone to create the WP plugin. I then copied the WP project into the VS 2012 solution and was able to work on both together.

Credit for the serialization code goes to Carlos Figueira on this thread .

PLUGIN CODE

In my Real DLL project, I have a class called FileIO, with 2 functions:

 using System.Runtime.Serialization;
 
 namespace UnityWindowsPhonePlugin
 {
 
     public class FileIO
     {
         
         public static byte[] SerializeObject<T>(T obj)
         {
             using (MemoryStream ms = new MemoryStream())
             {
                 using (XmlDictionaryWriter writer = XmlDictionaryWriter.CreateBinaryWriter(ms))
                 {
                     DataContractSerializer dcs = new DataContractSerializer(typeof(T));
 
                     dcs.WriteObject(writer, obj);
                     writer.Flush();
                     return ms.ToArray();
                 }
             }
         }
 
         public static T DeserializeObject<T>(byte[] obj )
         {
             using (MemoryStream memoryStream = new MemoryStream(obj))
             {
                 using (XmlDictionaryReader reader = XmlDictionaryReader.CreateBinaryReader(memoryStream, XmlDictionaryReaderQuotas.Max))
                 {
                     DataContractSerializer dcs = new DataContractSerializer(typeof(T));
                     return (T)dcs.ReadObject(reader);
                 }
             }
 
         }
 
     }
 }

For my Fake DLL, I implement fake versions of the same classes.

 namespace UnityWindowsPhonePlugin
 {
     public class FileIO
     {
 
         public static byte[] SerializeObject<T>(T obj) 
         {
             return new byte[0];
         }
 
         public static T DeserializeObject<T>(byte[] obj) 
         {
             return default(T);
         }
     }
 }

UNITY CODE

Copy your Real DLL to Assets\Plugins\WP8, and your fake DLL to Assets\Plugins.

In Unity, this is how I save my game progress:

 #if UNITY_WINRT
         byte[] bytes = UnityWindowsPhonePlugin.FileIO.SerializeObject<GameProgress>(this);
         File.WriteAllBytes(getSaveFile(), bytes);
 #else
         BinaryFormatter bf = new BinaryFormatter();
         FileStream fs = File.Create(getSaveFile());
         bf.Serialize(fs, this);
         fs.Close();
 #endif

And I load my Game Progress like this:

 byte[] bytes = File.ReadAllBytes(saveFile);        
 GameProgress gp = UnityWindowsPhonePlugin.FileIO.DeserializeObject<GameProgress>(bytes);

Hope that helps others who are stuck with the same problem I was. Let me know if you need help with the plugin. I might be able to send you my VS project.