title: Unknown type 'AppBarButton' in XML namespace - Windows 8 Store App XAML, issues adding app bars
body: <p>I'm new to developing Windows 8 apps, and am having trouble getting my XAML file to recognise generated code for adding AppBars and CommandBars.</p>
  
  <p>I am getting the error "Unknown type [something] in XML namespace" for a number of elements I am trying to add, here is my example below:</p>
  
  <p><img src="https://i.stack.imgur.com/EOkhx.png" alt="Unknown type &#39;AppBarButton&#39; in XML Namespace"></p>
  
  <p>If I re-open my solution this is temporarily displayed as a warning rather than an error. Also navigating to <a href="http://schemas.microsoft.com/winfx/2006/xaml/presentation" rel="nofollow noreferrer">http://schemas.microsoft.com/winfx/2006/xaml/presentation</a> just produces "The resource you are looking for has been removed, had its name changed, or is temporarily unavailable.".</p>
  
  <p>This is all default code. my class namespace (e.g. MyApp.MainPage) matches the MainPage code behind namespace. I am at a loss and have been battling this for hours. I also intermittently changes from this error to 'InitializeComponent' does not exist in the current context". I have spent hours on XAML permissions errors in the past few days and don't want to waste any more time with errors in default generated code! :(</p>
  
  <p>EDIT: I have tried this on three different machines, using both Windows 8 and Windows 8.1, both new projects and the existing project I have described in this post.</p>