How can I keep an offline and online application in sync using .NET?

Microsoft provides a great framework to keep things synchronized called Microsoft Sync Framework.

"Microsoft Sync Framework is a comprehensive synchronization platform that enables collaboration and offline access for applications, services and devices. It features technologies and tools that enable roaming, sharing, and taking data offline. Using Microsoft Sync Framework, developers can build sync ecosystems that integrate any application, with any data from any store using any protocol over any network. "

The framework comes ready to use in many cases with sync providers for

1. Sync Services for ADO.NET - Synchronization for ADO.NET enabled data sources
2. Sync Services for File Systems - Synchronization for files and folders
3. Sync Services for FeedSync - Synchronization for RSS and ATOM feeds
   

The beauty of MS Sync Framework is that it is completely agnostic of the communication protocol, business domain, or even data. You just tell it when to sync, and what to sync and it will tell you if anything needs to be synchronized. The framework will even synchronize the data for you, though you can provide custom conflict resolution rules.