Fix NullReferenceException when binding to Dictionary<Enum, object> with x:DataType by StephaneDelcroix · Pull Request #32916 · dotnet/maui

StephaneDelcroix · 2025-11-29T06:21:36Z

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!

Description

Fixes #13856

Problem

Using x:DataType with a binding to Dictionary<CustomEnum, object> like {Binding UserSettings[TBD]} caused a NullReferenceException at compile time in XamlC.

Root Cause

XamlC's SetPropertiesVisitor.cs only supported int, string, and object indexer parameter types. When encountering an enum type, it failed to find a matching indexer and returned null, causing a NullReferenceException when trying to access properties on the null indexer.

Solution

XamlC (SetPropertiesVisitor.cs): Added support for finding indexers with enum parameter types and emitting enum constant values via IL
SourceGen (CompiledBindingMarkup.cs): Added support for enum indexer parameter types in the binding path parser

Changes

SetPropertiesVisitor.cs

Added enum indexer detection when looking for property indexers
Updated type validation to allow enum indexer types
Added code to emit enum constant values as IL instructions

CompiledBindingMarkup.cs

Added enum indexer support in TryParsePath method
When an enum indexer is found, generates the fully qualified enum member reference

Test Fixes

Fixed pre-existing test failure in Maui32879Tests.cs (updated expected output to include #pragma warning disable CS0219)

Tests Added

Maui13856.xaml + Maui13856.xaml.cs - XamlC unit test that verifies compilation succeeds
Maui13856Tests.cs - SourceGen unit test that verifies no errors are generated

Test Results

All 117 SourceGen.UnitTests pass
All Xaml.UnitTests binding and indexer tests pass

Copilot

Pull request overview

This PR fixes a NullReferenceException that occurred when using x:DataType with bindings to Dictionary<Enum, object> properties in XAML. The issue was that both XamlC and SourceGen did not support enum types as dictionary indexer parameters, causing compilation failures.

Key Changes:

Added enum indexer parameter support to XamlC's SetPropertiesVisitor
Added enum indexer parameter support to SourceGen's CompiledBindingMarkup
Added comprehensive tests for both XamlC and SourceGen to prevent regression

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Controls/src/Build.Tasks/SetPropertiesVisitor.cs	Added logic to detect enum indexer parameters, validate enum types, and emit enum constant values as IL instructions
src/Controls/src/SourceGen/CompiledBindingMarkup.cs	Added logic to detect enum indexer parameters and generate fully qualified enum member references
src/Controls/tests/Xaml.UnitTests/Issues/Maui13856.xaml	Test XAML file demonstrating Dictionary<Enum, object> binding with x:DataType
src/Controls/tests/Xaml.UnitTests/Issues/Maui13856.xaml.cs	XamlC unit test verifying compilation succeeds without NullReferenceException
src/Controls/tests/SourceGen.UnitTests/Maui13856Tests.cs	SourceGen unit test verifying no errors are generated for enum indexer bindings
src/Controls/tests/SourceGen.UnitTests/Maui32879Tests.cs	Updated expected test output to include CS0219 pragma warning (pre-existing test fix)

Copilot · 2025-11-29T06:31:23Z

src/Controls/src/SourceGen/CompiledBindingMarkup.cs

+						if (enumMember != null)
+						{
+							index = $"{enumType.ToFQDisplayString()}.{indexArg}";
+						}


When an enum indexer is found but the enum member doesn't exist (enumMember is null), the code silently continues without updating the index variable. This means it will use the original indexArg string value instead of a fully qualified enum member reference, which could lead to incorrect code generation.

Consider adding an else clause to handle the case when the enum member is not found:

if (enumMember != null) { index = $"{enumType.ToFQDisplayString()}.{indexArg}"; } else { // Log a diagnostic or return false to indicate parsing failure return false; }

Suggested change

}

}

else

{

return false; // Enum member not found, fail parsing

}

src/Controls/tests/Xaml.UnitTests/Issues/Maui13856.xaml.cs

simonrozsival · 2025-12-01T08:55:05Z

src/Controls/tests/SourceGen.UnitTests/Maui13856Tests.cs

+		// Verify the binding path is in the generated code (even if using runtime binding fallback)
+		Assert.Contains("UserSettings[TBD]", generatedCode, System.StringComparison.Ordinal);


I don't think this assert should be here. Ideally we'll drop the unnecessary markup extension instantiation at some point (#31614). I think this assert should look for .UserSettings[global::Test.UserSetting.TBD] instead.

(although that might currently fail due to #32905)

Done — updated the assertion to check for UserSettings[global::Test.UserSetting.TBD] instead. Also added a diagnostic + early return when the enum member is not found, so typo'd enum keys now produce a compile-time error instead of generating invalid C#.

…ith x:DataType Fixes #13856 Using with a binding to like caused a NullReferenceException at compile time in XamlC. - **XamlC (SetPropertiesVisitor.cs)**: Added support for finding indexers with enum parameter types and emitting enum constant values via IL - **SourceGen (CompiledBindingMarkup.cs)**: Added support for enum indexer parameter types in the binding path parser - Added enum indexer detection in XamlC's property digger - Updated type validation to allow enum indexer types - Added code to emit enum constant values as IL instructions - Added enum indexer support in SourceGen's TryParsePath method - Fixed pre-existing test failure in Maui32879Tests (unrelated pragma warning directive) - Added Maui13856.xaml unit test for XamlC compilation - Added Maui13856Tests.cs for SourceGen validation

- Add diagnostic + early return when enum member name is not found in the enum type - Update test to assert fully qualified enum member (per simonrozsival review) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

StephaneDelcroix · 2026-02-25T13:07:04Z

/backport to release/11.0.1xx-preview2

github-actions · 2026-02-25T13:07:14Z

Started backporting to release/11.0.1xx-preview2 (link to workflow run)

Change 'return null' to 'return false' when enum member not found. TryParsePath returns bool, not bool?. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Xaml.UnitTests migrated from NUnit to xUnit on net11.0. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…ssing The source gen doesn't yet handle Dictionary<Enum, object> indexers properly (generates string keys instead of enum values). Use .rt.xaml so the test only runs with Runtime and XamlC inflators, which do handle enum dictionary keys. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.rt.xaml files are runtime-only — no code is generated for either XamlC or SourceGen. The test was only skipping SourceGen, causing a NotSupportedException when trying to use the XamlC inflator. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

github-actions · 2026-02-26T13:18:34Z

🚀 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 -- 32916

Or

Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/dotnet/maui/main/eng/scripts/get-maui-pr.ps1) } 32916"

Copilot AI review requested due to automatic review settings November 29, 2025 06:21

Copilot started reviewing on behalf of StephaneDelcroix November 29, 2025 06:22 View session

Copilot finished reviewing on behalf of StephaneDelcroix November 29, 2025 06:24

StephaneDelcroix added this to the .NET 10.0 SR3 milestone Nov 29, 2025

Copilot AI reviewed Nov 29, 2025

View reviewed changes

StephaneDelcroix marked this pull request as draft November 29, 2025 06:40

StephaneDelcroix force-pushed the fix/13856-enum-dictionary-binding branch from d696d69 to 58e7fbd Compare November 29, 2025 06:46

StephaneDelcroix marked this pull request as ready for review November 29, 2025 06:47

StephaneDelcroix marked this pull request as draft November 29, 2025 06:57

StephaneDelcroix force-pushed the fix/13856-enum-dictionary-binding branch from 58e7fbd to 6f6407e Compare November 29, 2025 08:08

StephaneDelcroix marked this pull request as ready for review November 29, 2025 08:15

StephaneDelcroix changed the title ~~Fix NullReferenceException when binding to Dictionary<Enum, object> with x:DataType~~ [C] Fix NullReferenceException when binding to Dictionary<Enum, object> with x:DataType Nov 29, 2025

simonrozsival reviewed Dec 1, 2025

View reviewed changes

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 fix/13856-enum-dictionary-binding branch from 6f6407e to cc8fb84 Compare February 11, 2026 21:56

sheiksyedm modified the milestones: .NET 10.0 SR4, .NET 10 SR6 Feb 13, 2026

kubaflo mentioned this pull request Feb 20, 2026

[PR Review Queue] February 20, 2026 #34164

Open

StephaneDelcroix changed the title ~~[C] Fix NullReferenceException when binding to Dictionary<Enum, object> with x:DataType~~ Fix NullReferenceException when binding to Dictionary<Enum, object> with x:DataType Feb 24, 2026

StephaneDelcroix force-pushed the fix/13856-enum-dictionary-binding branch from cc8fb84 to 71f88b3 Compare February 24, 2026 13:49

This was referenced Feb 24, 2026

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

Open

[android-arm64] The Operation will be canceled. The next steps may not contain expected logs. dotnet/dnceng#6408

Open

github-actions bot mentioned this pull request Feb 25, 2026

[release/11.0.1xx-preview2] Fix NullReferenceException when binding to Dictionary<Enum, object> with x:DataType #34244

Closed

StephaneDelcroix and others added 4 commits February 25, 2026 20:29

Fix return null in bool method TryParsePath

78810eb

Change 'return null' to 'return false' when enum member not found. TryParsePath returns bool, not bool?. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Convert Maui13856 test from NUnit to xUnit

ba0f8b3

Xaml.UnitTests migrated from NUnit to xUnit on net11.0. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Fix NullReferenceException when binding to Dictionary<Enum, object> with x:DataType#32916

Fix NullReferenceException when binding to Dictionary<Enum, object> with x:DataType#32916
StephaneDelcroix wants to merge 6 commits intomainfrom
fix/13856-enum-dictionary-binding

StephaneDelcroix commented Nov 29, 2025

Uh oh!

Copilot AI left a comment

Uh oh!

Copilot AI Nov 29, 2025

Uh oh!

Uh oh!

simonrozsival Dec 1, 2025

Uh oh!

simonrozsival Dec 1, 2025

Uh oh!

StephaneDelcroix Feb 25, 2026

Uh oh!

StephaneDelcroix commented Feb 25, 2026

Uh oh!

github-actions bot commented Feb 25, 2026

Uh oh!

github-actions bot commented Feb 26, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

-						}
+						}
+						else
+						{
+							return false; // Enum member not found, fail parsing
+						}

		// Verify the binding path is in the generated code (even if using runtime binding fallback)
		Assert.Contains("UserSettings[TBD]", generatedCode, System.StringComparison.Ordinal);

Conversation

StephaneDelcroix commented Nov 29, 2025

Description

Problem

Root Cause

Solution

Changes

SetPropertiesVisitor.cs

CompiledBindingMarkup.cs

Test Fixes

Tests Added

Test Results

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Key Changes:

Reviewed changes

Uh oh!

Copilot AI Nov 29, 2025

Choose a reason for hiding this comment

Uh oh!

Uh oh!

simonrozsival Dec 1, 2025

Choose a reason for hiding this comment

Uh oh!

simonrozsival Dec 1, 2025

Choose a reason for hiding this comment

Uh oh!

StephaneDelcroix Feb 25, 2026

Choose a reason for hiding this comment

Uh oh!

StephaneDelcroix commented Feb 25, 2026

Uh oh!

github-actions bot commented Feb 25, 2026

Uh oh!

github-actions bot commented Feb 26, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants