Add project files.

MauiApp1/Platforms/Windows/App.xaml.cs

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+using Microsoft.UI.Xaml;
+
+// To learn more about WinUI, the WinUI project structure,
+// and more about our project templates, see: http://aka.ms/winui-project-info.
+
+namespace MauiApp1.WinUI
+{
+    /// <summary>
+    /// Provides application-specific behavior to supplement the default Application class.
+    /// </summary>
+    public partial class App : MauiWinUIApplication
+    {
+        /// <summary>
+        /// Initializes the singleton application object.  This is the first line of authored code
+        /// executed, and as such is the logical equivalent of main() or WinMain().
+        /// </summary>
+        public App()
+        {
+            this.InitializeComponent();
+        }
+
+        protected override MauiApp CreateMauiApp() => MauiProgram.CreateMauiApp();
+    }
+
+}