Estou tentando escrever um método para salvar alguns dados básicos de um jogo. No momento, estou tentando salvar dados em um xml formato com um pai<data> elemento contendo uma matriz de<leveldata> elementos que possuem campos para índice, conclusão e tentativas. Esta é a primeira vez que tentei serializar qualquer coisa em c #, por isso, encontrei alguns problemas. No momento, ele está caindo com uminvalidoperationexception quando tentodeserialize o arquivo (detalhes da exceção na parte inferior

Aqui está o código.

public void SaveData()
{
    const string filename = "data.vision";

#if WINDOWS_PHONE
    IsolatedStorageFile dataFile = IsolatedStorageFile.GetUserStoreForApplication();
#else
    IsolatedStorageFile dataFile = IsolatedStorageFile.GetUserStoreForDomain();
#endif
    try
    {
        // Create an isolated storage stream and initialize it as null.
        IsolatedStorageFileStream isolatedFileStream = null;

        // Open the isolated storage stream, and write the save data file.
        if (dataFile.FileExists(filename))
        {
            using (isolatedFileStream = dataFile.OpenFile(filename, FileMode.Open,  FileAccess.ReadWrite))
            {
                // Read the data from the file.
                XmlSerializer serializer = new XmlSerializer(typeof(Data));
                // Store each of the deserialized data objects in the list.
                Data savedData = (Data)serializer.Deserialize(isolatedFileStream);


                // Loop through the saved data objects.
                for(int i = 0; i < savedData.Levels.Count; i++)
                {
                    // Get the data object in question.
                    LevelData levelData = savedData.Levels[i];

                    // Check to see if the index of the data object corresponds to the active level index.
                    if (levelData.Index == mLevelIndex)
                    {
                        // Check that the attempts already saved is less.
                        if (levelData.Attempts < mLevel.AttemptCounter)
                            levelData.Attempts = mLevel.AttemptCounter;

                        // Check that the 
                        if (levelData.PercentComplete < 50)
                            levelData.PercentComplete = 50;
                    }
                }
                serializer.Serialize(isolatedFileStream, savedData);
            }  
        }
        else
        {
            // If there is no data file, create a new one.
            using (isolatedFileStream = dataFile.CreateFile(filename))
            {
                // Check the file stream has been initialized.
                if (isolatedFileStream != null)
                {
                    // Create a new data object to store the meta data for the current level.
                    Data data = new Data();
                    // Create a list to store the data already saved.
                    data.Levels = new List<LevelData>();

                    // Initialize the new data values.
                    LevelData levelData = new LevelData();
                    levelData.Index = mLevelIndex;
                    levelData.Attempts = mLevel.AttemptCounter;
                    levelData.PercentComplete = 50;

                    // Add the level data.
                    data.Levels.Add(levelData);

                    // Convert the object to XML data and put it in the stream.
                    XmlSerializer serializer = new XmlSerializer(typeof(Data));
                    // Seriaize the data.
                    serializer.Serialize(isolatedFileStream, data);
                }
            }
        }
    }
    finally
    {
        // Dispose the storage file, in order to commit changes.
        dataFile.Dispose();
    }
}

DADOS

public struct Data
{
    /// <summary>
    /// The Level data object.
    /// </summary>
    [XmlArray(ElementName = "Levels")]
    public List<LevelData> Levels;
}

Level Data

public struct LevelData
{
    /// <summary>
    /// The index of the level.
    /// </summary>
    [XmlElement(ElementName = "Index")]
    public int Index;

    /// <summary>
    /// The number of attempts the player has made for a particular level.
    /// </summary>
    [XmlElement(ElementName = "Attempts")]
    public int Attempts;

    /// <summary>
    /// A value describing the furthest the player has ever got within the level.
    /// </summary>
    [XmlElement(ElementName = "PercentComplete")]
    public int PercentComplete;
}

Detalhe da exceção:

{System.InvalidOperationException: There is an error in XML document (10, 10). ---> System.Xml.XmlException: Unexpected XML declaration. The XML declaration must be the first node in the document, and no white space characters are allowed to appear before it. Line 10, position 10.
   at System.Xml.XmlTextReaderImpl.Throw(Exception e)
   at System.Xml.XmlTextReaderImpl.Throw(String res, String arg)
   at System.Xml.XmlTextReaderImpl.ParsePI(BufferBuilder piInDtdStringBuilder)
   at System.Xml.XmlTextReaderImpl.ParseDocumentContent()
   at System.Xml.XmlTextReaderImpl.Read()
   at System.Xml.XmlReader.ReadEndElement()
   at System.Xml.Serialization.XmlSerializationReader.ReadEndElement()
   at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReaderData.Read3_Data(Boolean checkType)
   at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReaderData.Read4_Data()
   --- End of inner exception stack trace ---
   at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle, Object events)
   at System.Xml.Serialization.XmlSerializer.Deserialize(Stream stream)
   at Vision.GameplayScreen.SaveData()
   at Vision.GameplayScreen.HandleInput(InputState input)
   at Vision.ScreenManager.Update(GameTime gameTime)
   at Microsoft.Xna.Framework.Game.<.cctor>b__19(IUpdateable updateable, GameTime gameTime)
   at Microsoft.Xna.Framework.Game.SortingFilteringCollection`1.ForEachFilteredItem[TUserData](Action`2 action, TUserData userData)
   at Microsoft.Xna.Framework.Game.Update(GameTime gameTime)
   at Vision.Game.Update(GameTime gameTime)
   at Microsoft.Xna.Framework.Game.DoUpdate(GameTime gameTime)
   at Microsoft.Xna.Framework.Game.Tick()
   at MonoGame.Framework.WindowsPhone.SurfaceUpdateHandler.Draw(Device device, DeviceContext context, RenderTargetView renderTargetView)
   at MonoGame.Framework.WindowsPhone.DrawingSurfaceUpdateHandler.DrawingSurfaceContentProvider.GetTexture(Size2F surfaceSize, DrawingSurfaceSynchronizedTexture& synchronizedTexture, RectangleF& textureSubRectangle)
   at SharpDX.Direct3D11.DrawingSurfaceContentProviderShadow.DrawingSurfaceContentProviderVtbl.GetTexture(IntPtr thisPtr, IntPtr surfaceSize, IntPtr synchronizedTexture, IntPtr textureSubRectangle)}