Add XC0067/MAUIX2015 warning for duplicate implicit content property assignments in XAML by StephaneDelcroix · Pull Request #32654 · dotnet/maui

StephaneDelcroix · 2025-11-16T06:57:44Z

Note

Are you waiting for the changes in this PR to be merged?
It would be very helpful if you could test the resulting artifacts from this PR and let us know in a comment if this change resolves your issue. Thank you!

Fixes #3059

Description of Change

Adds XC0067/MAUIX2006 warning diagnostic to detect when XAML implicit content properties are set multiple times (e.g., <Border><Label/><Label/></Border>).

This implementation specifically targets the issue described in #3059: detecting multiple children in single-child content properties.

Key Changes

Focused implementation: Only warns for implicit content property duplicates
Does NOT warn for: Explicit attribute duplicates like <Label Text="A" Text="B" /> (XAML parser already rejects these at parse time)
Collection-aware: Correctly handles collection properties (e.g., Style.Setters, VisualStateGroup.States) - no warnings when adding multiple items
Works for both implicit and explicit content property syntax:
- <Border><Label/><Label/></Border> → warns about Border.Content
- <Border><Border.Content><Label/><Label/></Border.Content></Border> → warns about Border.Content

Implementation Details

Duplicate detection only in Visit(ElementNode) for implicit content properties
Removed checks for explicit attribute assignments (ValueNode) - not needed
Added property type inspection to detect collection properties
Collections use Add() method, so multiple children are expected and don't trigger warnings
Improved NoWarn parsing to properly handle comma/semicolon-delimited codes
Implemented in both Build.Tasks and SourceGen

Testing

Added comprehensive unit tests in SourceGen.UnitTests
Tests verify warnings for single-value content properties
Tests verify NO warnings for collection properties
Tests verify NO warnings for single children
Removed unnecessary tests for explicit attribute duplicates

Issues Fixed

Fixes #3059

Copilot

Pull Request Overview

This PR adds a warning diagnostic (XC0067/MAUIX2006) to detect when XAML properties are assigned multiple times, addressing issue #3059. The implementation is generalized to check all property assignments, not just ContentProperty.

Key changes:

Added duplicate property detection in both Build.Tasks and SourceGen XAML processing
Introduced new warning diagnostic MAUIX2006 with resource strings for error messages
Added comprehensive unit tests covering duplicate property scenarios and proper NoWarn suppression

Reviewed Changes

Copilot reviewed 15 out of 16 changed files in this pull request and generated 14 comments.

Show a summary per file

File	Description
src/Controls/tests/Xaml.UnitTests/Issues/Issue3059b.xaml.cs	New test class verifying duplicate property assignment behavior (Text, Placeholder) - missing XamlInflator constructor
src/Controls/tests/Xaml.UnitTests/Issues/Issue3059b.xaml	Test XAML with duplicate property assignments for validation
src/Controls/tests/Xaml.UnitTests/Issues/Issue3059.xaml.cs	New test class verifying ContentProperty duplicate behavior - missing XamlInflator constructor
src/Controls/tests/Xaml.UnitTests/Issues/Issue3059.xaml	Test XAML with multiple children in Border to test ContentProperty duplication
src/Controls/tests/Xaml.UnitTests/Issues/Gh5095.rt.xaml.cs	Updated diagnostic count assertion (1→2) to account for new duplicate property warning
src/Controls/tests/Xaml.UnitTests/Controls.Xaml.UnitTests.csproj	Added NoWarn suppressions for test files and globally suppressed CS1030
src/Controls/tests/SourceGen.UnitTests/InitializeComponent/MultipleChildrenWarningTests.cs	Comprehensive unit tests for duplicate property warning in SourceGen
src/Controls/src/SourceGen/xlf/MauiGResources.Designer.cs	Added resource string accessors for new diagnostic messages
src/Controls/src/SourceGen/Visitors/SetPropertiesVisitor.cs	Implemented duplicate property tracking and warning emission logic
src/Controls/src/SourceGen/MauiGResources.resx	Added resource strings for warning title and message
src/Controls/src/SourceGen/InitializeComponentCodeWriter.cs	Minor formatting adjustment to raw string literal
src/Controls/src/SourceGen/Descriptors.cs	Added MAUIX2006 diagnostic descriptor for duplicate properties
src/Controls/src/SourceGen/AnalyzerReleases.Unshipped.md	Documented new MAUIX2006 warning in analyzer releases
src/Controls/src/Build.Tasks/SetPropertiesVisitor.cs	Implemented duplicate property tracking for Build.Tasks (with formatting fix)
src/Controls/src/Build.Tasks/ErrorMessages.resx	Added error message resource for Build.Tasks
src/Controls/src/Build.Tasks/BuildException.cs	Added XC0067 exception code for duplicate property warnings

Files not reviewed (1)

src/Controls/src/SourceGen/xlf/MauiGResources.Designer.cs: Language not supported

src/Controls/src/Build.Tasks/BuildException.cs

Copilot · 2025-11-16T07:08:47Z

src/Controls/src/Build.Tasks/ErrorMessages.resx

+  <data name="MultipleChildrenInContentProperty" xml:space="preserve">
+    <value>Property '{0}' is being set multiple times. Only the last value will be used.</value>
+    <comment>0 is property name (e.g. "Border.Content" or "Label.Text")</comment>


The error message resource key MultipleChildrenInContentProperty is misleading since this diagnostic now detects duplicate assignments for any property, not just ContentProperty. Consider renaming to something more accurate like DuplicatePropertyAssignment or PropertySetMultipleTimes to better reflect its general-purpose nature.

Suggested change

<data name="MultipleChildrenInContentProperty" xml:space="preserve">

<value>Property '{0}' is being set multiple times. Only the last value will be used.</value>

<comment>0 is property name (e.g. "Border.Content" or "Label.Text")</comment>

<data name="DuplicatePropertyAssignment" xml:space="preserve">

<value>Property '{0}' is being set multiple times. Only the last value will be used.</value>

<comment>0 is the property name (e.g. "Border.Content" or "Label.Text")</comment>

src/Controls/src/SourceGen/Visitors/SetPropertiesVisitor.cs

src/Controls/tests/Xaml.UnitTests/Controls.Xaml.UnitTests.csproj

src/Controls/tests/Xaml.UnitTests/Issues/Issue3059.xaml.cs

Copilot · 2025-11-16T07:08:49Z

src/Controls/src/SourceGen/AnalyzerReleases.Unshipped.md

 MAUIX2003 | XamlInflation | Error | Descriptors
 MAUIX2004 | XamlInflation | Error | Descriptors
 MAUIX2005 | XamlInflation | Warning | Descriptors
+MAUIX2006 | XamlInflation | Warning | PropertySetMultipleTimes


The description "PropertySetMultipleTimes" doesn't match the misleading internal identifier name MultipleChildrenInContentProperty. While the description is accurate for the feature, it should ideally align with the descriptor name used in code. Consider updating the descriptor name in code to match this more accurate description.

Suggested change

MAUIX2006 | XamlInflation | Warning | PropertySetMultipleTimes

MAUIX2006 | XamlInflation | Warning | MultipleChildrenInContentProperty

src/Controls/src/SourceGen/Visitors/SetPropertiesVisitor.cs

Copilot · 2025-11-16T07:08:50Z

src/Controls/src/Build.Tasks/SetPropertiesVisitor.cs

@@ -140,6 +174,10 @@ public void Visit(ElementNode node, INode parentNode)
 						return;
 					if (parentNode is ElementNode node2 && node2.SkipProperties.Contains(propertyName))


Line 175 checks node2.SkipProperties.Contains(propertyName) but propertyName is still the old value from the outer scope, not the newly assigned name variable from line 172. This should be checking name instead of propertyName.

Change to:

if (parentNode is ElementNode node2 && node2.SkipProperties.Contains(name)) return;

Suggested change

if (parentNode is ElementNode node2 && node2.SkipProperties.Contains(propertyName))

if (parentNode is ElementNode node2 && node2.SkipProperties.Contains(name))

src/Controls/tests/Xaml.UnitTests/Issues/Issue3059b.xaml.cs

src/Controls/src/SourceGen/MauiGResources.resx

- Renamed MultipleChildrenInContentProperty to DuplicatePropertyAssignment across all files - Updated diagnostic IDs: XC0067 (Build.Tasks) and MAUIX2006 (SourceGen) - Renamed Issue3059 to Maui3059.rt (runtime XAML - no sourcegen/xamlC) - Added tests to verify MockSourceGenerator and MockCompiler report warnings correctly - Removed NoWarn suppression to allow warning verification in tests Addresses review comments on PR #32654

Copilot

Pull request overview

Copilot reviewed 20 out of 21 changed files in this pull request and generated 4 comments.

Files not reviewed (1)

src/Controls/src/SourceGen/xlf/MauiGResources.Designer.cs: Language not supported

Copilot · 2025-11-28T21:11:37Z

src/Controls/src/SourceGen/Visitors/SetPropertiesVisitor.cs

+			var noWarn = Context.ProjectItem.NoWarn;
+			bool shouldSuppress = !string.IsNullOrEmpty(noWarn) &&
+				noWarn.Split(new[] { ',', ';', ' ' })
+					.Select(code => code.Trim())


The NoWarn parsing logic includes space (' ') as a delimiter, which could cause issues when combined with other delimiters. For example, NoWarn="67; 2006" would produce empty strings after splitting. The .Trim() call helps, but empty strings could still match warnings if not filtered. Consider adding .Where(code => !string.IsNullOrWhiteSpace(code)) before the .Any() check to filter out empty entries.

Suggested change

.Select(code => code.Trim())

.Select(code => code.Trim())

.Where(code => !string.IsNullOrWhiteSpace(code))

Copilot · 2025-11-28T21:11:37Z

src/Controls/tests/Xaml.UnitTests/Issues/Maui3059.rt.xaml.cs

+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+// Note: This is a .rt.xaml file (runtime) which skips source generation and XamlC compilation
+// This file tests that MockCompiler (XamlC) emits the XC0067 warning


[nitpick] This comment has a typo: "XamlC" should be "XamlC compilation" or "XamlC Build.Tasks" for clarity. The term "XamlC" alone might be confusing as it doesn't immediately convey that it refers to the Build.Tasks compilation path.

Suggested change

// This file tests that MockCompiler (XamlC) emits the XC0067 warning

// This file tests that MockCompiler (XamlC compilation) emits the XC0067 warning

Copilot · 2025-11-28T21:11:38Z

src/Controls/tests/Xaml.UnitTests/Issues/Issue3059.xaml

+<?xml version="1.0" encoding="UTF-8"?>
+<ContentPage
+	xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+	xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+	x:Class="Microsoft.Maui.Controls.Xaml.UnitTests.Issue3059">
+	<Border>
+		<Label Text="First" />
+		<Label Text="Second" />
+	</Border>
+</ContentPage>


[nitpick] The test file naming is inconsistent. Both Issue3059.xaml and Maui3059.xaml exist, testing the same issue (#3059). The repository convention typically uses the prefix "Maui" for MAUI-specific issues (e.g., Maui3059) rather than "Issue". Consider standardizing on one naming pattern. Since this PR references issue #3059 which is a .NET MAUI issue, Maui3059 would be more consistent with the repository's current naming conventions.

Copilot · 2025-11-28T21:11:38Z

src/Controls/tests/Xaml.UnitTests/Controls.Xaml.UnitTests.csproj

+
+    <!-- Suppress XC0067/MAUIX2006 (property set multiple times) for test files that intentionally test this scenario -->
+    <MauiXaml Update="Issues\Bz40906.xaml" NoWarn="67;2006" />
+    <MauiXaml Update="Issues\Issue3059b.xaml" NoWarn="67;2006" />


The file Issue3059b.xaml referenced in this NoWarn directive does not exist in the repository. This appears to be a typo or reference to a file that was never created. Consider removing this line or verifying if a different file name was intended.

Suggested change

<MauiXaml Update="Issues\Issue3059b.xaml" NoWarn="67;2006" />

StephaneDelcroix · 2026-03-04T07:47:05Z

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

azure-pipelines · 2026-03-04T07:47:21Z

Azure Pipelines successfully started running 2 pipeline(s).

StephaneDelcroix · 2026-03-06T17:06:35Z

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

azure-pipelines · 2026-03-06T17:06:56Z

Azure Pipelines successfully started running 2 pipeline(s).

simonrozsival · 2026-03-10T09:15:42Z

src/Controls/src/SourceGen/Visitors/SetPropertiesVisitor.cs

+				// Only check for duplicate property assignment if the property is not a collection
+				// Get the property type and check if it supports Add() for collection behavior
+				bool attached = false;
+				var localName = name.LocalName;
+				var bpFieldSymbol = parentVar.Type.GetBindableProperty(name.NamespaceURI, ref localName, out attached, context, node);
+				ITypeSymbol? propertyType = null;
+
+				// Try to get the property type
+				bool hasProperty = (bpFieldSymbol != null && SetPropertyHelpers.CanGetValue(parentVar, bpFieldSymbol, attached, context, out propertyType))
+					|| SetPropertyHelpers.CanGet(parentVar, localName, context, out propertyType, out _);
+
+				// Only warn if:
+				// 1. We can resolve the property and its type
+				// 2. The property type is NOT a collection (doesn't support Add)
+				// 3. The property type is not System.Object (unresolved generic)
+				bool isObject = propertyType != null && propertyType.SpecialType == Microsoft.CodeAnalysis.SpecialType.System_Object;
+				// Use parent namespace for duplicate tracking to match how explicit property assignments are keyed
+				var contentPropertyName = new XmlName(((ElementNode)parentNode).NamespaceURI, contentProperty);
+				if (hasProperty && propertyType != null && !isObject && !propertyType.CanAdd(context))
+					CheckForDuplicateProperty((ElementNode)parentNode, contentPropertyName, node);


This non-trivial code is basically identical to a previous snippet of code. I suggest we move the logic to a method to ensure the behavior stays in sync.

extracted the shared logic into a CheckImplicitContentForDuplicate helper method that's now called from both Visit(ValueNode) and Visit(ElementNode). The inline duplication is gone.Done

MauiBot · 2026-03-23T00:25:58Z

⚠️ Merge Conflict Detected — This PR has merge conflicts with its target branch. Please rebase onto the target branch and resolve the conflicts.

…assignments in XAML When a non-collection implicit content property (e.g. Border.Content, ContentPage.Content) is assigned more than once in XAML, only the last value is used and earlier assignments are silently discarded. This adds a compile-time warning to flag such cases. - Build.Tasks (XamlC): emits XC0067 for ElementNode duplicate assignments - SourceGen: emits MAUIX2015 for both ElementNode and ValueNode duplicates - Collection types (IEnumerable+Add) and System.Object are excluded from the check - Fix SkipProperties.Contains bug (was checking wrong XmlName variable) - Improve NoWarn parsing: support space separator and bare numeric suffixes - Add SourceGen, XAML, and visual-state regression tests - Suppress new warnings on existing test files that intentionally duplicate properties Fixes #3059

StephaneDelcroix · 2026-03-30T07:30:26Z

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

azure-pipelines · 2026-03-30T07:30:40Z

Azure Pipelines successfully started running 2 pipeline(s).

…assignments in XAML (#32654)  > [!NOTE] > Are you waiting for the changes in this PR to be merged? > It would be very helpful if you could [test the resulting artifacts](https://github.com/dotnet/maui/wiki/Testing-PR-Builds) from this PR and let us know in a comment if this change resolves your issue. Thank you! Fixes #3059 ## Description of Change Adds XC0067/MAUIX2006 warning diagnostic to detect when XAML **implicit content properties** are set multiple times (e.g., `<Border><Label/><Label/></Border>`). This implementation specifically targets the issue described in #3059: detecting multiple children in single-child content properties. ## Key Changes - **Focused implementation**: Only warns for implicit content property duplicates - **Does NOT warn for**: Explicit attribute duplicates like `<Label Text="A" Text="B" />` (XAML parser already rejects these at parse time) - **Collection-aware**: Correctly handles collection properties (e.g., `Style.Setters`, `VisualStateGroup.States`) - no warnings when adding multiple items - **Works for both implicit and explicit content property syntax**: - `<Border><Label/><Label/></Border>` → warns about `Border.Content` - `<Border><Border.Content><Label/><Label/></Border.Content></Border>` → warns about `Border.Content` ## Implementation Details - Duplicate detection only in `Visit(ElementNode)` for implicit content properties - Removed checks for explicit attribute assignments (ValueNode) - not needed - Added property type inspection to detect collection properties - Collections use `Add()` method, so multiple children are expected and don't trigger warnings - Improved NoWarn parsing to properly handle comma/semicolon-delimited codes - Implemented in both Build.Tasks and SourceGen ## Testing - Added comprehensive unit tests in SourceGen.UnitTests - Tests verify warnings for single-value content properties - Tests verify NO warnings for collection properties - Tests verify NO warnings for single children - Removed unnecessary tests for explicit attribute duplicates ## Issues Fixed Fixes #3059

Copilot AI review requested due to automatic review settings November 16, 2025 06:57

Copilot started reviewing on behalf of StephaneDelcroix November 16, 2025 06:58 View session

Copilot finished reviewing on behalf of StephaneDelcroix November 16, 2025 07:02

Copilot AI reviewed Nov 16, 2025

View reviewed changes

StephaneDelcroix marked this pull request as draft November 16, 2025 12:04

StephaneDelcroix force-pushed the dev/stdelc/fix3059 branch from f73e09a to 418703a Compare November 17, 2025 11:04

StephaneDelcroix marked this pull request as ready for review November 17, 2025 12:34

StephaneDelcroix added the area-xaml XAML, CSS, Triggers, Behaviors label Nov 17, 2025

StephaneDelcroix added this to the .NET 11 Planning milestone Nov 24, 2025

StephaneDelcroix force-pushed the dev/stdelc/fix3059 branch 3 times, most recently from 95d81ee to 1889047 Compare November 25, 2025 09:37

StephaneDelcroix requested a review from Copilot November 28, 2025 21:01

Copilot started reviewing on behalf of StephaneDelcroix November 28, 2025 21:02 View session

Copilot finished reviewing on behalf of StephaneDelcroix November 28, 2025 21:05

Copilot AI reviewed Nov 28, 2025

View reviewed changes

StephaneDelcroix modified the milestones: .NET 11 Planning, .NET 10.0 SR3 Nov 29, 2025

StephaneDelcroix changed the title ~~Add warning for duplicate property assignments in XAML~~ [X] Add warning for duplicate property assignments in XAML Nov 29, 2025

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 modified the milestones: .NET 10.0 SR3, .NET 10.0 SR4 Jan 21, 2026

StephaneDelcroix force-pushed the dev/stdelc/fix3059 branch from 1424bc2 to 0a3fad0 Compare February 11, 2026 14:14

StephaneDelcroix modified the milestones: .NET 10.0 SR4, .NET 11.0-preview2 Feb 11, 2026

StephaneDelcroix changed the base branch from main to net11.0 February 11, 2026 14:14

StephaneDelcroix force-pushed the dev/stdelc/fix3059 branch from 0a3fad0 to 9ec54c0 Compare February 11, 2026 15:11

build-analysis bot mentioned this pull request Feb 11, 2026

The Operation will be canceled. The next steps may not contain expected logs. dotnet/dnceng#3008

Open

3 tasks

StephaneDelcroix force-pushed the dev/stdelc/fix3059 branch 3 times, most recently from 47fbaa9 to 1697526 Compare March 3, 2026 16:50

StephaneDelcroix force-pushed the dev/stdelc/fix3059 branch from 7c58278 to 4e8a3ba Compare March 6, 2026 11:58

simonrozsival reviewed Mar 10, 2026

View reviewed changes

simonrozsival approved these changes Mar 10, 2026

View reviewed changes

github-project-automation bot moved this from Todo to Approved in MAUI SDK Ongoing Mar 10, 2026

PureWeen modified the milestones: .NET 11.0-preview2, .NET 11.0-preview3 Mar 12, 2026

StephaneDelcroix force-pushed the dev/stdelc/fix3059 branch from f6eea58 to ccfb7f4 Compare March 30, 2026 07:26

StephaneDelcroix merged commit 8bb9726 into net11.0 Mar 30, 2026
92 of 146 checks passed

StephaneDelcroix deleted the dev/stdelc/fix3059 branch March 30, 2026 08:42

github-project-automation bot moved this from Approved to Done in MAUI SDK Ongoing Mar 30, 2026

	MAUIX2006 \| XamlInflation \| Warning \| PropertySetMultipleTimes
	MAUIX2006 \| XamlInflation \| Warning \| MultipleChildrenInContentProperty

		@@ -140,6 +174,10 @@ public void Visit(ElementNode node, INode parentNode)
		return;
		if (parentNode is ElementNode node2 && node2.SkipProperties.Contains(propertyName))

	.Select(code => code.Trim())
	.Select(code => code.Trim())
	.Where(code => !string.IsNullOrWhiteSpace(code))

	// This file tests that MockCompiler (XamlC) emits the XC0067 warning
	// This file tests that MockCompiler (XamlC compilation) emits the XC0067 warning

Conversation

StephaneDelcroix commented Nov 16, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Description of Change

Key Changes

Implementation Details

Testing

Issues Fixed

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull Request Overview

Reviewed Changes

Uh oh!

Uh oh!

Copilot AI Nov 16, 2025

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Copilot AI Nov 16, 2025

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Copilot AI Nov 16, 2025

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Uh oh!

Copilot AI Nov 28, 2025

Choose a reason for hiding this comment

Uh oh!

Copilot AI Nov 28, 2025

Choose a reason for hiding this comment

Uh oh!

Copilot AI Nov 28, 2025

Choose a reason for hiding this comment

Uh oh!

Copilot AI Nov 28, 2025

Choose a reason for hiding this comment

Uh oh!

StephaneDelcroix commented Mar 4, 2026

Uh oh!

azure-pipelines bot commented Mar 4, 2026

Uh oh!

StephaneDelcroix commented Mar 6, 2026

Uh oh!

azure-pipelines bot commented Mar 6, 2026

Uh oh!

simonrozsival Mar 10, 2026

Choose a reason for hiding this comment

Uh oh!

StephaneDelcroix Mar 30, 2026

Choose a reason for hiding this comment

Uh oh!

MauiBot commented Mar 23, 2026

Uh oh!

StephaneDelcroix commented Mar 30, 2026

Uh oh!

azure-pipelines bot commented Mar 30, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

StephaneDelcroix commented Nov 16, 2025 •

edited

Loading