MainForm.cs

using System;

namespace EtoApp
{
  class MainForm : Eto.Forms.Form
  {
    static Rhino.Runtime.InProcess.RhinoCore _rhinoCore;

    public static void Run()
    {
      _rhinoCore = new Rhino.Runtime.InProcess.RhinoCore(null, Rhino.Runtime.InProcess.WindowStyle.Hidden);
      MainForm mf = new MainForm();
      mf.Show();

      for (int i = 0; i < System.Windows.Application.Current.Windows.Count; i++)
      {
        var window = System.Windows.Application.Current.Windows[i];
        if (window.IsVisible)
        {
          window.Closed += (s, e) => Shutdown();
          break;
        }
      }
      System.Windows.Forms.Application.Run();
    }

    static void Shutdown()
    {
      _rhinoCore.Dispose();
      System.Windows.Forms.Application.Exit();
    }

    Rhino.UI.Controls.ViewportControl _viewportControl;
    public MainForm()
    {
      Title = "Rhino.Inside";
      ClientSize = new Eto.Drawing.Size(400, 400);
      _viewportControl = new Rhino.UI.Controls.ViewportControl();
      Content = _viewportControl;

      var viewMenu = new Eto.Forms.ButtonMenuItem { Text = "&View" };
      BuildDisplayModesMenu(viewMenu.Items);
      Menu = new Eto.Forms.MenuBar()
      {
        Items =
        {
          new Eto.Forms.ButtonMenuItem
          {
            Text = "&File",
            Items =
            {
              new Eto.Forms.ButtonMenuItem(new Eto.Forms.Command((s,e)=>OpenFile())) { Text = "Open..." }
            }
          },
          viewMenu

        }
      };

    }

    void BuildDisplayModesMenu(Eto.Forms.MenuItemCollection collection)
    {
      Rhino.Display.DisplayModeDescription[] modes = Rhino.Display.DisplayModeDescription.GetDisplayModes();
      foreach(var mode in modes)
      {
        var menuitem = new Eto.Forms.ButtonMenuItem((s, e) =>
        {
          _viewportControl.Viewport.DisplayMode = mode;
          _viewportControl.Refresh();
        });
        menuitem.Text = mode.EnglishName;
        collection.Add(menuitem);
      }
    }

    void OpenFile()
    {
      var ofd = new Eto.Forms.OpenFileDialog();
      ofd.Filters.Add(new Eto.Forms.FileFilter("Rhino 3dm", ".3dm"));
      if( ofd.ShowDialog(this) == Eto.Forms.DialogResult.Ok )
      {
        Title = $"Rhino.Inside ({ofd.FileName})";
        Rhino.RhinoDoc.Open(ofd.FileName, out bool alreadyOpen);
        _viewportControl.Viewport.ZoomExtents();
        _viewportControl.Refresh();
      }
    }


  }
}