feat: Warn when Window constructor is used in Uno Platform targets

Author: MartinZikmund

src/SolutionTemplate/UnoSolutionTemplate/Shared/App.xaml.cs

@@ -54,7 +54,7 @@ protected override void OnLaunched(LaunchActivatedEventArgs args)
             }
 #endif
 
-#if NET5_0 && WINDOWS
+#if NET5_0 && WINDOWS && !HAS_UNO
             _window = new Window();
             _window.Activate();
 #else

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml/Given_Window.cs

@@ -15,7 +15,7 @@ public class Given_Window
 		public void When_CreateNewWindow()
 		{
 			// This used to crash on wasm which was trying to create a second D&D extension
-			var sut = new Window();
+			var sut = new Window(true);
 		}
 #endif
 	}

src/Uno.UI/UI/Xaml/Window.Android.cs

@@ -25,15 +25,13 @@ public sealed partial class Window
 	{
 		private Border _rootBorder;
 
-		public Window()
+		partial void InitPlatform()
 		{
 			Dispatcher = CoreDispatcher.Main;
 			CoreWindow = CoreWindow.GetOrCreateForCurrentThread();
 
 			CoreApplication.GetCurrentView().TitleBar.ExtendViewIntoTitleBarChanged
 				+= RaiseNativeSizeChanged;
-
-			InitializeCommon();
 		}
 
 		internal Thickness Insets { get; set; }

src/Uno.UI/UI/Xaml/Window.Desktop.cs

@@ -17,11 +17,9 @@ public sealed partial class Window
 	{
 		private bool _isActive;
 
-		public Window()
+		partial void InitPlatform()
 		{
 			CoreWindow = CoreWindow.GetOrCreateForCurrentThread();
-
-			InitializeCommon();
 		}
 
 		partial void InternalActivate()

src/Uno.UI/UI/Xaml/Window.cs

@@ -22,7 +22,7 @@ namespace Windows.UI.Xaml
 	public sealed partial class Window
 	{
 		private static Window _current;
-		
+
 		private UIElement _content;
 		private RootVisual _rootVisual;
 
@@ -36,6 +36,32 @@ public sealed partial class Window
 		public global::Microsoft.UI.Dispatching.DispatcherQueue DispatcherQueue { get; } = global::Microsoft.UI.Dispatching.DispatcherQueue.GetForCurrentThread();
 #endif
 
+#if HAS_UNO_WINUI
+		public Window() : this(false)
+		{
+		}
+#endif
+
+		internal Window(bool internalUse)
+		{
+			if (!internalUse)
+			{
+				if (this.Log().IsEnabled(LogLevel.Warning))
+				{
+					this.Log().LogWarning(
+						"Creating a secondary Window instance is currently not supported in Uno Platform targets. " +
+						"Use the Window.Current property instead (you can use #if HAS_UNO to differentiate " +
+						"between Uno Platform targets and Windows App SDK).");
+				}
+			}
+			
+			InitPlatform();
+
+			InitializeCommon();
+		}
+
+		partial void InitPlatform();
+
 #pragma warning disable 67
 		/// <summary>
 		/// Occurs when the window has successfully been activated.
@@ -96,7 +122,7 @@ public UIElement Content
 						oldRoot.SizeChanged -= RootSizeChanged;
 					}
 				}
-				
+
 				if (value != null)
 				{
 					value.IsWindowRoot = true;
@@ -106,7 +132,7 @@ public UIElement Content
 
 				if (value is FrameworkElement newRoot)
 				{
-					newRoot.SizeChanged += RootSizeChanged;					
+					newRoot.SizeChanged += RootSizeChanged;
 				}
 
 				oldContent?.XamlRoot?.NotifyChanged();
@@ -282,7 +308,7 @@ internal IDisposable RegisterBackgroundChangedEvent(EventHandler handler)
 					(h as EventHandler)?.Invoke(s, (EventArgs)e)
 			);
 
-		private static Window InternalGetCurrentWindow() => _current ??= new Window();
+		private static Window InternalGetCurrentWindow() => _current ??= new Window(true);
 
 		private UIElement InternalGetContent() => _content!;
 

src/Uno.UI/UI/Xaml/Window.iOS.cs

@@ -31,7 +31,7 @@ public sealed partial class Window
 		/// </summary>
 		public static Func<RootViewController> ViewControllerGenerator { get; set; }
 
-		public Window()
+		partial void InitPlatform()
 		{
 			_nativeWindow = new Uno.UI.Controls.Window();
 
@@ -45,7 +45,6 @@ public Window()
 			CoreWindow = new CoreWindow(_nativeWindow);
 
 			_nativeWindow.SetOwner(CoreWindow);
-			InitializeCommon();
 		}
 
 		internal Uno.UI.Controls.Window NativeWindow => _nativeWindow;

src/Uno.UI/UI/Xaml/Window.macOS.cs

@@ -31,7 +31,7 @@ public sealed partial class Window
 		/// </summary>
 		public static Func<RootViewController> ViewControllerGenerator { get; set; }
 
-		public Window()
+		partial void InitPlatform()
 		{
 			var style = NSWindowStyle.Closable | NSWindowStyle.Resizable | NSWindowStyle.Titled | NSWindowStyle.Miniaturizable;
 
@@ -55,8 +55,6 @@ public Window()
 			CoreWindow = new CoreWindow(_window);
 
 			_window.CoreWindowEvents = CoreWindow;
-
-			InitializeCommon();
 		}
 
 		internal NSWindow NativeWindow => _window;

src/Uno.UI/UI/Xaml/Window.skia.cs

@@ -19,19 +19,12 @@ public sealed partial class Window
 		// private ScrollViewer _rootScrollViewer;
 		private Border? _rootBorder;
 
-		public Window()
-		{
-			Init();
-
-			Compositor = new Compositor();
-		}
-
-		public void Init()
+		partial void InitPlatform()
 		{
 			Dispatcher = CoreDispatcher.Main;
 			CoreWindow = CoreWindow.GetOrCreateForCurrentThread();
 
-			InitializeCommon();
+			Compositor = new Compositor();
 		}
 
 		internal void OnNativeSizeChanged(Size size)
@@ -54,7 +47,7 @@ internal void OnNativeSizeChanged(Size size)
 			}
 		}
 
-		public Compositor Compositor { get; }
+		public Compositor Compositor { get; private set; }
 
 		private void InternalSetContent(UIElement content)
 		{

src/Uno.UI/UI/Xaml/Window.wasm.cs

@@ -29,14 +29,7 @@ public sealed partial class Window
 		private Border _rootBorder;
 		private bool _invalidateRequested;
 
-		public Window()
-		{
-			Init();
-
-			InitializeCommon();
-		}
-
-		private void Init()
+		partial void InitPlatform()
 		{
 			Dispatcher = CoreDispatcher.Main;
 			CoreWindow = CoreWindow.GetOrCreateForCurrentThread();