As a quick note for other developers that may be getting the same (difficult to understand) error, the Microsoft Sync Framework version 2 is not as thread-friendly as one might expect. The API documentation makes it clear that class instances in the framework are not thread-safe, however, this thread-unsafety goes farther. Even when the instance is protected by proper locking to prevent concurrent access, it may still error when accessed from multiple threads. For example, if a SyncOrchestrator and 2 FileSyncProviders are initialized on one thread and (later) Synchronize is called from another thread, the following exception will be thrown:
System.InvalidCastException: Specified cast is not valid.
at Microsoft.Synchronization.CoreInterop.SyncServicesClass.CreateSyncSession(ISyncProvider pDestinationProvider, ISyncProvider pSourceProvider)
at Microsoft.Synchronization.KnowledgeSyncOrchestrator.DoOneWaySyncHelper(SyncIdFormatGroup sourceIdFormats, SyncIdFormatGroup destinationIdFormats, KnowledgeSyncProviderConfiguration destinationConfiguration, SyncCallbacks DestinationCallbacks, ISyncProvider sourceProxy, ISyncProvider destinationProxy, ChangeDataAdapter callbackChangeDataAdapter, SyncDataConverter conflictDataConverter, Int32& changesApplied, Int32& changesFailed)
at Microsoft.Synchronization.KnowledgeSyncOrchestrator.DoOneWayKnowledgeSync(SyncDataConverter sourceConverter, SyncDataConverter destinationConverter, SyncProvider sourceProvider, SyncProvider destinationProvider, Int32& changesApplied, Int32& changesFailed)
at Microsoft.Synchronization.KnowledgeSyncOrchestrator.Synchronize()
at Microsoft.Synchronization.SyncOrchestrator.Synchronize()
at DigitalEngine.SyncMgrFileSync.SyncItem.Synchronize() in File.cs:line num
To work around such errors, make sure all sync instances are only accessed from a single thread.
