MVVM Saving xDoc file exception

I'm trying to learn c#/WPF/MVVM by converting a sample WPF app to MVVM. This app opens a xml file for editing and then saves it. The app preforms well until I try to save the file, I'm getting an InvalidCastException. Here is some code

mDataSource.cs

public static List<MediaItem> Load(string filename)
    {
        var mediafiles = XDocument.Load(filename).Root.Elements("style").Elements("item").Select(
            x => new MediaItem(
                (string)x.Element("title"),
                (string)x.Element("artist"),
                (string)x.Element("year")));

        return mediafiles.ToList();
    }

MainViewModel.cs - Load the xml file

public void LoadList(string filename)
    {
        this.mediafiles = new ObservableCollection<MediaItemViewModel>();

        List<MediaItem> mediabaseList = mDataSource.Load(filename);
        foreach (MediaItem mediaitem in mediabaseList)
        {
            this.mediafiles.Add(new MediaItemViewModel(mediaitem));
        }

        this.collectionView = CollectionViewSource.GetDefaultView(mediafiles);
        if (this.collectionView == null)
            throw new NullReferenceException("collectionView");

        this.collectionView.CurrentChanged += new EventHandler(this.OnCollectionViewCurrentChanged);
    }

Saving the file

 private void Save(ICollectionView collectionView)
    {
        mDataSource mds = new mDataSource();
        mds.Save(this.collectionView); 
    }

mDataSource - Saving the file, during debugging data shows up properly everywhere but the exception comes on the line - MediaItem mi = (MediaItem)mediaitem; {"Unable to cast object of type 'mList.ViewModels.MediaItemViewModel' to type 'mList.Models.MediaItem'."}

public void Save(ICollectionView collectionView)
    {
        XDocument xdoc = new XDocument();
        XElement xeRoot = new XElement("art");
        XElement xeSubRoot = new XElement("style");

        foreach (var mediaitem in collectionView)
        {
            MediaItem mi = (MediaItem)mediaitem; 

            XElement xRow = new XElement("item"); 
            xRow.Ad开发者_C百科d(new XElement("title", mi.Title));
            xRow.Add(new XElement("artist", mi.Artist));
            xRow.Add(new XElement("year", mi.Year));
            xeSubRoot.Add(xRow);
        }
        xeRoot.Add(xeSubRoot);
        xdoc.Add(xeRoot);
        xdoc.Save(filename);
    }

Thank You

ICollectionView (which shouldn't be referenced in your VM) contains a bunch of MediaItemViewModels.

this.mediafiles.Add(new MediaItemViewModel(mediaitem));

So, you need to get the MediaItem that is wrapped by the given MediaItemViewModel. You didn't include that code, so I can't tell you where the original MI is stored.

foreach (var mediaitem in collectionView.OfType<MediaItemViewModel>())
{
    MediaItem mi = mediaitem.ThisPropertyContainsTheWrappedMediaItem;