title: DebuggerVisualizer not showing up when debugging but works on test application
body: <p>I have made a DebuggerVisualizer to show a simple Image of an abstract Puzzle class - the issue I'm having is that in my test application that simply creates a puzzle and calls TestShowVisualizer() the image is displayed correctly on screen. However when debugging the project and hovering over the variable there is no visualizer (or magnifying glass icon)</p>
  
  <pre><code>    public abstract class Puzzle
      {
           // Lots of properties and methods here //
  
           public static Puzzle Load(StreamReader sr)
           {
             // Reads the stream reader and creates the correct derived class
           }
  
           public abstract Image GenerateNet();
      }
  
  
          [DebuggerVisualizer(typeof(PuzzleVisualizer), typeof(PuzzleVisualizerObjectSource), Description = "Image Visualizer", Target = typeof(Puzzle))]
  public class PuzzleVisualizer : DialogDebuggerVisualizer
  {
      protected override void Show(IDialogVisualizerService windowService, IVisualizerObjectProvider objectProvider)
      {
          var puz = Puzzle.Load(new StreamReader(objectProvider.GetData()));
          var image = puz.GenerateNet();
  
          var form = new Form
          {
              Text = $"Width: {image.Width}, Height: {image.Height}",
              ClientSize = new Size(image.Width, image.Height),
              FormBorderStyle = FormBorderStyle.FixedToolWindow
          };
  
          var pictureBox = new PictureBox
          {
              Image = image,
              Parent = form,
              Dock = DockStyle.Fill
          };
  
          windowService.ShowDialog(form);
      }
  
      public static void TestShowVisualizer(object objectToVisualize)
      {
          var visualizerHost = new VisualizerDevelopmentHost(objectToVisualize, typeof(PuzzleVisualizer), typeof(PuzzleVisualizerObjectSource));
          visualizerHost.ShowVisualizer();
      }
  }
  </code></pre>
  
  <p>I made a test application which all the examples for making visualizers use too:</p>
  
  <pre><code>public class Program
  {
      public static void Main(string[] args)
      {
          var p = new Cube222();
          p.Setup();
          PuzzleVisualizer.TestShowVisualizer(p);
          return;
      }
  }
  </code></pre>
  
  <p>The visualizer works when TestShowVisualizer() is called but not for just general debugging.</p>
  
  <p>I've made sure that the visualizer class is in its own DLL and that this gets copied to C:\Program Files\Microsoft Visual Studio\2017\Community\Common7\Packages\Debugger\Visualizers</p>
  
  <p>Any ideas on what I'm doing wrong?</p>