Windows 8 C# Serialization of ObservableCollection. Getting error: System.InvalidOperationException

Question

I am trying to Serialize an ObservableCollection. But I am getting following error: Let me know how can I fix this?

Error:

{System.InvalidOperationException: There was an error reflecting type 'System.Collections.ObjectModel.ObservableCollection1[eText.DataModel.BooksDownloadedData]'. ---> System.InvalidOperationException: There was an error reflecting type 'eText.DataModel.BooksDownloadedData'. ---> System.InvalidOperationException: Cannot serialize member 'eText.DataModel.BooksDownloadedData.DownloadedBookFileDetails' of type 'Windows.Storage.StorageFile', see inner exception for more details. ---> System.InvalidOperationException: Windows.Storage.StorageFile cannot be serialized because it does not have a parameterless constructor. --- End of inner exception stack trace --- at System.Xml.Serialization.StructModel.CheckSupportedMember(TypeDesc typeDesc, MemberInfo member, Type type) at System.Xml.Serialization.StructModel.GetPropertyModel(PropertyInfo propertyInfo) at System.Xml.Serialization.StructModel.GetFieldModel(MemberInfo memberInfo) at System.Xml.Serialization.XmlReflectionImporter.InitializeStructMembers(StructMapping mapping, StructModel model, Boolean openModel, String typeName, RecursionLimiter limiter) at System.Xml.Serialization.XmlReflectionImporter.ImportStructLikeMapping(StructModel model, String ns, Boolean openModel, XmlAttributes a, RecursionLimiter limiter) at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping(TypeModel model, String ns, ImportContext context, String dataType, XmlAttributes a, Boolean repeats, Boolean openModel, RecursionLimiter limiter) --- End of inner exception stack trace --- at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping(TypeModel model, String ns, ImportContext context, String dataType, XmlAttributes a, Boolean repeats, Boolean openModel, RecursionLimiter limiter) at System.Xml.Serialization.XmlReflectionImporter.CreateArrayElementsFromAttributes(ArrayMapping arrayMapping, XmlArrayItemAttributes attributes, Type arrayElementType, String arrayElementNs, RecursionLimiter limiter)
at System.Xml.Serialization.XmlReflectionImporter.ImportArrayLikeMapping(ArrayModel model, String ns, RecursionLimiter limiter) at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping(TypeModel model, String ns, ImportContext context, String dataType, XmlAttributes a, Boolean repeats, Boolean openModel, RecursionLimiter limiter) --- End of inner exception stack trace --- at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping(TypeModel model, String ns, ImportContext context, String dataType, XmlAttributes a, Boolean repeats, Boolean openModel, RecursionLimiter limiter) at System.Xml.Serialization.XmlReflectionImporter.ImportElement(TypeModel model, XmlRootAttribute root, String defaultNamespace, RecursionLimiter limiter) at System.Xml.Serialization.XmlReflectionImporter.ImportTypeMapping(Type type, XmlRootAttribute root, String defaultNamespace) at System.Xml.Serialization.XmlSerializer..ctor(Type type, String defaultNamespace) at System.Xml.Serialization.XmlSerializer..ctor(Type type) at eText.Common.Xml.Serialize[T](Object obj, Type[] extraTypes) in f:\TFS\New TFS\PDFReader\Posh.PdfReader.Win8\eText\Common\Utility.cs:line 81
at eText.Common.ApplicationSettings.<SaveDataToFileAsync>d__01.MoveNext() in f:\TFS\New TFS\PDFReader\Posh.PdfReader.Win8\eText\Common\ApplicationSettings.cs:line 16 --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at System.Runtime.CompilerServices.TaskAwaiter.GetResult()
at eText.ViewModel.MainViewModel.d__a.MoveNext() in f:\TFS\New TFS\PDFReader\Posh.PdfReader.Win8\eText\ViewModel\MainViewModel.cs:line 268}

Code That I am using to serialize:

Book Class:

[DataContract] 
public class Book : ViewModelBase
{
    #region Constructor

    /// <summary>
    /// Initializes a new instance of the <see cref="Book" /> class.
    /// </summary>
    public Book(){}

    /// <summary>
    /// Initializes a new instance of the <see cref="Book" /> class.
    /// </summary>
    /// <param name="BookTitle">The book title.</param>
    /// <param name="BookCategory">The book category.</param>
    /// <param name="DownloadURL">The download URL.</param>
    /// <param name="TotalNumberOfPages">The total number of pages.</param>
    /// <param name="BookAuthor">The book author.</param>
    /// <param name="ImageURL">The image URL.</param>
    public Book(string BookTitle,string BookCategory,string DownloadURL, string TotalNumberOfPages, string BookAuthor, string ImageURL)
    {
        this.BookCategory = BookCategory;
        this.BookTitle = BookTitle;
        this.BookAuthor = BookAuthor;
        this.TotalNumberOfPages = TotalNumberOfPages;
        this.DownloadURL = DownloadURL;
        this.ImageURL = ImageURL;
    }

    #endregion    
}

BooksDownloadedData class

 /// <summary>
    /// Class to store downloaded books metaData
    /// </summary>
    [DataContract] 
    public class BooksDownloadedData
    {

        /// <summary>
        /// The downloaded books detail
        /// </summary>
        [DataMember]
        public Book DownloadedBooks { get; set; }

        /// <summary>
        /// Gets or sets a value indicating whether this instance is downloaded completed.
        /// </summary>
        /// <value>
        /// <c>true</c> if this instance is downloaded completed; otherwise, <c>false</c>.
        /// </value>
       [DataMember]
        public bool IsDownloadedCompleted { get; set; }
}

Created an ObservableCollection for BooksDownloadedData and then saving it in Local Storage after serializing but at time of serializing getting error:

// Create object to get the saved meta data of files
  ObservableCollection<BooksDownloadedData> downLoadedFiles;

 // Save the data to the local storage
                await ApplicationSettings.SaveDataToFileAsync<ObservableCollection<BooksDownloadedData>>(fileName, downLoadedFiles);

 public static async Task SaveDataToFileAsync<T>(string key, T value, bool roaming = false, Type[] extraTypes = null)
        {
            var file = roaming ? await ApplicationData.Current.RoamingFolder.CreateFileAsync(key + ".xml", CreationCollisionOption.ReplaceExisting) :
                await ApplicationData.Current.LocalFolder.CreateFileAsync(key + ".xml", CreationCollisionOption.ReplaceExisting);

            var xml = Xml.Serialize<T>(value, extraTypes);
            await FileIO.WriteTextAsync(file, xml, UnicodeEncoding.Utf8);
        }

 public static string Serialize<T>(object obj, Type[] extraTypes = null)
        {
            using (var sw = new StringWriter())
            {
                var serializer = extraTypes == null ? new XmlSerializer(typeof(T)) : new XmlSerializer(typeof(T), extraTypes);
                serializer.Serialize(sw, obj);
                return sw.ToString();
            }
        }

Answer 1

From your stack trace I see:

Cannot serialize member 'eText.DataModel.BooksDownloadedData.DownloadedBookFileDetails' of type 'Windows.Storage.StorageFile', see inner exception for more details. ---> System.InvalidOperationException: Windows.Storage.StorageFile cannot be serialized because it does not have a parameterless constructor.

One of the limitations of the XmlSerializer class is that the class being serialised must have a parameterless constructor.

Are you able to use the DataContractSerializer instead?

Answer 2

The problem is the use of Windows.Storage.StorageFile, which clearly isn't intended to play nicely with serialization (or at least: not XmlSerializer).

You might be able to refactor your model so that you don't need to use StorageFile. You might be able to mark that member (DownloadedBookFileDetails) with [XmlIgnore], and re-create the StorageFile manually after deserialization. You might be able to use a different serializer (DataContractSerializer, JSON.NET, protobuf-net, etc).

However! In most cases I see (and I deal with serialization a lot) the key mistake people make is trying to fight the serializer so that they can keep using their existing domain model. When actually, the simplest thing to do is usually: create a DTO: a separate model that is designed solely to represent the data as a bridge to serialization, and then map to/from the DTO model and your main domain model.

Not only does this invariably work really easily every time, it also puts you in a very strong position if you ever want to:

• revise the domain model without affecting serialized data (you only tweak the mapping)
• add a separate version / API that has a different layout (public vs private APIs, maybe)
• use a different serializer (in addition, or instead of)