title: pack uri validation
body: <p>What is the simplest way to verify that a pack uri can be found? </p>
  
  <p>For example, given</p>
  
  <pre><code>pack://application:,,,Rhino.Mocks;3.5.0.1337;0b3305902db7183f;component/SomeImage.png
  </code></pre>
  
  <p>how would I check whether the image actually exists?</p>
  
  <p>Cheers,<br>
  Berryl</p>