title: ASP.NET MVC Bundling cache. (Detecting css files changes) (internal behaviour)
body: <p>I've been diving into ASP.NET MVC internal functionality much (different reasons), but still can not cover all the behaviour. One of those which I did not is subj.</p>
  
  <p>The way it works is the following:</p>
  
  <p>if I <strong>bundle some files</strong> (css files for instance), <strong>the framework detects those changes</strong> and <strong>generates new id</strong> for the new bundle (to make it easy for browsers to refresh the changes) like href="/Content/css?v=qartPE4jGe-l1U0I7kNDZPZzVTdh0kT8VBZZA_uURjI1".</p>
  
  <p>What I am actually trying to understand:</p>
  
  <ol>
  <li><p>How exactly the framework (that's possibly not MVC but .NET stuff) <strong>detects that the files are changed</strong> (as there are <em>no directory watchers active</em> (as I can change the file even when web-server if off-line) to see the file changes live, and also the system detects actually the file content changes (I tried just to <em>re-save files without changing their contents and the bundle number did not change as well</em>))?
  (I consider that obviously the system can not compare every file content to detect its changings on every request came).</p></li>
  <li><p>Where (and how) the frameworks <strong>stores current bundle id</strong> and how it <strong>stores previous versions</strong> (as previous bundles are still available when go to their urls)?</p></li>
  </ol>
  
  <p>Thanks a lot!</p>