Add CA1876 analyzer to detect misuse of AsParallel() in foreach loops

[Copilot]

Plan for AsParallel() Misuse Analyzer

Creating a new analyzer to detect incorrect usage of AsParallel() in LINQ queries.

Problem

• AsParallel() at the end of a LINQ query directly in a foreach is a no-op
• Example: foreach (var item in src.Select(...).Where(...).AsParallel()) { } does NOT parallelize the foreach
• Example: foreach (var item in src.AsParallel()) { } also does NOT parallelize the foreach

Implementation Complete ✅

• Explore existing analyzer structure and resource files
• Create new diagnostic rule CA1876
• Create analyzer to detect foreach over AsParallel() result
• Add resource strings for the diagnostic
• Create comprehensive unit tests (11 tests, all passing)
• Build and test the analyzer
• Run dotnet format on changes
• Address code review feedback
• Revert unintended formatting changes (479 files)
• Regenerate auto-generated documentation files
• Convert test strings to raw string literals

Changes Made

1. New Analyzer: DoNotUseAsParallelInForEachLoop.cs

   • Detects when AsParallel() is called directly in a foreach loop
   • Rule ID: CA1876
   • Category: Performance
   • Severity: Info (IdeSuggestion)
   • Uses SymbolEqualityComparer.Default for proper symbol comparison

2. Resource Strings: Added to MicrosoftNetCoreAnalyzersResources.resx

   • Title: "Do not use 'AsParallel' in 'foreach'"
   • Message: "Using 'AsParallel()' directly in a 'foreach' loop has no effect and the loop is not parallelized"
   • Description explains the issue and suggests alternatives

3. Configuration Files Updated:

   • WellKnownTypeNames.cs: Added SystemLinqParallelEnumerable
   • DiagnosticCategoryAndIdRanges.txt: Extended Performance range to CA1876
   • AnalyzerReleases.Unshipped.md: Added CA1876 entry

4. Auto-generated Documentation:

   • Regenerated .md and .sarif files using msbuild /t:pack

5. Test Coverage: 11 comprehensive tests covering:

   • Basic AsParallel in foreach (C# and VB)
   • AsParallel with LINQ chains (Select, Where)
   • Valid usage patterns (AsParallel with ToList/ToArray)
   • No false positives for regular foreach loops
   • Updated: All test strings now use raw string literals (""") for better code maintainability

All tests passing ✓
Build succeeds without warnings ✓

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Use AsParallel() correctly</issue_title>
<issue_description>Using .AsParallel() at the end of a LINQ query, e.g. foreach (var item in src.Select(...).Where(...).AsParallel(...)), is a nop and should either be removed or the AsParallel() moved earlier in the query. I've even seen developers write foreach (var item in src.AsParallel()) thinking it parallelizes the foreach loop, which it doesn't... it'd be good to warn about such misuse.

Category: Performance</issue_description>

<agent_instructions>Please create an analyzer in the net analyzers project that flags AsParallel() being used incorrectly, in particular when its result is directly foreach'd over. In such cases, it's a nop. You don't need to create a fixer for the analyzer right now, we'll start just with the analyzer. The details about the misuse can highlight that if it's used in a LINQ query, the developer should consider moving the AsParallel earlier in the sequence, or the developer might instead consider using Parallel.ForEach.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@stephentoub Here's an example of the kinds of things we'd like to catch: https://github.com//issues/10937 @jeffhandley Estimates: * Analyzer: Medium * Fixer: Medium @stephentoub > guess the analyzer only triggers when the AsParallel is directly inside a foreach

Yes

can only be resolved by removing the code. I guess that is the intended fix

That's the safe fix, in that it doesn't change the semantics. It'd be fine to offer multiple fixes, though, with a second option being to move the call to the beginning of the chain, and if the developer chooses that, it's their choice.</comment_new>
<comment_new>@jeffhandley
@carlossanlop @buyaa-n Can you follow up with @Mrnikbobjeff on this? We would need to bring this in for API Review if we want to move it forward.</comment_new>
<comment_new>@stephentoub

I do not think that would be advisable

If someone added the AsParallel(), they intended for something to be run in parallel. And while we can't know for sure how much of the query that comes before it was intended, as an option I think it's reasonable to offer to fix it with our best guess for where it would go, which is at the beginning. The only 100% safe fix is to remove it, but both can be offered, and if we have any control which is the "default" / first option, it should be the removal.</comment_new>
<comment_new>@eiriktsarpalis
Hey @Mrnikbobjeff, would you still be interested in picking this up?</comment_new>
<comment_new>@
Tagging subscribers to this area: @tarekgh, @dotnet/area-system-linq-parallel
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Using .AsParallel() at the end of a LINQ query, e.g. foreach (var item in src.Select(...).Where(...).AsParallel(...)), is a nop and should either be removed or the AsParallel() moved earlier in the query. I've even seen developers write foreach (var item in src.AsParallel()) thinking it parallelizes the foreach loop, which it doesn't... it'd be good to warn about such misuse.

Category: Performance

Author:	terrajobst
Assignees:	Mrnikbobjeff
Labels:	api-approved, area-System.Linq, area-System.Linq.Parallel, code-analyzer, code-fixer, in pr
Milestone:	7.0.0

Fixes dotnet/runtime#33769

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.

[stephentoub]

@copilot, it looks like you checked in edits to hundreds of unrelated files. Did you accidentally run dotnet format and commit those changes? Please revert those.

Also, there's a build failure to address:

  One or more auto-generated documentation files were either edited manually, or not updated. Please revert changes made to the following files (if manually edited) and run `msbuild /t:pack` at the root of the repo to automatically update them:
      /mnt/vss/_work/1/s/src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers.md
      /mnt/vss/_work/1/s/src/Microsoft.CodeAnalysis.NetAnalyzers/src/Microsoft.CodeAnalysis.NetAnalyzers.sarif

[Copilot]

@copilot, it looks like you checked in edits to hundreds of unrelated files. Did you accidentally run dotnet format and commit those changes? Please revert those.

Also, there's a build failure to...

Reverted the 479 unintended formatting changes from dotnet format and regenerated the documentation files using msbuild /t:pack. Commit: 7ba18ec

[Copilot]

Pull Request Overview

This PR implements analyzer CA1876 to detect incorrect usage of AsParallel() in foreach loops, where it has no effect on parallelization. The analyzer identifies when AsParallel() is called directly in a foreach statement's collection expression, as the foreach statement itself always iterates serially regardless of the collection type.

Key Changes:

• Added new performance analyzer CA1876 with comprehensive test coverage
• Extended diagnostic ID range and added necessary localization resources
• Regenerated auto-generated documentation files

Reviewed Changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
DoNotUseAsParallelInForEachLoop.cs	Core analyzer implementation detecting AsParallel() misuse in foreach loops
DoNotUseAsParallelInForEachLoopTests.cs	11 test cases covering various scenarios (basic usage, LINQ chains, valid patterns)
WellKnownTypeNames.cs	Added SystemLinqParallelEnumerable type constant
DiagnosticCategoryAndIdRanges.txt	Extended Performance category range to include CA1876
MicrosoftNetCoreAnalyzersResources.resx	Added English resource strings for title, message, and description
MicrosoftNetCoreAnalyzersResources.*.xlf	Added localization placeholders for 13 languages
AnalyzerReleases.Unshipped.md	Documented new CA1876 analyzer
Microsoft.CodeAnalysis.NetAnalyzers.sarif	Auto-generated SARIF metadata for CA1876
Microsoft.CodeAnalysis.NetAnalyzers.md	Auto-generated documentation for CA1876

[tarekgh]

LGTM!