Obsolete setting mainpage via application

src/Compatibility/ControlGallery/src/iOS/AppDelegate.cs

@@ -162,7 +162,7 @@ public void StartPressed40911()
 
 			button.TouchUpInside += (sender, e) =>
 			{
-				Maui.Controls.Application.Current.MainPage = new ContentPage { Content = new Label { Text = "40911 Success" } };
+				Maui.Controls.Application.Current.Windows[0].Page = new ContentPage { Content = new Label { Text = "40911 Success" } };
 				loginViewController.DismissViewController(true, null);
 			};
 

src/Controls/samples/Controls.Sample/Pages/Base/BasePage.cs

@@ -23,7 +23,7 @@ public BasePage()
 			{
 				if (SelectedItem != null)
 				{
-					if (Application.Current.MainPage is FlyoutPage fp)
+					if (this.Window.Page is FlyoutPage fp)
 						await fp.Detail.Navigation.PushAsync(PreparePage(SelectedItem));
 					else
 						await Navigation.PushAsync(PreparePage(SelectedItem));

src/Controls/samples/Controls.Sample/Pages/Compatibility/TabbedPageGalleryMainPage.xaml.cs

@@ -50,7 +50,7 @@ void OnSetToBottomTabs(object sender, EventArgs e)
 
 			SetNewMainPage(bottomTabs);
 			AndroidSpecific.TabbedPage.SetToolbarPlacement(bottomTabs, AndroidSpecific.ToolbarPlacement.Bottom);
-			Application.Current!.MainPage = bottomTabs;
+			this.Window!.Page = bottomTabs;
 		}
 
 		void OnChangeTabIndex(object sender, EventArgs e)

src/Controls/samples/Controls.Sample/Pages/Core/FlyoutPageGallery.xaml.cs

@@ -6,7 +6,7 @@ namespace Maui.Controls.Sample.Pages
 	public partial class FlyoutPageGallery
 	{
 
-		FlyoutPage? FlyoutPage => Application.Current!.MainPage as FlyoutPage;
+		FlyoutPage? FlyoutPage => this.Window!.Page as FlyoutPage;
 		public FlyoutPageGallery()
 		{
 			InitializeComponent();

src/Controls/samples/Controls.Sample/Pages/Core/ShellGalleries/ShellChromeGallery.cs

@@ -12,7 +12,7 @@ namespace Maui.Controls.Sample.Pages.ShellGalleries
 {
 	public partial class ShellChromeGallery
 	{
-		AppShell? AppShell => Application.Current!.MainPage as AppShell;
+		AppShell? AppShell => this.Window!.Page as AppShell;
 
 		public ShellChromeGallery()
 		{

src/Controls/samples/Controls.Sample/Pages/PlatformSpecifics/iOS/iOSTranslucentNavigationBarPage.xaml.cs

@@ -18,7 +18,7 @@ public iOSTranslucentNavigationBarPage(ICommand restore)
 
 		void OnTranslucentNavigationBarButtonClicked(object sender, EventArgs e)
 		{
-			(Microsoft.Maui.Controls.Application.Current!.MainPage as Microsoft.Maui.Controls.NavigationPage)!.On<iOS>().SetIsNavigationBarTranslucent(!(Microsoft.Maui.Controls.Application.Current!.MainPage as Microsoft.Maui.Controls.NavigationPage)!.On<iOS>().IsNavigationBarTranslucent());
+			(this.Window!.Page as Microsoft.Maui.Controls.NavigationPage)!.On<iOS>().SetIsNavigationBarTranslucent(!(this.Window!.Page as Microsoft.Maui.Controls.NavigationPage)!.On<iOS>().IsNavigationBarTranslucent());
 		}
 
 		void OnReturnButtonClicked(object sender, EventArgs e)

src/Controls/samples/Controls.Sample/Pages/SettingsPage.xaml.cs

@@ -18,7 +18,7 @@ void OnTapGestureRecognizerTapped(object sender, EventArgs args)
 
 		void OnRTLToggled(object sender, ToggledEventArgs e)
 		{
-			var mainPage = Application.Current!.MainPage;
+			var mainPage = this.Window!.Page;
 
 			if (mainPage == null)
 				return;

src/Controls/src/Core/Application/Application.cs

@@ -85,13 +85,15 @@ public IAppLinks AppLinks
 		/// <include file="../../docs/Microsoft.Maui.Controls/Application.xml" path="//Member[@MemberName='MainPage']/Docs/*" />
 		public Page? MainPage
 		{
+			[Obsolete("This property has been deprecated. For single-window applications, use Windows[0].Page. For multi-window applications, identify and use the appropriate Window object to access the desired Page. Additionally, each element features a Window property, accessible when it's part of the current window.")]
 			get
 			{
 				if (Windows.Count == 0)
 					return _singleWindowMainPage;
 
 				return Windows[0].Page;
 			}
+			[Obsolete("This property is deprecated. Initialize your application by overriding Application.CreateWindow rather than setting MainPage. To modify the root page in an active application, use Windows[0].Page for applications with a single window. For applications with multiple windows, use Application.Windows to identify and update the root page on the correct window.  Additionally, each element features a Window property, accessible when it's part of the current window.")]
 			set
 			{
 				if (MainPage == value)
@@ -513,15 +515,19 @@ protected virtual Window CreateWindow(IActivationState? activationState)
 				return window;
 
 			if (Windows.Count > 1)
+#pragma warning disable CS0618 // Type or member is obsolete
 				throw new NotImplementedException($"Either set {nameof(MainPage)} or override {nameof(Application.CreateWindow)}.");
+#pragma warning restore CS0618 // Type or member is obsolete
 
 			if (Windows.Count > 0)
 				return Windows[0];
 
 			if (_singleWindowMainPage is not null)
 				return new Window(_singleWindowMainPage);
 
+#pragma warning disable CS0618 // Type or member is obsolete
 			throw new NotImplementedException($"Either set {nameof(MainPage)} or override {nameof(Application.CreateWindow)}.");
+#pragma warning restore CS0618 // Type or member is obsolete
 		}
 
 		void AddWindow(Window window)

src/Controls/src/Core/Shell/Shell.cs

@@ -950,14 +950,51 @@ public static Shell Current
 		{
 			get
 			{
-				if (Application.Current == null)
+				if (Application.Current is null || Application.Current.Windows.Count == 0)
 					return null;
 
+				if (Application.Current.Windows.Count == 1)
+				{
+					return Application.Current.Windows[0].Page as Shell;
+				}
+
+				// Check if shell is activated
+				Shell currentShell = null;
+				Shell returnIfThereIsJustOneShell = null;
+				bool tooManyShells = false;
 				foreach (var window in Application.Current.Windows)
-					if (window is Window && window.IsActivated && window.Page is Shell shell)
-						return shell;
+				{
+					if (window.Page is Shell shell)
+					{
+						if (window.IsActivated)
+						{
+							if(currentShell is not null)
+							{
+								currentShell = null;
+								break;
+							}
+
+							currentShell = shell;
+						}
+
+						if (returnIfThereIsJustOneShell is not null)
+						{
+							tooManyShells = true;					
+						}
+					}
+				}
+
+				if (currentShell is not null)
+				{
+					return currentShell;
+				}
+
+				if (!tooManyShells &&  returnIfThereIsJustOneShell is not null)
+				{
+					return returnIfThereIsJustOneShell;
+				}
 
-				return Application.Current?.MainPage as Shell;
+				throw new InvalidOperationException($"Unable to determine the current Shell instance you want to use. Please access Shell via the Windows property on {Application.Current.GetType()}.");
 			}
 		}
 

src/Controls/tests/DeviceTests/Elements/Window/WindowTests.cs

@@ -73,7 +73,9 @@ public async Task ChangingToNewMauiContextDoesntCrash(bool useAppMainPage, Type
 			if (useAppMainPage)
 			{
 				var app = ApplicationServices.GetService<IApplication>() as ApplicationStub;
+#pragma warning disable CS0618 // Type or member is obsolete
 				app.MainPage = rootPage;
+#pragma warning restore CS0618 // Type or member is obsolete
 				window = await InvokeOnMainThreadAsync(() => (app as IApplication).CreateWindow(null));
 
 			}

src/Controls/tests/TestCases.HostApp/CoreViews/CorePageView.cs

@@ -136,7 +136,7 @@ public CorePageView(Page rootPage)
 					var realize = page.Realize();
 					if (realize is Shell)
 					{
-						Application.Current.MainPage = realize;
+						this.Window.Page = realize;
 					}
 					else
 					{