C#

[Serializable]
public class _ModelBase
{
    public Guid Id { get; set; }
    public string Caption { get; set; }
    public string Description { get; set; }
    public bool IsActive { get; set; }
    public int Sequence { get; set; }
    public ImageSource Image { get; set; }
}

[Serializable]
public class FolderModel : _ModelBase
{
    public List<FolderModel> Folders { get; set; }
    public List<FileModel> Files { get; set; }
    public bool IsExpanded { get; set; }

    public FolderModel()
    {
        Folders = new List<FolderModel>();
        Files = new List<FileModel>();
    }
}

[Serializable]
public class FileModel : _ModelBase
{
    public string FileName { get; set; }
}

private void saveAppDataToFile()
{
    using (TextWriter textWriter = new StreamWriter(xmlDataFile))
    {
        XmlSerializer serializer = new XmlSerializer(typeof(FolderModel));
        serializer.Serialize(textWriter, Folders);
        textWriter.Close();
    }
}

"There was an error generating the XML document."

"Unable to cast object of type 'System.Collections.Generic.List`1[MyApp.Models.FolderModel]' to type 'MyApp.Models.FolderModel'"}