Your question isn't clear as to whether you want to write a client from scratch or use an existing client - iirc, Dropbox has a client you can install on your machine to sync a folder to the cloud
If you with to build your own client, then you're looking at something like :-
DropNet/DropNet at master · DropNet/DropNet · GitHub[
^]
or
GitHub - saguiitay/DropboxRestAPI: Dropbox REST API client in C#[
^]
[Edit]
you will need to know how to do things like :-
a) how to read the filesystem to get the initial file details
b) how to read a dropbox folder and get file details
c) how to detect file system changes ? FileSystemWatcher
d) how to use the REST client(s) shown above to interact with DropBox
e) how to get the differences in the filesystems to sync to dropbox using the info from a), b)
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