title: Xamarin Studio 6.0
body: <p>I've just updated my Xamarin installation to Studio 6.0.
  In order to install all the update packages (three in total), I've had to install MsBuild tools 2013 and 2015, and now none of the old solutions compiles anymore. Starting from the first directive "using System;" I've an error message saying "using directive is not necessary", almost my code with the red underline. 
  If I create a new solution from scratch, I encounter the very same issues.
  The following in an excerpt from the "blank" solution:</p>
  
  <pre><code>using Android.App;
  using Android.Widget;
  using Android.OS;
  
  namespace contrepair
  {
      [Activity (Label = "contrepair", MainLauncher = true, Icon = "@mipmap/icon")]
      public class MainActivity : Activity
      {
          int count = 1;
  </code></pre>
  
  <p>All the using directives are marked as unnecessary, the <em>Activity</em> word is marked with "The type or namespace ActivityAttribute could not be found (are you missing a using directive or an assembly reference ?)", ending with the <em>int count = 1</em> declaration, where <em>int</em> is marked with "Predefined type System.Int32 is not defined or imported".
  I've also upgraded the android-sdk with the latest available packages, but nothing changes.
  Anyone has any ideas ? In the meanwhile I'll deactivate my notebook installation and I'll activate the one on my desktop machine, in order to continue working.
  Thanks.
  Rodolfo.</p>