title: VS 2015 IntelliSense: Assembly Not Referenced Error
body: <p>I just switched to VS 2015. I have an older MVC 5 app that runs against 4.52. In VS 2013 it's perfectly fine.</p>
  
  <p>In VS 2015 I'm getting red squigglies under my <code>@Html.TextBoxFor()</code> with an error indicating: </p>
  
  <blockquote>
    <p>The type 'Expression&lt;>' is defined in an assembly that is not
    referenced. You must add a reference to assembly 'System.Core,
    Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.</p>
  </blockquote>
  
  <p>The project builds and runs just fine -- but I am concerned about the IntelliSense error that never happened in VS 2013. Okay, so I try to add the reference to System.Core as recommended in the error above and then I get this error:</p>
  
  <blockquote>
    <p>A reference to 'System.Core' could not be added. This component is
    already automatically referenced by the build system.</p>
  </blockquote>
  
  <p>Again, this is fine in VS 2013.</p>