title: Upgrade existing project to use Roslyn compiler
body: <p>I installed VS 2015 and imported an existing solution. This solution contains a Razor helper in the App_Code folder.  The Error List complains:</p>
  
  <blockquote>
    <p>The type or namespace name 'global_asax' does not exist in the
    namespace 'ASP' (are you missing an assembly reference?)</p>
  </blockquote>
  
  <p>After a bit of research, I found that this is (or was) a known issue (cf. <a href="https://stackoverflow.com/a/30502263/1637105">This SO answer</a> and <a href="http://blogs.msdn.com/b/webdev/archive/2014/05/12/enabling-the-net-compiler-platform-roslyn-in-asp-net-applications.aspx" rel="nofollow noreferrer">this article</a>). The solution given in those articles is to manually install the <code>Microsoft.CodeDom.Providers.DotNetCompilerPlatform</code> NuGet package.</p>
  
  <p>So I installed that NuGet package in my web app project, and the error went away.</p>
  
  <p>But now I get an error when I open the Package Manager Console complaining that it can't copy some file. The (incorrect) path to the missing file is given in the <code>Microsoft.CodeDom.Providers.DotNetCompilerPlatform</code> NuGet package configuration info.</p>
  
  <p><strong>All of this leads me to believe that there must be a better way!</strong></p>
  
  <p>Am I missing something? Shouldn't I get the new Roslyn magic automagically via VS 2015? If not, is there some better way for me to install it into my existing project? Or am I just stumbling over broken infrastructure bits that someone needs to fix?</p>