how to keep track of activities within a folder???

Question

i am creating a website using ASP.NET (C#) using FTP
problem:
i have assigned a folder with a storage space let say 1GB to a user now what i want is to keep track of the storage space ulitilized by the user. for eg if a user saves/deletes a file in his folder, how to know that user has saved/deleted something to/from his folder.

any help(coding in c#) in this regard will be highly appreciated.
thanx.