From where to start in making an application like Dropbox ?
Exists any code snippets or some api ?
If not possible in C++, C# is good ...

Thanks :D.

My guess is that's it's just a cloud system The originator of dropbox has a server computer that stores the data and a program (dropbox) on each computer that can upload/download data to/from the server.

How to write such a program in c/c++? The server side is probably running PHP. The client programs could be written using sockets. So if you don't know how to use sockets then that's the place to start.

Yes ... sockets but what to do ?
To check on server every file by an identifier like the last date modified then compare to one of them in my computer and download / update the files ?

I think that would take some time ...

boost::filesystem may be useful to you because its platform independent which means you write your program once for both MS-Windows and *nix operating systems. You'll have to download and install boost onto your local computer.