How to debug serialization error in tombstoned WP7 app

Question

I am handling the current state of my WP7 app in the OnNavigatedFrom and To events so that if the app is tombstoned OR they navigate to a different page it saves the state to the inbuilt PhoneApplicationService state.

I have a simple ViewModel which I dump to the state to make life easier. When navigate to a different page within my app, the state is stored fine and restores fine (ie. I assume it is serializing at this point). However, when I tombstone the app (i.e. Start button) I am getting an unhandled error with serialization and the stacktrace is not giving me any clues as to why it is failing.

I have tried wrapping the actual call in a try catch block to try and see what is wrong but it doesn't help - it's something that the runtime is doing different on tombstoning with my object than when it just stores it between pages.

Here is my code:

protected override void OnNavigatedFrom(NavigationEventArgs args)
{
   appService.State["TournamentViewModel"] = tournamentViewModel;
   base.OnNavigatedFrom(args);
}

protected override void OnNavigatedTo(NavigationEventArgs args)
{
    if (appService.State.ContainsKey("TournamentViewModel"))
    {
        tournamentViewModel = (TournamentViewModel)appService.State["TournamentViewModel"];
    }
    base.OnNavigatedTo(args);
}

This is the error that is generated - I really can't figure out where the problem is - how can I debug this better?

Exception Info: Message: "SecurityException" InnerException: "Could not evaluate expression"

StackTrace:

   at System.Runtime.Serialization.DataContract.DataContractCriticalHelper.CreateGetOnlyCollectionDataContract(Int32 id, RuntimeTypeHandle typeHandle, Type type)
   at System.Runtime.Serialization.DataContract.DataContractCriticalHelper.GetGetOnlyCollectionDataContractSkipValidation(Int32 id, RuntimeTypeHandle typeHandle, Type type)
   at System.Runtime.Serialization.DataContract.GetGetOnlyCollectionDataContractSkipValidation(Int32 id, RuntimeTypeHandle typeHandle, Type type)
   at System.Runtime.Serialization.XmlObjectSerializerContext.GetDataContractSkipValidation(Int32 typeId, RuntimeTypeHandle typeHandle, Type type)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithXsiType(XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle objectTypeHandle, Type objectType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle, Type declaredType)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerialize(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerializeReference(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
   at System.Reflection.RuntimeMethodInfo.InternalInvoke(RuntimeMethodInfo rtmi, Object obj, BindingFlags invokeAttr, Binder binder, Object parameters, CultureInfo culture, Boolean isBinderDefault, Assembly caller, Boolean verifyAccess, StackCrawlMark& stackMark)
   at System.Reflection.RuntimeMethodInfo.InternalInvoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, StackCrawlMark& stackMark)
   at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
   at System.Runtime.Serialization.XmlFormatWriter.InternalSerialize(MethodInfo methodInfo, Object memberValue, Type memberType, Boolean writeXsiType, XmlObjectSerializerWriteContext context, XmlWriterDelegator xmlWriter)
   at System.Runtime.Serialization.XmlFormatWriter.WriteValue(Type memberType, Object memberValue, Boolean writeXsiType, XmlObjectSerializerWriteContext context, XmlWriterDelegator xmlWriter)
   at System.Runtime.Serialization.XmlFormatWriter.WriteMember(SerializingObject serObj, Int32 memberIndex, ClassDataContract derivedMostClassContract)
   at System.Runtime.Serialization.XmlFormatWriter.WriteClass(CallStackElement`1 callStackElement)
   at System.Runtime.Serialization.XmlFormatWriter.Serialize(XmlObjectSerializerWriteContext context)
   at System.Runtime.Serialization.XmlFormatWriter.InitializeCallStack(XmlWriterDelegator xmlWriterDel, Object obj, XmlObjectSerializerWriteContext writeContext, DataContract contract)
   at System.Runtime.Serialization.CollectionDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.WriteDataContractValue(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle)
   at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithoutXsiType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle)
   at System.Runtime.Serialization.DataContractSerializer.InternalWriteObjectContent(XmlWriterDelegator writer, Object graph)
   at System.Runtime.Serialization.DataContractSerializer.InternalWriteObject(XmlWriterDelegator writer, Object graph)
   at System.Runtime.Serialization.XmlObjectSerializer.WriteObjectHandleExceptions(XmlWriterDelegator writer, Object graph)
   at System.Runtime.Serialization.XmlObjectSerializer.WriteObject(XmlDictionaryWriter writer, Object graph)
   at System.Runtime.Serialization.XmlObjectSerializer.WriteObject(Stream stream, Object graph)
   at Microsoft.Phone.Shell.StreamPersister.Serialize(IDictionary`2 dictionary, IEnumerable`1 knownTypes)
   at Microsoft.Phone.Shell.StreamPersister.Save(ShellPageManager shellPageManager, String key, IDictionary`2 dictionary, IEnumerable`1 knownTypes)
   at Microsoft.Phone.Shell.PhoneApplicationService.FireDeactivated()
   at Microsoft.Phone.Execution.NativeEmInterop.FireOnPause()

Update: By trial and error I found the problem to be the AppBar, which does not seem to be serializable. I marked it with [IgnoreDataMember] and it now throws a more meaningful error on a different problem. I would still like to know if there are any easy ways of catching this info...

Answer 1

Well, how does your ViewModel express the way in which it's serializable? Personally I tend to try to steer clear of potentially-intricate serialization: one fairly simple option is to give your ViewModel the ability to explicitly convert itself to/from XML, and then perform that step manually yourself, saving an XDocument (or its string representation) in the application state. That way it's easy to debug the serialization step, see the exact XML being generated etc.

Of course, the use of XML here is incidental - if you can easily put everything in a CSV string, that would be fine too. Anything that you can easily inspect in terms of the serialized form before putting in application state.

I realise this is side-stepping some of the supposed benefits of the "automatic" serialization, but I've run into enough hard-to-diagnose problems with automatic serialization of arbitrary objects that I don't think the benefits outweigh the drawbacks.

Answer 2

Rodney,

My understanding is that when you put an object in one of the two State Dictionaries (the one off of PhoneApplicationPage and the other off of PhoneApplicationServices) it does not immediately serialize or deserialize.

If you use the Page.State to store data, it serializes your state bag when you leave the page, and deserializes when you return to the page (rather than when you add or read an object from the state bag).

If you use the PhoneApplicationService.State, serialization happens when you tombstone and deserialization when the application is re-activated (though I'm not sure if this is tied to the PhoneApplicationService.Activated event or not).

Page.State, by the way, doesn't allow sharing data between pages. Saving to PhoneApplicationService.State does allow you to do so.

IsolatedStorage.AppSettings seems to handle deserialization problems silently, so I'm not sure when that happens. Serialization, however, occurs when you call Save().

James