Program.cs

using Microsoft.UI.Dispatching;
using Microsoft.UI.Xaml;
using Microsoft.UI.Xaml.Markup;
using Microsoft.UI.Xaml.XamlTypeInfo;

namespace CSharp_WinUI3_DesktopWindowXamlSource
{
    internal class Program : Microsoft.UI.Xaml.Application, IXamlMetadataProvider
    {
        private static XamlControlsXamlMetaDataProvider? xamlMetaDataProvider = null;

        public Program()
        {
            System.Windows.Forms.Application.Run(new Form1());
        }

        protected override void OnLaunched(Microsoft.UI.Xaml.LaunchActivatedEventArgs args)
        {
            XamlControlsXamlMetaDataProvider.Initialize();
            xamlMetaDataProvider = new();
            this.Resources.MergedDictionaries.Add(new Microsoft.UI.Xaml.Controls.XamlControlsResources());
        }

        /// <summary>
        ///  The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            // To customize application configuration such as set high DPI settings or default font,
            // see https://aka.ms/applicationconfiguration.
            ApplicationConfiguration.Initialize();

            Microsoft.UI.Xaml.Application.Start((p) =>
            {
                var syncContext = new DispatcherQueueSynchronizationContext(DispatcherQueue.GetForCurrentThread());
                SynchronizationContext.SetSynchronizationContext(syncContext);

                new Program();            

                var currentApp = Microsoft.UI.Xaml.Application.Current;
                if (currentApp is not null)
                    currentApp.Exit();
            });
        }

        public IXamlType GetXamlType(Type type)
        {
            return xamlMetaDataProvider.GetXamlType(type);
        }

        public IXamlType GetXamlType(string fullName)
        {
            return xamlMetaDataProvider.GetXamlType(fullName);
        }

        public XmlnsDefinition[] GetXmlnsDefinitions()
        {
            return xamlMetaDataProvider.GetXmlnsDefinitions();
        }     
    }
}