Upgrade from .NET 8 to .NET 9 (#2608)

* Upgrade to .NET MAUI 9.

* Fix link.

* Edits.

* Fix linting error.

* Edits.

* Fix links.

docs/whats-new/dotnet-9.md

@@ -1,7 +1,7 @@
 ---
 title: What's new in .NET MAUI for .NET 9
 description: Learn about the new features introduced in .NET MAUI for .NET 9.
-ms.date: 11/07/2024
+ms.date: 11/08/2024
 ---
 
 # What's new in .NET MAUI for .NET 9
@@ -424,15 +424,15 @@ Full trimming is now supported by setting the `$(TrimMode)` MSBuild property to
 
 When debugging and deploying a new .NET MAUI project to Windows, the default behavior in .NET MAUI 9 is to deploy an unpackaged app. For more information, see [Deploy and debug your .NET MAUI app on Windows](~/windows/setup.md).
 
-## XAML compiler
+## XAML compiler error codes
 
 In .NET MAUI 9, the XAML compiler error codes have changed their prefix from `XFC` to `XC`. Ensure that you update the `$(WarningsAsErrors)`, `$(WarningsNotAsErrors)`, and `$(NoWarn)` build properties in your app's project files, if used, to reference the new prefix.
 
 ## XAML markup extensions
 
 All classes that implement <xref:Microsoft.Maui.Controls.Xaml.IMarkupExtension>, <xref:Microsoft.Maui.Controls.Xaml.IMarkupExtension`1>, <xref:Microsoft.Maui.Controls.Xaml.IValueProvider>, and <xref:Microsoft.Maui.Controls.IExtendedTypeConverter> need to be annotated with either the <xref:Microsoft.Maui.Controls.Xaml.RequireServiceAttribute> or <xref:Microsoft.Maui.Controls.Xaml.AcceptEmptyServiceProviderAttribute>. This is required due to a XAML compiler optimization introduced in .NET MAUI 9 that enables the generation of more efficient code, which helps reduce the app size and improve runtime performance.
 
-For information about annotating markup extensions with these attributes, see [Service providers](~/xaml/markup-extensions/create.md?view=net-maui-9&preserve-view=true).
+For information about annotating markup extensions with these attributes, see [Service providers](~/xaml/markup-extensions/create.md?view=net-maui-9&preserve-view=true#service-providers).
 
 ## Xcode sync
 
@@ -500,6 +500,71 @@ As a replacement, the <xref:Microsoft.Maui.Controls.VisualElement.Measure(System
 
 In addition, the <xref:Microsoft.Maui.SizeRequest> struct is obsoleted. Instead, <xref:Microsoft.Maui.Graphics.Size> should be used.
 
+## Upgrade from .NET 8 to .NET 9
+
+To upgrade your .NET MAUI projects from .NET 8 to .NET 9, first install .NET 9 and the .NET MAUI workload with [Visual Studio 17.12+](https://visualstudio.microsoft.com/vs/), or with [Visual Studio Code and the .NET MAUI extension and .NET and the .NET MAUI workloads](~/get-started/installation.md?tabs=visual-studio-code), or with the [standalone installer](https://dotnet.microsoft.com/en-us/download/dotnet/9.0) and the `dotnet workload install maui` command.
+
+### Update project file
+
+To update your .NET MAUI app from .NET 8 to .NET 9 open the app's project file (*.csproj*) and change the Target Framework Monikers (TFMs) from 8 to 9. If you're using a TFM such as `net8.0-ios15.2` be sure to match the platform version or remove it entirely. The following example shows the TFMs for a .NET 8 project:
+
+```xml
+<TargetFrameworks>net8.0-android;net8.0-ios;net8.0-maccatalyst;net8.0-tizen</TargetFrameworks>
+<TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('windows'))">$(TargetFrameworks);net8.0-windows10.0.19041.0</TargetFrameworks>
+```
+
+The following example shows the TFMs for a .NET 9 project:
+
+```xml
+<TargetFrameworks>net9.0-android;net9.0-ios;net9.0-maccatalyst;net9.0-tizen</TargetFrameworks>
+<TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('windows'))">$(TargetFrameworks);net9.0-windows10.0.19041.0</TargetFrameworks>
+```
+
+If your app's project file references a .NET 8 version of the [`Microsoft.Maui.Controls`](https://www.nuget.org/packages/Microsoft.Maui.Controls/) NuGet package, either directly or through the `$(MauiVersion)` build property, update this to a .NET 9 version. Then, remove the package reference for the [`Microsoft.Maui.Controls.Compatibility`](https://www.nuget.org/packages/Microsoft.Maui.Controls.Compatibility/) NuGet package, provided that your app doesn't use any types from this package. In addition, update the package reference for the [`Microsoft.Extensions.Logging.Debug`](https://www.nuget.org/packages/Microsoft.Extensions.Logging.Debug/) NuGet package to the latest .NET 9 release.
+
+If your app targets iOS or Mac Catalyst, update the `$(SupportedOSPlatformVersion)` build properties for these platforms to 15.0:
+
+```xml
+<SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'ios'">15.0</SupportedOSPlatformVersion>
+<SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'maccatalyst'">15.0</SupportedOSPlatformVersion>
+```
+
+When debugging and deploying a new .NET MAUI project to Windows, the default behavior in .NET 9 is to deploy an unpackaged app. To adopt this behavior, see [Convert a packaged .NET MAUI Windows app to unpackaged](~/windows/setup.md#convert-a-packaged-net-maui-windows-app-to-unpackaged).
+
+Prior to building your upgraded app for the first time, delete the `bin` and `obj` folders. Any build errors and warnings will guide you towards next steps.
+
+### Update XAML compiler error codes
+
+XAML compiler error codes have changed their prefix from `XFC` to `XC`, so update the `$(WarningsAsErrors)`, `$(WarningsNotAsErrors)`, and `$(NoWarn)` build properties in your app's project file, if used, to reference the new prefix.
+
+### Address new XAML compiler warnings for compiled bindings
+
+Build warnings will be produced for bindings that don't use compiled bindings, and these will need to be addressed. For more information, see [XAML compiled bindings warnings](~/fundamentals/data-binding/compiled-bindings.md?view=net-maui-9.0&preserve-view=true#xaml-compiled-bindings-warnings).
+
+### Update XAML markup extensions
+
+XAML markup extensions will need to be annotated with either the <xref:Microsoft.Maui.Controls.Xaml.RequireServiceAttribute> or <xref:Microsoft.Maui.Controls.Xaml.AcceptEmptyServiceProviderAttribute>. This is required due to a XAML compiler optimization that enables the generation of more efficient code, which helps reduce the app size and improve runtime performance. For more information, see [Service providers](~/xaml/markup-extensions/create.md?view=net-maui-9&preserve-view=true#service-providers).
+
+### Address deprecated APIs
+
+.NET MAUI 9 deprecates some APIs, which will be completely removed in a future release. Therefore, address any build warnings about deprecated APIs. For more information, see [Deprecated APIs](#deprecated-apis).
+
+### Adopt compiled bindings that set the Source property
+
+You can opt into compiling bindings that set the `Source` property, to take advantage of better runtime performance. For more information, see [Compile bindings that define the `Source` property](~/fundamentals/data-binding/compiled-bindings.md?view=net-maui-9.0&preserve-view=true#compile-bindings-that-define-the-source-property).
+
+### Adopt compiled bindings in C\#
+
+You can opt into compiling binding expressions that are declared in code, to take advantage of better runtime performance. For more information, see [Compiled bindings in code](~/fundamentals/data-binding/compiled-bindings.md#compiled-bindings-in-code).
+
+### Adopt full trimming
+
+You can adopt into using full trimming, to reduce the overall size of your app, by setting the `$(TrimMode)` MSBuild property to `full`. For more information, see [Trim a .NET MAUI app](~/deployment/trimming.md).
+
+### Adopt NativeAOT deployment on supported platforms
+
+You can opt into Native AOT deployment on iOS and Mac Catalyst. Native AOT deployment produces a .NET MAUI app that's been ahead-of-time (AOT) compiled to native code. For more information, see [Native AOT deployment on iOS and Mac Catalyst](~/deployment/nativeaot.md).
+
 ## .NET for Android
 
 .NET for Android in .NET 9, which adds support for API 35, includes work to reduce build times, and to improve the trimability of apps to reduce size and improve performance. For more information about .NET for Android in .NET 9, see the following release notes: