Doesn't tallavor address this in the first sentence?

> In Windows, even with a filter limiting which directories to process, a FileSystemWatcher object has to examine each event and decide whether or not to call the event handler or let it pass without action.

Sounds like the very act of checking whether the file falls within the "agreed location" is what caught the author's attention. I can't say whether that's true, but it doesn't seem unreasonable.

It depends on the method they're using to monitor for changes to files and the type of filter being used. They're probably trying to reuse code across clients, so I'm not sure what method they're using. --ReadDirectoryChangesW makes the most sense, but maybe they've done it differently. If someone really wants to figure it out, it's probably possible to attach a debugger to try and catch the calls being used, but since we don't have symbols, it wouldn't be the easiest exercise (although it might be fun if I could find the time).

Well, it's not 'scanning files'. It's scanning meta-data about directories and files.

You can see three calls for each file in the ProcMon logs:

1. CreateFile

2. QueryBasicInformationFile

3. CloseFile

Let's go through these in order:

1. Opens a handle to the file, allowing the application to query meta information, read, and write

2. Queries meta information about the file.

3. Closes the handle to the file before doing anything else, including reading data.

There's no scanning of files, because no data is read. Its common for applications to traverse directories like this, and the behavior could even be a consequence of a file system library or API that dropbox is using. If you think of your file system as a filing cabinet, what DropBox is doing might be equivalent to opening every drawer looking for a single file.

ProcMon detects CreateFile and QueryInformationFile using these function codes:

https://msdn.microsoft.com/en-us/library/ff548630(v=vs.85).a...

https://msdn.microsoft.com/en-us/library/ff549283.aspx

As you can see, neither of these indicates a file being read. It also monitors a number of other codes (I'm not sure how specifically it detects CloseFile), including this one, which is triggered when data is read from a file:

https://msdn.microsoft.com/en-us/library/ff549327(v=vs.85).a...

If that was being called, ProcMon would show it, but it doesn't