Kapcha/AvaloniaApplication2/ViewModels/MainWindowViewModel.cs

using Avalonia.Controls;
using AvaloniaApplication2.Views;
using ReactiveUI;

namespace AvaloniaApplication2.ViewModels
{
    public class MainWindowViewModel : ViewModelBase
    {
        UserControl us = new UserControl1();
        public UserControl US { get => us; set => this.RaiseAndSetIfChanged(ref us,value); }
    }

}