[net11.0] Trimmable element handlers

[simonrozsival]

This is a follow-up to #28357

Summary

All built-in controls now use [ElementHandler] attributes instead of DI registration for handler resolution. This removes the need for AppHostBuilderExtensions.AddControlsHandlers() and makes handler associations visible at the type level, improving trimmability.

Key changes

[ElementHandler] attribute

• New public ElementHandlerAttribute on view types (e.g., [ElementHandler(typeof(ButtonHandler))]) declaratively associates a view with its handler
• Inherited = false — each concrete type that needs a different handler must declare its own attribute
• The attribute walks the BaseType chain at resolution time, so derived types without their own attribute inherit the nearest ancestor's handler

Handler resolution in MauiHandlersFactory

Both GetHandler(Type) and GetHandlerType(Type) follow the same 4-step resolution order:

1. Exact DI registration — if the concrete type (e.g., typeof(Button)) is registered via AddHandler<Button, MyButtonHandler>(), that wins. DI-registered handlers are instantiated through MauiFactory.GetService().
2. [ElementHandler] attribute — walk the type's BaseType chain looking for the attribute. Attribute-based handlers are instantiated via Activator.CreateInstance().
3. Interface-based DI registration — map the concrete type to its virtual view interface (e.g., Button → IButton) using RegisteredHandlerServiceTypeSet, then look up the interface in DI. This is the path for third-party/custom handler overrides registered via AddHandler<IButton, ...>().
4. IContentView fallback — if the type implements IContentView, return ContentViewHandler.

GetHandler() is the primary API — it returns an instantiated IElementHandler. GetHandlerType() returns only the Type and is used by code that needs to compare handler types without instantiating them (e.g., Compatibility layer cell renderers).

ElementExtensions.ToHandler() / SetHandler()

These methods call GetHandler() directly. When a handler requires constructor parameters (no parameterless constructor), ToHandler() catches the MissingMethodException and falls back to ActivatorUtilities.CreateInstance() with DI injection. This fallback path is cached per-type in a HashSet<Type> to avoid repeated exception throwing.

Mapper remapping via static constructors

• Controls-layer types override Core-layer mapper entries in their static constructors
• Base-class static constructor ordering is enforced via RemappingHelper.EnsureBaseTypeRemapped(), which accesses a s_forceStaticConstructor field on the parent type to force the runtime to run the parent's static constructor first
• RemappingDebugHelper.AssertBaseClassForRemapping (DEBUG-only) validates that no intermediate type in the hierarchy is accidentally skipped

Cleanup

• Removed IElementHandlerWithAndroidContext interface and ElementHandlerWithAndroidContextAttribute
• Removed dead RemapForControls(this MauiAppBuilder) extension method and old RemapForControls()/RemapIfNeeded() pattern
• Fixed Slider not having a static constructor (was only called explicitly from the removed extension method)
• Restored IMauiHandlersFactory.GetHandler() to its original signature (GetHandler(Type) / GetHandler<T>()) — no longer requires IMauiContext parameter
• Removed unused MauiContext/HandlersContextStub declarations from tests and benchmarks

[Copilot]

Pull Request Overview

This PR transitions built-in controls from DI registration to using [ElementHandler] attributes, introduces IElementHandlerWithAndroidContext<T> for Android renderers, and adds a new MSBuild property to toggle CSS support.

• Apply [ElementHandler] to core controls and cells instead of DI
• Introduce IElementHandlerWithAndroidContext<T> and CreateHandler(Context) on Android renderers
• Add MauiCssEnabled MSBuild property and corresponding runtime host configuration

Reviewed Changes

Copilot reviewed 120 out of 120 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/Controls/src/Core/ContentPage/ContentPage.Mapper.cs	Added static ctor calling RemapForControls and VisualElement.RemapIfNeeded
src/Controls/src/Core/Compatibility/Handlers/TableView/Android/TableViewRenderer.cs	Implemented IElementHandlerWithAndroidContext<TableViewRenderer> and CreateHandler
src/Controls/src/Core/Compatibility/Handlers/ListView/Android/ListViewRenderer.cs	Implemented IElementHandlerWithAndroidContext<ListViewRenderer> and CreateHandler
src/Controls/src/Core/Compatibility/Handlers/Android/FrameRenderer.cs	Implemented IElementHandlerWithAndroidContext<FrameRenderer> and CreateHandler
src/Controls/src/Core/Cells/*.cs	Added [ElementHandler<...>] attributes for compatibility cell renderers
src/Controls/src/Core/Button/Button.cs	Added [ElementHandler<ButtonHandler>]
src/Controls/src/Core//.Mapper.cs	Added static ctors calling RemapForControls / RemapIfNeeded
src/Controls/src/Build.Tasks/nuget/.../Microsoft.Maui.Controls.targets	Added MauiCssEnabled property and runtime host configuration option
src/Compatibility/Core/src/{iOS,Android}/Platform.cs	Updated GetHandler calls to pass MauiContext

Comments suppressed due to low confidence (1)

src/Controls/src/Build.Tasks/nuget/buildTransitive/netstandard2.0/Microsoft.Maui.Controls.targets:240

• Added the MauiCssEnabled build property but no corresponding public documentation under /docs/; please update the XML docs to describe this new feature.

<MauiCssEnabled Condition="'$(MauiCssEnabled)' == '' and '@(MauiCss->Count())' == '0'">false</MauiCssEnabled>

[Copilot]

Implementing IElementHandlerWithAndroidContext introduces a new public interface on this renderer, which is a breaking API change; please assess version impact.

[Copilot]

Implementing IElementHandlerWithAndroidContext introduces a new public interface on this renderer, which is a breaking API change; please assess version impact.

[Copilot]

Implementing IElementHandlerWithAndroidContext introduces a new public interface on this renderer, which is a breaking API change; please assess version impact.

[mattleibow]

/azp run

[azure-pipelines]

Azure Pipelines will not run the associated pipelines, because the pull request was updated after the run command was issued. Review the pull request again and issue a new run command.

[simonrozsival]

@copilot there are merge conflicts in this branch. please resolve them. When you're done, write summary of what this PR does and what functionality is missing to fully convert the "old" way of implementing handlers to the "new" way.

[Copilot]

@simonrozsival I've opened a new pull request, #32861, to work on those changes. Once the pull request is ready, I'll request review from you.

[simonrozsival]

@copilot oops, try now!

[Copilot]

@simonrozsival I've opened a new pull request, #32862, to work on those changes. Once the pull request is ready, I'll request review from you.

[simonrozsival]

@copilot there are several errors popping up:

• src\Controls\src\Core\Shell\Shell.cs(29,25): Error CS0246: The type or namespace name 'ShellHandler' could not be found (are you missing a using directive or an assembly reference?)
• src\Controls\src\Core\Shell\ShellItem.cs(18,25): Error CS0246: The type or namespace name 'ShellItemHandler' could not be found (are you missing a using directive or an assembly reference?)
• buildtasks/Microsoft.Maui.Controls.Build.Tasks.After.targets(15,3): error MSB4024: The imported project file "/mnt/vss/_work/1/s/.buildtasks/Microsoft.Maui.Controls.targets" could not be loaded. Name cannot begin with the '<' character, hexadecimal value 0x3C. Line 325, position 2. [/mnt/vss/_work/1/s/src/BlazorWebView/tests/DeviceTests/MauiBlazorWebView.DeviceTests.csproj]

Fix them and make sure the product can be built.

[Copilot]

@simonrozsival I've opened a new pull request, #32933, to work on those changes. Once the pull request is ready, I'll request review from you.

[rmarinho]

/rebase

[simonrozsival]

@copilot this PR needs a proper rebase with conflict resolution. open a PR which addresses this issue. make sure it is up to date with the base net11.0 branch.

[Copilot]

@simonrozsival I've opened a new pull request, #34008, to work on those changes. Once the pull request is ready, I'll request review from you.

[albyrock87]

It's not clear to me @simonrozsival how one can create their own Button handler.
Would they still use the DI?
If not, how can the system know which one to use? (The MAUI one or the custom one)

Edit: yes it seems DI win, thanks!

So this is basically a feature for library authors to make their handlers trimmable.

[simonrozsival]

@albyrock87 exactly. If you don't use webview in your app, there should be no webview related code in the app.

This change should be as transparent as possible. Any custom handler registered via DI should still work.

[github-actions]

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/dotnet/maui/main/eng/scripts/get-maui-pr.sh | bash -s -- 29952

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/dotnet/maui/main/eng/scripts/get-maui-pr.ps1) } 29952"