Avalonia/samples/ControlCatalog/MainView.xaml.cs

using System.Collections;
using Avalonia;
using Avalonia.Controls;
using Avalonia.Markup.Xaml;
using Avalonia.Platform;
using ControlCatalog.Pages;

namespace ControlCatalog
{
    public class MainView : UserControl
    {
        public MainView()
        {
            this.InitializeComponent();
            if (AvaloniaLocator.Current.GetService<IRuntimePlatform>().GetRuntimeInfo().IsDesktop)
                ((IList) this.FindControl<TabControl>("Sidebar").Items).Add(new TabItem()
                {
                    Header = "Dialogs",
                    Content = new DialogsPage()
                });
        }

        private void InitializeComponent()
        {
            AvaloniaXamlLoader.Load(this);
        }
    }
}