I've been working on an update to the Tasks application which adds in background synchronisation using the new background task support in Windows Phone 7.5. As with any app when you submit your XAP static analysis is performed on it to check, for example, that you have declared the correct capabilities based on the functionality in your app. Within a background agent you are only allow to call a subset of APIs - as you would expect you can't play sound effects or draw to the UI (with the exception of toast notifications and tile updates). Because the main app has to include a reference to the dll which provides the background task, and the functionality within the dll is used in both the background task and the main app it made sense to move all of the logic into the background dll. This meant including a number of third-party references. I was very careful to ensure that nowhere in the background task was any restricted API called and it worked perfectly in testing.
The first problem is that upon submitting the static anaylsis looks at all of the referenced dlls and sees that they contain APIs not permitted in a background task and blocks the XAP. One of the culprits was InTheHand.Phone.dll (Part of Mobile In The Hand) because it provides a mixture of helper classes for logic and for UI. As I've been working on the 7.1 version in tandem I've now split this into two assemblies for Windows Phone 7.1 SDK projects. This means we can continue to use InTheHand.Phone in background tasks and add a reference to the second assembly in the main application only.
The second problem is that code within the sync logic calls ScheduledActionService.Add which is disallowed within a background task. This API is used within Tasks to add system-wide reminders (Another new feature for this version) for new task items. The code also has the ability to update and delete reminders based on modified or deleted tasks but these do not seem to have been flagged up as errors. This is a pain because it means that if the device syncs a new task with a reminder set to show before you next open the foreground app we won't have had a chance to register it.
The other issue with the NeutralResourceLanguage is an issue with projects originally created with an older version of the development tools. You can add this setting from the projects properties, Application tab and click "Assembly Information...".
This has been an interesting learning experience and shows that the validation picks up not just APIs used in the background agent code-path but also in referenced assemblies.