title: UWP app build failure "Microsoft.UI.Xaml.Markup could not be found"
body: <p>I've downloaded a UWP solution that consists of 8 projects. Upon building in VS 2017 I get the following error each of the projects in the solution:</p>
  
  <p>"The referenced component 'Microsoft.UI.Xaml.Markup' could not be found."</p>
  
  <p>Sure enough, when I look under the "References" node for each project in the Solution Explorer window "Microsoft.UI.Xaml.Markup" is listed but with an exclamation mark next to it. If I click on it and look at its properties in the properties window there is nothing listed in the "Path" field (and Copy Local is set to false, if that's of interest to anyone).</p>
  
  <p>I've tried right clicking on it and selecting "Remove". I then use another UWP app that builds just fine (that's also referencing "Microsoft.UI.Xaml.Markup") to locate the path of "Microsoft.UI.Xaml.Markup". In the project that's giving the error I right-click "References" and select "Add References...". I browse to the location of "Microsoft.UI.Xaml.Markup" and add it to the project. It shows up in the "References" node but again displays an exclamation mark and nothing in the Path field. I can't seem to add it to any of the projects in the downloaded solution. What am I doing wrong?</p>