Fix implicit styles with BasedOn chains breaking in 10.0.20 by StephaneDelcroix · Pull Request #33218 · dotnet/maui

StephaneDelcroix · 2025-12-18T16:08:49Z

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Description of Change

This fixes issue #33203 where implicit styles using BasedOn with keyed styles stopped working in 10.0.20.

Root Cause

The regression was introduced by PR #32711 which fixed #9648 (ApplyToDerivedTypes not working for implicit styles). That fix created wrapper styles to merge multiple applicable implicit styles, but in doing so it replaced the original BasedOn chain with a new one, losing the connection to keyed styles that defined the actual property values.

The Fix

Instead of creating wrapper styles that break BasedOn chains, we now store the list of applicable implicit styles and apply them sequentially during Apply(). This preserves each style's BasedOn chain intact while still supporting the ApplyToDerivedTypes functionality from #9648.

Test Results

✅ All 6 tests for ApplyToDerivedTypes not working for implicit styles #9648 (ApplyToDerivedTypes fix) still pass
✅ All 1776 XAML unit tests pass (8 skipped, 0 failed)
✅ All 164 Style-related Controls.Core unit tests pass
✅ User's reproduction scenario (keyed style with BasedOn) now works correctly

Issues Fixed

Fixes #33203

Copilot

Pull request overview

This PR fixes a regression in .NET MAUI 10.0.20 where implicit styles using BasedOn chains with keyed styles stopped working. The issue was introduced by PR #32711 which fixed ApplyToDerivedTypes functionality.

Key Changes

Changed merging strategy: Instead of creating wrapper styles that broke BasedOn chains, the fix now stores multiple applicable implicit styles in a list and applies them sequentially
Preserved BasedOn chains: Each style's BasedOn chain remains intact, ensuring property values from keyed styles are properly resolved
Maintained ApplyToDerivedTypes support: The fix continues to support multiple implicit styles from the type hierarchy while fixing the regression

Copilot · 2025-12-18T16:19:17Z

+			// Apply implicit styles from most base to most derived
+			// This ensures derived styles override base styles for conflicting properties
+			if (_applicableImplicitStyles != null && _applicableImplicitStyles.Count > 0)
+			{
+				// Apply in reverse order (most base first, most derived last)
+				// so derived styles override base styles
+				for (int i = _applicableImplicitStyles.Count - 1; i >= 0; i--)
+				{
+					//NOTE specificity could be more fine grained (using distance)
+					((IStyle)_applicableImplicitStyles[i]).Apply(bindable, new SetterSpecificity(SetterSpecificity.StyleImplicit, 0, 0, 0));
+				}
+			}
+			else
+			{
+				//NOTE specificity could be more fine grained (using distance)
+				ImplicitStyle?.Apply(bindable, new SetterSpecificity(SetterSpecificity.StyleImplicit, 0, 0, 0));
+			}


The logic for applying and unapplying multiple implicit styles is duplicated across multiple methods (Apply() at lines 113-127, UnApply() at lines 145-155, and SetStyle() at lines 254-264 and 275-286). Consider extracting this logic into private helper methods like ApplyImplicitStyles(BindableObject target) and UnApplyImplicitStyles(BindableObject target) to eliminate duplication and ensure consistency. This would improve maintainability and reduce the risk of introducing bugs when updating the style application logic in the future.

Copilot · 2025-12-18T16:19:17Z

+			// Set ImplicitStyle to the most derived style for compatibility
+			// (some code may check if ImplicitStyle is set)
+			ImplicitStyle = applicableStyles[0];


Setting ImplicitStyle to the most derived style (applicableStyles[0]) when multiple applicable styles exist may lead to unexpected behavior or confusion. Code that checks if ImplicitStyle is set may not realize there are multiple styles being applied. Consider either:

Setting ImplicitStyle to null when multiple styles are present to make it clear that the _applicableImplicitStyles list should be used instead

Adding a property or method to indicate when multiple implicit styles are active
This would make the multi-style state more explicit and prevent potential misuse of the ImplicitStyle property.

Suggested change

// Set ImplicitStyle to the most derived style for compatibility

// (some code may check if ImplicitStyle is set)

ImplicitStyle = applicableStyles[0];

// When multiple implicit styles are active, leave ImplicitStyle null

// so callers know to consult _applicableImplicitStyles instead.

ImplicitStyle = null;

PureWeen · 2025-12-29T21:38:15Z

/rebase

PureWeen · 2025-12-30T15:48:27Z

/rebase

This fixes issue #33203 where implicit styles using BasedOn with keyed styles stopped working in 10.0.20. The regression was introduced by PR #32711 which fixed #9648 (ApplyToDerivedTypes not working for implicit styles). The problem was that when merging multiple applicable implicit styles, the code created wrapper styles that replaced the original BasedOn chain, losing the connection to keyed styles that defined the actual property values. The fix changes the approach: instead of creating wrapper styles that break BasedOn chains, we now store the list of applicable implicit styles and apply them sequentially during Apply(). This preserves each style's BasedOn chain intact while still supporting the ApplyToDerivedTypes functionality from #9648. Fixes #33203

Tests verify that: 1. Implicit styles with BasedOn to keyed styles work correctly 2. Derived implicit styles properly override base styles' properties

When a base style uses AppThemeBinding for a property and a derived style overrides that property with a static value, the AppThemeBinding would re-apply when theme changes or resources propagate, overwriting the derived style's value. Changes: - AppThemeBinding.Apply(fromTarget): Skip re-apply when value is set from target - AppThemeBinding.ApplyCore(): Check if current value was set by this binding before re-applying (using IsValueFromThisBinding helper) - MergedStyle: Apply graduated specificities for multiple implicit styles Added unit test Maui33203AppTheme to verify derived style setters override base style's AppThemeBinding in both Light and Dark modes.

PureWeen

I'm pretty sure the failures on this one are legit UI test failures.

Co-authored-by: PureWeen <5375137+PureWeen@users.noreply.github.com>

…" (#33397) - [x] Revert `MergedStyle.cs` to original state (restore simple `OnImplicitStyleChanged()` method) - [x] Delete test file `Maui9648.xaml` - [x] Delete test file `Maui9648.xaml.cs` - [x] Delete benchmark file `ImplicitStylesBenchmark.cs` - [x] Add passing tests from PR #33218 for issue #33203 - [x] Remove failing AppTheme tests ### Description of Change Reverts the changes from #32711 which modified how implicit styles with `ApplyToDerivedTypes=true` are applied. The original behavior returns early with the first applicable implicit style instead of merging multiple styles from the type hierarchy. ### Issues Fixed Fixes #33203 ### Test Results **Maui33203 Tests (6/6 PASS):** - `ImplicitStyleWithBasedOnKeyedStyleWorks` - All 3 inflators PASSED ✅ - `DerivedImplicitStyleOverridesBaseStyle` - All 3 inflators PASSED ✅   <details> <summary>Original prompt</summary> > Revert this PR #32711 </details>  --- ✨ Let Copilot coding agent [set things up for you](https://github.com/dotnet/maui/issues/new?title=✨+Set+up+Copilot+instructions&body=Configure%20instructions%20for%20this%20repository%20as%20documented%20in%20%5BBest%20practices%20for%20Copilot%20coding%20agent%20in%20your%20repository%5D%28https://gh.io/copilot-coding-agent-tips%29%2E%0A%0A%3COnboard%20this%20repo%3E&assignees=copilot) — coding agent works faster and does higher quality work when set up for your repo. --------- Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com> Co-authored-by: PureWeen <5375137+PureWeen@users.noreply.github.com>

Copilot AI review requested due to automatic review settings December 18, 2025 16:08

StephaneDelcroix added this to the .NET 10.0 SR3 milestone Dec 18, 2025

StephaneDelcroix added t/bug Something isn't working area-xaml XAML, CSS, Triggers, Behaviors i/regression This issue described a confirmed regression on a currently supported version labels Dec 18, 2025

Copilot started reviewing on behalf of StephaneDelcroix December 18, 2025 16:09 View session

Copilot AI reviewed Dec 18, 2025

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StephaneDelcroix marked this pull request as draft December 18, 2025 16:22

StephaneDelcroix marked this pull request as ready for review December 18, 2025 16:48

StephaneDelcroix mentioned this pull request Dec 18, 2025

10.0.20 breaks implicit styles #33203

Closed

This was referenced Dec 18, 2025

System.InvalidOperationException: Failed to copy test results from simulator to host. dotnet/dnceng#6442

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"We stopped hearing from agent Azure Pipelines 32. Verify the agent machine is running and has a healthy network connection" dotnet/dnceng#1886

Open

jfversluis previously approved these changes Dec 19, 2025

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StephaneDelcroix modified the milestones: .NET 10.0 SR3, .NET 10.0 SR2 Dec 19, 2025

rmarinho mentioned this pull request Dec 19, 2025

System.TimeoutException #33245

Open

PureWeen modified the milestones: .NET 10.0 SR2, .NET 10.0 SR2.1 Dec 19, 2025

StephaneDelcroix dismissed jfversluis’s stale review via cfba365 December 19, 2025 19:52

build-analysis Bot mentioned this pull request Dec 19, 2025

CI UItests failure System.TimeoutException : Timed out waiting for element... #33220

Open

github-actions Bot force-pushed the fix/issue-33203-implicit-styles-regression branch from cfba365 to 6ac9db6 Compare December 29, 2025 21:39

PureWeen added this to MAUI SDK Ongoing Dec 30, 2025

github-project-automation Bot moved this to Todo in MAUI SDK Ongoing Dec 30, 2025

PureWeen approved these changes Dec 30, 2025

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github-project-automation Bot moved this from Todo to Approved in MAUI SDK Ongoing Dec 30, 2025

StephaneDelcroix added 3 commits December 30, 2025 15:49

Add unit tests for issue #33203

53360fb

Tests verify that: 1. Implicit styles with BasedOn to keyed styles work correctly 2. Derived implicit styles properly override base styles' properties

github-actions Bot force-pushed the fix/issue-33203-implicit-styles-regression branch from 6ac9db6 to 46d0b16 Compare December 30, 2025 15:49

PureWeen requested changes Dec 30, 2025

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github-project-automation Bot moved this from Approved to Changes Requested in MAUI SDK Ongoing Dec 30, 2025

PureWeen closed this Jan 6, 2026

github-project-automation Bot moved this from Changes Requested to Done in MAUI SDK Ongoing Jan 6, 2026

PureWeen mentioned this pull request Jan 6, 2026

Revert "Fix ApplyToDerivedTypes not working for implicit styles (#9648)" #33397

Merged

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Copilot AI added a commit that referenced this pull request Jan 6, 2026

Add tests from PR #33218 for issue #33203

f7ba907

Co-authored-by: PureWeen <5375137+PureWeen@users.noreply.github.com>

github-actions Bot pushed a commit that referenced this pull request Jan 7, 2026

Add tests from PR #33218 for issue #33203

0cc60e6

Co-authored-by: PureWeen <5375137+PureWeen@users.noreply.github.com>

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Fix implicit styles with BasedOn chains breaking in 10.0.20#33218

Fix implicit styles with BasedOn chains breaking in 10.0.20#33218
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StephaneDelcroix commented Dec 18, 2025

Description of Change

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The Fix

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