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Windows Explorer Question

Posted: 09/08/2008 12:57:58
by Sid Schipper (Standard support level)
Joined: 03/14/2008
Posts: 285

I have a question that is related to CBFS but may just be a Windows Explorer thing that I don't understand.

In my CBFS application I have implemented an internal tracing facility that selectively writes messages to a log file. I write messages whenever I get calls to any of the CBFS callback routines. When I had my tracing system turned on I found some interesting things that I'd like to run by you and let you tell me what they mean, because I cannot understand some of them.

For example, in a folder on my virtual disk I have two .wmv (Windows Media Viewer files, usually these are video clips). When I click on the folder that contains these files, in Windows explorer, it shows the contents of the folder in the right hand pane of the Explorer window just as I'd expect, but if I then look at my log file I find this curious symptom. The CbFsReadFile callback gets called to read a very small part of one of the .wmv files. I attributed that to, maybe, Windows Explorer reading the thumbnail for that file. The curious thing is that the callback only gets called for that file and no other files in the folder? Why would that be? I would have thought that if Windows Explorer was reading the thumbnail for that file then it would try to read the thumbnails for all the files in the folder, but there are 4 other files in the folder including one other .wmv file and the CbFsReadFile callback does not get called for any of the other files. The name of the file that it does get called for is last alphabetically so it is listed last by the EnumerateDirectory callback, but I can't see why that would make any difference? I found this behavior to be quite curious and I was wondering if any of you out there could explain it to me?
Posted: 09/08/2008 13:08:26
by Eugene Mayevski (Team)

Do you mean that reading occured for only one of several wmv files? I think that maybe Explorer attempted to read the video metadata from the file and either failed and decided not to continue, or, in opposite, found out that it can read that data and stopped. This could be done in order to enable or disable some menu command or something like that.

Sincerely yours
Eugene Mayevski
Posted: 09/08/2008 15:43:33
by Sid Schipper (Standard support level)
Joined: 03/14/2008
Posts: 285

OK, I see what you mean. Maybe it does read the metadata and stops when it sees it can.

I have some other examples of traces that I don't quite get, but I don't have complete details of them now. I will reply back to this thread when I come up with one that I can explain fully.
Posted: 09/12/2008 07:31:43
by Dan Cooper (Basic support level)
Joined: 06/27/2007
Posts: 125

We have noticed something similar on Windows Vista where multiple images files all end up with the same thumbnail, even though when they are opened they are completely different images. Is this similar to what you are seeing?



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