title: Linq does not exist in the namespace System
body: <p>I am trying to run an asp.net application that has been created in Visual Studio 2013 in Visual Studio 2015, but I am having a lot of build errors. All errors are in the two website projects and can be traced back to the following error:</p>
  
  <blockquote>
    <p>CS0234    The type or namespace name 'Linq' does not exist in the
    namespace 'System' (are you missing an assembly reference?)</p>
  </blockquote>
  
  <p>All projects are being build using .net4.0 or higher, but still I recieve this error. When the project is build in vs2013 everything builds fine.</p>
  
  <pre><code>&lt;compilation debug="true" targetFramework="4.5"&gt;
  </code></pre>
  
  <p>Does someone know what could be wrong here?</p>