Question: I need to create an application that when a user copies a file into an specific folder, my application will be activated and the file will be changed, also, when the user reads back any file in the folder, changes will also be made to the file.

Is it possible?

I'll use .net for this.

I think the folder C:\WINDOWS\assembly is somehow like this?

Any help will be appreciated!
Thanks!

Answer: 
You can detect users copying files into a folder with a service that uses FileSystemWatcher. Altering the contents of a file when an app reads the file requires viral techniques that patch the operating system. Like a rootkit or a file system filter driver. Not something I'd recommend considering, alter the file after it was written.

c:\windows\assembly contains the GAC. .NET installs a shell extension handler that hides the contents of the folder. The folder is otherwise quite accessible, the shell extension handler only works when you look at the folder with Windows Explorer. Writing shell extension handlers with .NET is strongly discouraged by Microsoft, the framework also completely lacks support.
You can monitor for changes with [FileSystemWatcher](http://msdn.microsoft.com/en-us/library/ch2s8yd7.aspx).

> 
> You can configure the component to
>  watch either an entire directory and
>  its contents or a specific file or set
>  of files within a given directory. The
>  FileSystemWatcher component raises an
>  event whenever a file or subdirectory
>  within the specified root directory is
>  created, deleted, renamed, or changed
>  in some other way. The types of
>  changes that the component monitors
>  include changes in the file's or
>  subdirectory's attributes, size, last
>  write time, last access time, and
>  security settings.
> 
> 
>