[net11.0] Trimmable StyleSheets by simonrozsival · Pull Request #33160 · dotnet/maui

simonrozsival · 2025-12-15T12:33:58Z

Description of Change

This PR will allow trimming CSS-related code and default styles when there are no CSS stylesheets used in the MAUI app.

When @(MauiCss) item is added, the CSS feature is enabled
When the app does not have any @(MauiCss) but devs want to use Style.Parse to parse styles at runtime, they must enable $(MauiCssEnabled)=true

Most of the changes in this PR are around moving assembly-level attributes into a local list. This will allow us to shave of some unnecessary code from most apps:

// before
[assembly: StyleProperty("background-color", typeof(VisualElement), nameof(VisualElement.BackgroundColorProperty))]

// after
new StylePropertyAttribute("background-color", typeof(VisualElement), nameof(VisualElement.BackgroundColorProperty)),

Related to #29952

simonrozsival · 2025-12-15T12:34:11Z

/azp run

azure-pipelines · 2025-12-15T12:34:26Z

Azure Pipelines successfully started running 3 pipeline(s).

Copilot

Pull request overview

This PR introduces a trimming optimization for CSS-related code in .NET MAUI applications. The feature allows CSS stylesheets code to be trimmed away when not used, reducing app size. The main approach involves moving assembly-level StylePropertyAttribute declarations from AssemblyInfo.cs into a local array in Registrar.cs, which enables better trimming behavior. A new runtime feature flag IsCssEnabled controls whether CSS functionality is available at runtime, with MSBuild logic automatically disabling it in trimmed builds when no CSS files are present.

Key Changes

Adds IsCssEnabled runtime feature flag with default value of true
Moves 100+ assembly-level StyleProperty attributes into a local array to enable trimming
Adds runtime checks that throw NotSupportedException when CSS is disabled but CSS APIs are called

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
`src/Core/src/RuntimeFeature.cs`	Adds `IsCssEnabled` feature flag with default value of `true` and feature switch definition for .NET 9+
`src/Controls/src/Core/VisualElement/VisualElement_StyleSheet.cs`	Adds runtime check in `GetProperty` to throw helpful error when CSS is disabled
`src/Controls/src/Core/StyleSheets/Style.cs`	Adds runtime checks in `Parse` and `Apply` methods to prevent CSS usage when disabled
`src/Controls/src/Core/Registrar.cs`	Converts 100+ assembly-level attributes to local array and adds early return when CSS is disabled
`src/Controls/src/Core/Properties/AssemblyInfo.cs`	Removes all assembly-level `StylePropertyAttribute` declarations and unused using statements
`src/Controls/src/Build.Tasks/nuget/buildTransitive/netstandard2.0/Microsoft.Maui.Controls.targets`	Sets `MauiCssEnabled=false` in trimmed builds when no `@(MauiCss)` items exist, and configures runtime option

Copilot · 2025-12-15T12:43:44Z

+				new StylePropertyAttribute("width", typeof(VisualElement), nameof(VisualElement.WidthRequestProperty)),
+				new StylePropertyAttribute("letter-spacing", typeof(ITextElement), nameof(TextElement.CharacterSpacingProperty)) { Inherited = true },
+#pragma warning disable CS0618 // Type or member is obsolete
+				new StylePropertyAttribute("line-height", typeof(ILineHeightElement), nameof(LineHeightElement.LineHeightProperty)) { Inherited = true },


The type reference for ILineHeightElement in this attribute is incorrect. Since this code is in the Microsoft.Maui.Controls.Internals namespace, typeof(ILineHeightElement) will resolve to the internal obsolete Microsoft.Maui.Controls.Internals.ILineHeightElement instead of the public Microsoft.Maui.Controls.ILineHeightElement that was used in the original assembly attribute. This needs to be changed to use the fully qualified type name or add a using alias to ensure the correct type is referenced.

Suggested change

new StylePropertyAttribute("line-height", typeof(ILineHeightElement), nameof(LineHeightElement.LineHeightProperty)) { Inherited = true },

new StylePropertyAttribute("line-height", typeof(Microsoft.Maui.Controls.ILineHeightElement), nameof(LineHeightElement.LineHeightProperty)) { Inherited = true },

simonrozsival · 2026-01-12T09:52:04Z

/azp run maui-pr-devicetests,maui-pr-uitests

azure-pipelines · 2026-01-12T09:52:23Z

Azure Pipelines successfully started running 2 pipeline(s).

simonrozsival · 2026-01-16T06:37:38Z

Follow-up reminder: Documentation update needed

After this PR is merged, we need to document the MauiCssEnabled feature switch in the official .NET MAUI docs at dotnet/maui-docs.

The feature switch documentation should explain:

CSS is automatically disabled when no MauiCss items exist in the project
How to manually enable CSS with <MauiCssEnabled>true</MauiCssEnabled>
Trimming benefits when CSS is disabled

The in-repo documentation has been updated in docs/design/FeatureSwitches.md.

simonrozsival · 2026-01-16T10:27:38Z

/azp run maui-pr-uitests

azure-pipelines · 2026-01-16T10:27:50Z

Azure Pipelines successfully started running 1 pipeline(s).

simonrozsival · 2026-01-16T11:14:48Z

/azp run maui-pr-devicetests

azure-pipelines · 2026-01-16T11:15:01Z

Azure Pipelines successfully started running 1 pipeline(s).

rmarinho · 2026-01-16T13:54:49Z

/rebase

rmarinho · 2026-01-16T13:55:59Z

/azp run maui-pr-devicetests

azure-pipelines · 2026-01-16T13:56:10Z

Azure Pipelines successfully started running 1 pipeline(s).

rmarinho · 2026-01-31T01:39:55Z

/rebase

simonrozsival · 2026-02-12T09:31:29Z

@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 · 2026-02-12T09:31:40Z

@simonrozsival I've opened a new pull request, #34010, to work on those changes. Once the pull request is ready, I'll request review from you.

…hanges These changes belong in separate PRs: - #33611 (CSS StyleSheet trimming) - #33561 (EventTrigger trimming) - #33160 (HybridWebView trimming) Reverted files to their main branch state, keeping only the [ElementHandler] attribute addition on HybridWebView. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

github-actions · 2026-02-27T15:25:04Z

🚀 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 -- 33160

Or

Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/dotnet/maui/main/eng/scripts/get-maui-pr.ps1) } 33160"

…ed on @(MauiCss) item count

- Add MauiCssEnabled=true to TestCases.HostApp.csproj since some UI tests use CSS functionality - Add unit tests for CSS-disabled scenarios in StyleTests.cs

- Update NotSupportedException messages to explain why CSS is disabled and how to enable it - Add MauiCssEnabled documentation to docs/design/FeatureSwitches.md - Added follow-up reminder comment to PR for dotnet/maui-docs update

typeof(ILineHeightElement) was resolving to the obsolete Microsoft.Maui.Controls.Internals.ILineHeightElement. Use fully qualified Microsoft.Maui.Controls.ILineHeightElement and remove the CS0618 pragma suppression. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- IsCssEnabled: use FeatureSwitchPrefix + nameof() to match other feature switches in RuntimeFeature.cs - FeatureSwitches.md: clarify that CSS auto-disable only applies to optimized builds (PublishAot=true or TrimMode=full) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

simonrozsival · 2026-03-20T10:37:50Z

@rmarinho @jfversluis could you please have a look at this PR and let me know what you think about it? could we merge this into net11.0 soon?

Copilot AI review requested due to automatic review settings December 15, 2025 12:33

simonrozsival added the perf/app-size Application Size / Trimming (sub: perf) label Dec 15, 2025

Copilot started reviewing on behalf of simonrozsival December 15, 2025 12:34 View session

Copilot AI reviewed Dec 15, 2025

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simonrozsival force-pushed the dev/simonrozsival/css-trimmable branch from 3c28640 to db2f075 Compare January 16, 2026 07:03

github-actions Bot force-pushed the dev/simonrozsival/css-trimmable branch from db2f075 to 74375c4 Compare January 16, 2026 13:55

simonrozsival changed the title ~~[net11.0] Trim CSS-related code when not used~~ [net11.0] Trimmable StyleSheets Jan 20, 2026

simonrozsival force-pushed the dev/simonrozsival/css-trimmable branch from 22761a2 to 77c461a Compare January 20, 2026 13:33

github-actions Bot force-pushed the dev/simonrozsival/css-trimmable branch from 77c461a to 7ded7f1 Compare January 31, 2026 01:40

Copilot AI mentioned this pull request Feb 12, 2026

Rebase CSS trimmable PR onto latest net11.0 #34010

Closed

simonrozsival force-pushed the dev/simonrozsival/css-trimmable branch from 7ded7f1 to 66d1243 Compare February 13, 2026 08:09

build-analysis Bot mentioned this pull request Feb 13, 2026

System.Exception : Failed to launch Test AVD. #33862

Closed

StephaneDelcroix force-pushed the dev/simonrozsival/css-trimmable branch from 66d1243 to 564c244 Compare February 27, 2026 15:24

simonrozsival force-pushed the dev/simonrozsival/css-trimmable branch from 564c244 to afbda13 Compare March 11, 2026 09:31

simonrozsival and others added 7 commits March 20, 2026 10:07

Introduce IsCssEnabled feature switch and infer its default value bas…

62431eb

…ed on @(MauiCss) item count

Skip loading CSS style sheets when CSS support is trimmed

fd2c514

Throw in CSS-related APIs when CSS is disabled

122b1b7

Enable CSS in TestCases.HostApp and add unit tests

1a9e611

- Add MauiCssEnabled=true to TestCases.HostApp.csproj since some UI tests use CSS functionality - Add unit tests for CSS-disabled scenarios in StyleTests.cs

simonrozsival force-pushed the dev/simonrozsival/css-trimmable branch from 96755c5 to fa6afda Compare March 20, 2026 09:08

jfversluis approved these changes Mar 20, 2026

View reviewed changes

jfversluis added this to the .NET 11.0-preview3 milestone Mar 20, 2026

jfversluis merged commit 7d39822 into net11.0 Mar 20, 2026
29 checks passed

jfversluis deleted the dev/simonrozsival/css-trimmable branch March 20, 2026 14:37

github-actions Bot locked and limited conversation to collaborators Apr 20, 2026

	new StylePropertyAttribute("line-height", typeof(ILineHeightElement), nameof(LineHeightElement.LineHeightProperty)) { Inherited = true },
	new StylePropertyAttribute("line-height", typeof(Microsoft.Maui.Controls.ILineHeightElement), nameof(LineHeightElement.LineHeightProperty)) { Inherited = true },

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simonrozsival commented Dec 15, 2025

Description of Change

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Pull request overview

Key Changes

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simonrozsival commented Jan 16, 2026

Follow-up reminder: Documentation update needed

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