manage Offline+Online Database for winform application

Question

i am new in winform and i am developing Point of sale win form(with c# desktop)application .this application has some special requirement.
The application should work in Online and Offline environment. If user is working and suddenly Application goes offline User
should be able to work offline. All offline work is stored in local database and sync next time to central database
when Application connects to the internet

is there any sample code .?