title: When does Silverlight show "Debugging resource strings are unavailable." message?
body: <p>I have had a few reports of a message box showing up on application launch with the following contents:</p>
  
  <pre><code>[Arg_NullReferenceException]
  Arguments:
  Debugging resource strings are unavailable. Often the key and arguments provide
  sufficient information to diagnose the problem. See http://go.microsoft...
  </code></pre>
  
  <p>I understand the link provided gives me the exception detail, and I obviously have a null ref somewhere. I haven't been able to reproduce this, so I am trying to find where this message is being thrown and hopefully add some error handling.</p>
  
  <p>Is this specific message box something baked into Silverlight, or is it following whatever exception handling is in place when such an exception occurs? In the app constructor, I have</p>
  
  <pre><code>this.UnhandledException += this.Application_UnhandledException;
  </code></pre>
  
  <p>which is:</p>
  
  <pre><code>private void Application_UnhandledException(object sender,
      ApplicationUnhandledExceptionEventArgs e)
      {
          MessageBox.Show(e.ExceptionObject.Message + "\n" + e.ExceptionObject.StackTrace);
      }
  </code></pre>
  
  <p>So it looks like this message box is not coming from the <code>Application_UnhandledException</code> or it would have a message, newline, then a stack trace. So now my two options for how this message box is being generated are: 1) Silverlight is doing it in the background, or 2) this is coming from an existing <code>catch</code> block, although I have found none that would display in this way.</p>
  
  <p>I'm doing offline caching of XAPs in Isolated Storage using <a href="http://csharperimage.jeremylikness.com/2010/05/silverlight-out-of-browser-dynamic.html" rel="nofollow" title="OfflineCatalog by Jeremy Likeness"><code>OfflineCatalog</code></a>, which involves some asynchronous <code>WebClient</code> downloads and fall-backs to Isolated Storage when requests fail. It makes sense that a null ref might be happening somewhere in this process.</p>
  
  <p>To summarize, I'm wanting to add some exception handling for this Null Reference, but cannot reproduce it locally, and cannot find where it may be coming from. <strong>If this is coming from an unhandled exception, why does it not display the message and stack trace as specified?</strong></p>