title: Missing System DLL's in .NET project
body: <p>I have a .NET solution consisting of several projects - most of these are just simple c# libraries. One of them is an MVC web application.</p>
  
  <p>All the references of every project is missing references. Here is an example:</p>
  
  <p><img src="https://i.stack.imgur.com/02mJR.png" alt=""></p>
  
  <p>I have de-selected all projects for build except this one (shown above) simple c# library project which has no dependencies on any other in the solution.</p>
  
  <p>I've checked things like the .proj file in Notepad but can't see anything unusal. </p>
  
  <p>If I remove one of the references such as System.XML and then re-add it, it gets re-added but still with the missing warning icon! So the Dll's are in the .NET frameowrk directory as you'd expect.</p>
  
  <p>I'm really quite stumped with this.</p>
  
  <p>It's .NET framework 4.5, Windows 7, x64. </p>
  
  <p>Thanks in advance for any thoughts / help!</p>
  
  <p>Kind Regards</p>
  
  <p>Richard</p>