How Explorer Thumbnails Work
When you are browsing in Windows Explorer the thumbnail cache which is managed by Windows is queried for thumbnails when required.
If Explorer wants a thumbnail it asks it's cache (which is managed explictly by Windows) for a thumbnail for a thumbnail on a file when a file is about to be displayed - either as an icon (list view, detail view etc.) or an actual thumbnail from it's cache system.
The cache system first checks to see if a thumbnail handler exists for the file extension for the file type, and if it does, checks to see if there is a thumbnail already existing for the file.
If a thumbnail exists for the file the cache then checks to see if the file has been modified since the thumbnail was created. If the file has been modified, the cache requests a new thumbnail from the thumbnail handler associated for that file extension.
If a thumbnail already exists for the requested file at the requested thumbnail size, and it is newer than the the file, then that thumbnail is used.
If no thumbnail exists, or the file has been modified since the cached thumbnail was created, a new thumbnail is created.
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