[perf-improver] perf: eliminate LINQ allocations in terminal progress render hot path

[Evangelink]

🤖 This is an automated contribution from Perf Improver.

Goal and Rationale

The live-progress render loop runs every 500 ms while tests are executing. Two methods in this path were creating unnecessary LINQ iterator chains and iterator state machines per frame:

1. TestNodeResultsState.GetRunningTasks — built a Select + OrderByDescending + ToList LINQ pipeline to sort running test tasks, then used yield return to expose them. Each call produced: a DictionaryEnumerator, a SelectIterator<>, an OrderedEnumerable<,>, a List<TestDetailState>, a second array from [.. Take(n)] (when too many items), and an IEnumerator state machine. Called once per assembly per frame.

2. AnsiTerminalTestProgressFrame.GenerateLinesToRender — used progress.OfType<TestProgressState>() spread (OfTypeIterator + array copy) to filter nulls, and Enumerable.Range(0, n).OrderBy(...) (RangeIterator + OrderedEnumerable<,>) to sort indices by detail count. Called once per frame.

Approach

GetRunningTasks

• Changed return type from IEnumerable<TestDetailState> to List<TestDetailState> — caller gets the list directly, no iterator state machine needed.
• Build the list with a direct foreach over the ConcurrentDictionary instead of Select(d => d.Value).
• Sort with List.Sort and a static lambda (no closure allocation) instead of LINQ OrderByDescending.
• Truncate in-place via List.RemoveRange instead of [.. Take(n)] (avoids second array allocation).
• Remove yield return — items returned from the list directly.

GenerateLinesToRender

• Replace [.. progress.OfType<TestProgressState>()] with two null-counting passes over the array — avoids OfTypeIterator and collection-spread copy.
• Replace Enumerable.Range(0, n).OrderBy(i => ...) with a pre-filled int[] sorted via Array.Sort — avoids RangeIterator and OrderedEnumerable allocations.
• Update detailItems array type from IEnumerable<TestDetailState>[] to List<TestDetailState>[] to match the new return type.

Performance Evidence

Site	Before (per frame)	After
GetRunningTasks (per assembly)	DictionaryEnumerator + SelectIterator + OrderedEnumerable + List + array (Take) + state machine = ~5–6 objects	List only = 1 object
GenerateLinesToRender — null filter	OfTypeIterator + array spread = 2 objects	0 (in-place loops)
GenerateLinesToRender — index sort	RangeIterator + OrderedEnumerable = 2 objects	int[] + closure = 2 objects (same count but avoids LINQ machinery overhead)

For a test run with 10 assemblies, this eliminates roughly 4–5 allocations per assembly per frame = ~40–50 fewer heap allocations per 500 ms tick in the terminal render loop.

Methodology: code inspection + diff review. The render loop is easily verified by running tests and observing a steady terminal.

Trade-offs

• Slightly more verbose code in GenerateLinesToRender (two loops to count + fill the array vs. a one-liner spread).
• GetRunningTasks return type change is internal — no public API impact.
• The Array.Sort comparison lambda still captures progressItems (a local), but this is one allocation per frame total rather than per-item LINQ machinery.

Test Status

• Microsoft.Testing.Platform.UnitTests (net8.0): 1003 passed, 0 failed, 3 skipped ✅
• Build: 0 warnings, 0 errors ✅
• Note: Full repo build has a pre-existing infrastructure failure (XLF files out of sync with resx — unrelated to these changes).

Reproducibility

./build.sh
artifacts/bin/Microsoft.Testing.Platform.UnitTests/Debug/net8.0/Microsoft.Testing.Platform.UnitTests

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[Evangelink]

🔍 Build Failure Analysis

Summary — The build fails with 3 identical errors (one per target framework) because Resources/xlf/PlatformResources.cs.xlf (the Czech localization file) is out-of-date with Resources/PlatformResources.resx in Microsoft.Testing.Platform. This is not caused by this PR's code changes, which only touch AnsiTerminalTestProgressFrame.cs and TestNodeResultsState.cs.

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Root cause: Localization XLF out-of-sync with RESX

The Microsoft.DotNet.XliffTasks targets enforce that every .xlf file's <source> strings exactly match the values in the corresponding .resx. The Czech XLF (PlatformResources.cs.xlf) appears to have drifted from PlatformResources.resx — likely because the OneLocBuild automation check-in (PR #8798, commit 726cc8a) updated the resx and xlf together, but a subtle mismatch remains (e.g., a changed source string value or whitespace/encoding difference in a recent addition).

The same error fires for all three TFMs (net8.0, netstandard2.0, net9.0) because the check runs once per build of Microsoft.Testing.Platform.csproj.

Affected project / errors

Code	Project	Message
(no code)	Microsoft.Testing.Platform (net8.0)	'Resources/xlf/PlatformResources.cs.xlf' is out-of-date with 'Resources/PlatformResources.resx'
(no code)	Microsoft.Testing.Platform (netstandard2.0)	same
(no code)	Microsoft.Testing.Platform (net9.0)	same

Proposed fix

Regenerate the XLF files locally and commit the result. From the repository root:

dotnet msbuild src/Platform/Microsoft.Testing.Platform/Microsoft.Testing.Platform.csproj /t:UpdateXlf

Then commit the updated src/Platform/Microsoft.Testing.Platform/Resources/xlf/PlatformResources.cs.xlf (and any other updated xlf files). Do not hand-edit the xlf files directly.

Note: Since this PR doesn't modify any resource strings, it's also possible the mismatch is a pre-existing condition on the base branch. If UpdateXlf produces no diff, the fix is a simple re-run of the CI.

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Build overview

• Project: src/Platform/Microsoft.Testing.Platform/Microsoft.Testing.Platform.csproj
• Target frameworks: net8.0, netstandard2.0, net9.0
• Target that failed: CheckForOutdatedXlf (via Microsoft.DotNet.XliffTasks.targets:84)
• Build time: 1m 28s
• Exit code: 1
• Errors: 3 | Warnings: 0

All MSBuild errors (3)

Code	Project	TFM	File	Message
—	Microsoft.Testing.Platform	net8.0	XliffTasks.targets:84	'Resources/xlf/PlatformResources.cs.xlf' is out-of-date with 'Resources/PlatformResources.resx'
—	Microsoft.Testing.Platform	netstandard2.0	XliffTasks.targets:84	same
—	Microsoft.Testing.Platform	net9.0	XliffTasks.targets:84	same

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_{🤖 Generated by the Build Failure Analysis workflow · commit a435e21}

Generated by Build Failure Analysis for issue #8799 · sonnet46 2.7M · ◷