wp7 odata v2 dataservicestate save and restore methods tombstoning example needed

I am looking for 开发者_JS百科an example of how to use the new DataServiceState Save and Restore methods in a WP7 application in order to tombstone a datacontext - I cannot find any examples and the approach I used resulted in an exception

this saves the data context correctly

PhoneApplicationService.Current.State["DataContext"] = DataServiceState.Save(this.Model.Entities);

this attempts to restore it after the app is re-activated

var dc = (PhoneApplicationService.Current.State["DataContext"] as DataServiceState).Restore();

but throws an exception

An item could not be added to the collection. When items in a DataServiceCollection are tracked by the DataServiceContext, new items cannot be added before items have been loaded into the collection.

This is the same exception I get if I try to reload a datacontext that I stored "directly" (without using the DataServiceState.Save method) in the PhoneApplicationService.Current.State. I cannot find any offical documentation on the new ODATA v2 DataServiceState class or examples.

thanks Michael

I am looking for an example of how to use the new DataServiceState Save and Restore methods in a WP7 application in order to tombstone a datacontext

Are you sure? First link - bottom of the page.

Have you tried the walkthrough here?

How to: Preserve and Restore Application State for Windows Phone

Storing and retrieving datacontext is tricky due to serialization and object reference issues. The MSFT team is working on improving DataServiceState. I've succeeded in saving and restoring the context in a WP7 app, by storing the DataServiceState returned by Save() in the app state (just like you've done). Then on activated, I first instantiate my DataServiceClient (which contains the context and the DataServiceCollection), and then I call a RestoreData method in the client and pass the retrieved DataServiceState to it. The method restores the context and DSC within the DataServiceClient.