title: Determine when files in folder change on demand
body: <p>I am creating an RSS feed on-demand from a folder of images. The feed creation is pretty expensive on large sets of files obviously, so I am caching the feed after first creation but I'd like to know if I need to re-create the feed when someone calls for it. </p>
  
  <p>I'm thinking that if I could figure out a way to cheaply create a unique hash of all the files and file dates, then when the hash changed I would know to re-generate the feed.</p>
  
  <p>Does anyone have any ideas on how to cheaply create this hash? Is a hash even the right name for what I'm looking to do?</p>
  
  <p><strong>Edit:</strong></p>
  
  <p>I know a FileSystemWatcher exists exactly for this type of thing. But I'm more curious about an on-demand technique to do this. I've had other instances where I've done the same thing and used a file watcher to get the notifications but I figure someone has an idea of what to do when you can't get file changed notifications...</p>