Logging.EventSource tests failing on iOS with NullReferenceException in OnEventWritten

[github-actions]

Description

Microsoft.Extensions.Logging.EventSource.Tests failing on iOS and likely all mobile platforms with NullReferenceExceptions in EventSource.OnEventWritten.

Failure Pattern

All 8 failing tests report:

ERROR: Exception during EventSource.OnEventWritten: Object reference not set to an instance of an object.
Assert.Collection() Failure: Item comparison failure
Error: Event name does not match. Expected Message, event data is '{"__EVENT_NAME":"EventSourceMessage","message":"ERROR: Exception during EventSource.OnEventWritten: Object reference not set to an instance of an object."}'

Failing Tests

1. Logs_OnlyMessage_IfKeywordSet
2. Logs_TracingDetailsAsExpected_WithDefaults (4 theory variants)
3. Logs_OnlyFormattedMessage_IfKeywordSet
4. Logs_AsExpected_WithDefaults
5. Logs_AsExpected_WithDefaults_EnabledEarly

Platform Coverage

Confirmed on:

• iossimulator-arm64 CoreCLR

Likely affects all mobile platforms (tvOS, MacCatalyst, Android) based on failure pattern.

Failure Details

Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1397424
Date: 2026-04-27

Helix Job: 7da59944-242f-4296-8a21-1a36fe1d0562
Work Item: Microsoft.Extensions.Logging.EventSource.Tests

Console log shows the EventSource implementation itself is throwing a NullReferenceException during event writing, not the test code.

Related

Likely related to recent changes around trimming, reflection, or EventSource handling on mobile/NativeAOT platforms. See also #127448 for concurrent DI/Hosting failures on the same build.

Note

🔒 Integrity filter blocked 3 items

The following items were blocked because they don't meet the GitHub integrity level.

• #126023 search_pull_requests: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
• #124461 search_pull_requests: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
• #108513 search_pull_requests: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".

To allow these resources, lower min-integrity in your GitHub frontmatter:

tools:
  github:
    min-integrity: approved  # merged | approved | unapproved | none

Generated by Mobile Platform Failure Scanner · ● 4.2M · ◷

[dotnet-policy-service]

Tagging subscribers to 'os-ios': @vitek-karas, @kotlarmilos, @steveisok, @akoeplinger
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

Tagging subscribers to 'os-maccatalyst': @vitek-karas, @kotlarmilos, @steveisok, @akoeplinger
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-extensions-logging
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[dotnet-policy-service]

Tagging subscribers to 'arch-android': @vitek-karas, @simonrozsival, @steveisok, @akoeplinger
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

Tagging subscribers to 'os-tvos': @vitek-karas, @kotlarmilos, @steveisok, @akoeplinger
See info in area-owners.md if you want to be subscribed.

[tarekgh]

Looks the condition in

runtime/src/libraries/tests.proj

Line 381 in 14d3e32

<ItemGroup Condition="('$(TargetOS)' != 'browser' and '$(RunAOTCompilation)' == 'true' and '$(MonoForceInterpreter)' != 'true') or ('$(TargetOS)' == 'browser' and '$(BuildAOTTestsOnHelix)' == 'true' and '$(RunDisabledWasmTests)' != 'true') or (('$(TargetOS)' == 'ios' or '$(TargetOS)' == 'tvos' or '$(TargetOS)' == 'maccatalyst') and '$(BuildTestsOnHelix)' == 'true')">

is not correctly excluding the iossimulator and tvossimulator.

#127448 maybe similar root cause.

[tarekgh]

@vitek-karas @MichalStrehovsky do you think the trimming is aggressive? should preserve EventSource event metadata. The [DynamicDependency] and [UnconditionalSuppressMessage] attributes on LoggingEventSource may not be sufficient under aggressive trimming?

[MichalStrehovsky]

@vitek-karas @MichalStrehovsky do you think the trimming is aggressive? should preserve EventSource event metadata. The [DynamicDependency] and [UnconditionalSuppressMessage] attributes on LoggingEventSource may not be sufficient under aggressive trimming?

The core invariant we have for trimming is that if no trimming warnings are generated, the code needs to work. The test itself doesn't seem to be triggering trimming warnings. The trimming analyzer on the test itself is enabled (default value is now enabled and I don't see <EnableTrimAnalyzer>false</EnableTrimAnalyzer> in this test project; there is a <EnableAotAnalyzer>false</EnableAotAnalyzer> in the test project but that's irrelevant for non-native AOT testing). So if there was a trimming issue in the test code itself, the analyzer would warn and break the build.

So if the test is hitting issues after trimming, it's a product bug. We likely put [UnconditionalSuppressMessage] on something in the product that is not actually safe with trimming. We either need to make it safe, or do a breaking change and declare this library as not compatible with trimming so that users get a proper build time warning when using it.