mainwindow.cs

using System;
using System.Reactive.Linq;
using Gtk;
using ReactiveUI;
using ReactiveUI.Routing;

namespace GitHub
{
    public partial class MainWindow : Gtk.Window, IScreen
    {   
        public IRoutingState Router { get; protected set; }
    
        public MainWindow() : base (Gtk.WindowType.Toplevel)
        {
            Build();

            Router = new RoutingState();
            routedviewhost1.Router = Router;
			routedviewhost1.Child = new TestView();
            Router.Navigate.Execute(new TestViewModel(this));   // This eventually sets routedviewHost.Child to TestView
        }
        
        protected void OnDeleteEvent (object sender, DeleteEventArgs a)
        {
            Application.Quit ();
            a.RetVal = true;
        }
    }

}

routedviewhost.cs

using System;
using Gtk;
using System.Reactive;
using ReactiveUI;
using ReactiveUI.Routing;

namespace ReactiveUI.Gtk
{
    [System.ComponentModel.ToolboxItem(true)]
    public class RoutedViewHost : Bin
    {
        IRoutingState _Router;
        public IRoutingState Router {
            get { return _Router; }
            set {
                _Router = value;
                updateRouter();
            }
        }
        
        public Widget DefaultContent { get; set;}
    
        IDisposable currentRouterSub = null;
        void updateRouter()
        {
            if (currentRouterSub != null) {
                currentRouterSub.Dispose();
                currentRouterSub = null;
            }
            
            if (Router == null)
                return;
            
            currentRouterSub = Router.ViewModelObservable().Subscribe(vm => {
                if (vm == null) {
                    Child = DefaultContent;
                    return;
                }
                
                var view = RxRouting.ResolveView(vm);
                view.ViewModel = vm;
                Child = (Widget)view;
            });
        }
    }
}

testview.cs

using System;
using ReactiveUI.Routing;

namespace GitHub
{
    [System.ComponentModel.ToolboxItem(true)]
    public partial class TestView : Gtk.Bin, IViewForViewModel<TestViewModel>
    {
        public TestView()
        {
            this.Build();
        }
        
        public TestViewModel ViewModel { get; set; }
        object IViewForViewModel.ViewModel {
            get { return ViewModel; }
            set { ViewModel = (TestViewModel)value; }
        }
    }
}