perf: fix dictionary double-lookups, Collection.Contains, and LINQ allocations by Evangelink · Pull Request #15533 · microsoft/vstest

Amaury Levé (Evangelink) · 2026-03-20T10:11:16Z

Summary

Fix several obvious performance issues related to data types and redundant iterations across the codebase.

Changes

1. Dictionary `ContainsKey` + indexer → `TryGetValue`

Eliminates double hash lookups in ~15 locations. Each ContainsKey + dict[key] pair performs two hash operations when TryGetValue suffices with one.

Files changed:

ParallelRunDataAggregator.cs — GetAggregatedRunStats()
TestSessionPool.cs — KillSession(), TryTakeProxy(), ReturnProxy()
ProxyTestSessionManager.cs — DequeueProxy()
InProcDataCollectionSink.cs — AddKeyValuePairToDictionary(), AddOrUpdateData()
PortableSymbolReader.cs — GetNavigationData()
FullSymbolReader.cs — GetTypeSymbol(), GetMethodSymbol()
TestSessionStartArgs.cs — GetPropertyValue<T>()
SessionEvents.cs — GetPropertyValue<T>()
SimpleJSON.cs — indexer getter/setter, Add(), Remove()

2. `Collection<InvokedDataCollector>` → `HashSet<InvokedDataCollector>`

ParallelRunDataAggregator.InvokedDataCollectors was a Collection<T> with O(n) .Contains() called inside a foreach loop (O(n²) behavior). Changed to HashSet<T> for O(1) lookups. The type already implements IEquatable<T> and GetHashCode(). Updated call sites in ParallelRunEventsHandler and ParallelDataCollectionEventsHandler to wrap with .ToList() when passing to APIs expecting Collection<T>.

3. `new[]` allocation inside LINQ predicate → static `HashSet`

In EnableBlameArgumentProcessor.cs, new[] { "CollectAlways", "DumpType" } was allocated per element inside .Where() predicates. Extracted to file-scoped static HashSet<string> fields with StringComparer.OrdinalIgnoreCase.

4. String `+=` → string interpolation

Consolidated two sequential options += calls into a single interpolated string in TestRunnerConnectionInfoExtensions.cs.

Testing

Build: ✅ build.cmd -c Release — 0 errors, 0 warnings
ParallelRunDataAggregator tests: ✅ 44 passed
TestSessionPool tests: ✅ 8 passed
InProcDataCollection tests: ✅ 30 passed
EnableBlame tests: ✅ 26 passed

Copilot

Pull request overview

Performance-focused refactor to reduce redundant dictionary lookups/iterations and cut per-call allocations, with a small concurrency-safety improvement in run stats aggregation.

Changes:

Replaced ContainsKey + indexer patterns with TryGetValue to avoid double lookups.
Switched invoked data collector tracking to a HashSet to avoid O(n²) .Contains() patterns.
Removed a per-element array allocation in LINQ predicates by using static precomputed key sets.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
test/Microsoft.TestPlatform.CrossPlatEngine.UnitTests/Client/Parallel/ParallelRunDataAggregatorTests.cs	Updated assertions for `HashSet` semantics and strengthened thread-safety test by adding a concurrent reader task.
src/vstest.console/Processors/EnableBlameArgumentProcessor.cs	Replaced per-element `new[]` allocations in LINQ filters with static key sets.
src/Microsoft.TestPlatform.ObjectModel/Navigation/PortableSymbolReader.cs	Simplified nested dictionary lookups via `TryGetValue`.
src/Microsoft.TestPlatform.ObjectModel/Navigation/FullSymbolReader.cs	Eliminated redundant dictionary lookups using `TryGetValue`.
src/Microsoft.TestPlatform.ObjectModel/DataCollector/InProcDataCollector/TestSessionStartArgs.cs	Replaced double-lookup dictionary access with `TryGetValue`.
src/Microsoft.TestPlatform.ObjectModel/DataCollector/Events/SessionEvents.cs	Replaced double-lookup dictionary access with `TryGetValue`.
src/Microsoft.TestPlatform.ObjectModel/ConnectionInfo/TestRunnerConnectionInfoExtensions.cs	Reduced string concatenation operations by consolidating into one interpolated assignment.
src/Microsoft.TestPlatform.CrossPlatEngine/TestSession/TestSessionPool.cs	Reduced redundant dictionary lookups in session pool access.
src/Microsoft.TestPlatform.CrossPlatEngine/TestSession/ProxyTestSessionManager.cs	Reduced redundant dictionary lookups when resolving proxy indices.
src/Microsoft.TestPlatform.CrossPlatEngine/DataCollection/ParallelDataCollectionEventsHandler.cs	Adapted to `HashSet` by materializing to `Collection<T>` for downstream APIs.
src/Microsoft.TestPlatform.CrossPlatEngine/DataCollection/InProcDataCollectionSink.cs	Reduced redundant dictionary lookups and simplified update flow.
src/Microsoft.TestPlatform.CrossPlatEngine/Client/Parallel/ParallelRunEventsHandler.cs	Adapted to `HashSet` by materializing to `Collection<T>` for downstream APIs.
src/Microsoft.TestPlatform.CrossPlatEngine/Client/Parallel/ParallelRunDataAggregator.cs	Switched invoked collectors to `HashSet`, added locking in stats aggregation, and reduced redundant lookups.
src/Microsoft.TestPlatform.Common/Utilities/SimpleJSON.cs	Removed redundant dictionary lookups and simplified add/update/remove logic.

Jakub Jareš (nohwnd) · 2026-03-20T17:17:56Z

brain is not braining anymore, will have a look on monday, sorry!

Jakub Jareš (nohwnd) · 2026-03-23T14:58:02Z

        var testOutcomeMap = new Dictionary<TestOutcome, long>();
        long totalTests = 0;
-        if (_testRunStatsList.Count > 0)
+        lock (_dataUpdateSyncObject)


FYI: This was reverted before, this is called once at the end, not in parallel. Same with part of the test reverted before on the bottom.

Won't hurt most likely just sets unrealistic expectations from the api, that will confuse us if we end up replacing the method with other code.

Jakub Jareš (nohwnd) · 2026-03-23T14:58:30Z

+        // Also start a reader thread that calls GetAggregatedRunStats concurrently
+        var readerTask = Task.Run(() =>
+        {
+            barrier.SignalAndWait();
+            for (int i = 0; i < iterationsPerThread; i++)
+            {
+                // This must not throw InvalidOperationException due to collection modification
+                var runStats = aggregator.GetAggregatedRunStats();
+                Assert.IsTrue(runStats.ExecutedTests >= 0, "Executed tests count should be non-negative");
+            }
+        });
+
+        Task.WaitAll(aggregateTasks.Append(readerTask).ToArray());


This is the part I talk about above. Also the barrier.

Jakub Jareš (nohwnd) · 2026-03-24T07:14:54Z

looking

Address review comments from PR microsoft#15533: - Revert HashSet<InvokedDataCollector> back to Collection with Contains, as this code runs once at end, not in parallel (nohwnd feedback) - Add clarifying comment to Aggregate method about non-parallel usage - Revert barrier-based parallel test back to simpler sequential version (nohwnd feedback - was reverted before) - Revert test assertions to index-based access (order is deterministic with Collection) - Change BlameParameterNames from string[] to HashSet<string> with OrdinalIgnoreCase comparer for O(1) Contains lookups (Copilot/Youssef) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Address review comments from PR #15533: - Revert HashSet<InvokedDataCollector> back to Collection with Contains, as this code runs once at end, not in parallel (nohwnd feedback) - Add clarifying comment to Aggregate method about non-parallel usage - Revert barrier-based parallel test back to simpler sequential version (nohwnd feedback - was reverted before) - Revert test assertions to index-based access (order is deterministic with Collection) - Change BlameParameterNames from string[] to HashSet<string> with OrdinalIgnoreCase comparer for O(1) Contains lookups (Copilot/Youssef) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 4 comments.

Copilot · 2026-03-24T07:55:57Z

        IDiaEnumSymbols? enumSymbols = null;
        IDiaSymbol? methodSymbol;
-        Dictionary<string, IDiaSymbol> methodSymbolsForType;
+        Dictionary<string, IDiaSymbol>? methodSymbolsForType;


Making methodSymbolsForType nullable here reduces clarity: it is expected to be non-null in the TryGetValue(...) true-branch, and (based on the structure) likely initialized in the else branch later. To keep nullability accurate and avoid downstream nullability noise, consider using a non-null local inside the if (e.g., out var methodSymbolsForType) and/or ensuring the variable is always assigned a non-null dictionary before any later use.

Suggested change

Dictionary<string, IDiaSymbol>? methodSymbolsForType;

Dictionary<string, IDiaSymbol> methodSymbolsForType;

Copilot · 2026-03-24T07:55:57Z

+            if (_methodSymbols.TryGetValue(symbolName, out methodSymbolsForType))
            {
-                methodSymbolsForType = _methodSymbols[symbolName];
-                if (methodSymbolsForType.ContainsKey(methodName))
+                if (methodSymbolsForType.TryGetValue(methodName, out var cachedMethodSymbol))
                {
-                    return methodSymbolsForType[methodName];
+                    return cachedMethodSymbol;
                }
            }


Making methodSymbolsForType nullable here reduces clarity: it is expected to be non-null in the TryGetValue(...) true-branch, and (based on the structure) likely initialized in the else branch later. To keep nullability accurate and avoid downstream nullability noise, consider using a non-null local inside the if (e.g., out var methodSymbolsForType) and/or ensuring the variable is always assigned a non-null dictionary before any later use.

Copilot · 2026-03-24T07:55:57Z

+    // Note: This method is called once at the end of the test run to aggregate results.
+    // It is NOT called in parallel, so thread-safety optimizations here would be misleading.


The new note contradicts the XML doc immediately above it: the doc states this must be thread-safe and expected to be called by parallel managers, while the note says it is not called in parallel. Please reconcile these (either update the XML doc to match actual usage, or remove/adjust the note) so future changes don’t rely on conflicting guidance.

Suggested change

// Note: This method is called once at the end of the test run to aggregate results.

// It is NOT called in parallel, so thread-safety optimizations here would be misleading.

note belongs to GetAggregatedRunStats

Copilot · 2026-03-24T07:55:58Z

+            if (CollectionData.ContainsKey(key))
            {
                EqtTrace.Warning("The data for in-proc data collector with key {0} has already been set. Will be reset with new value", key);
-                CollectionData[key] = value;
            }
+            CollectionData[key] = value;


This path still does a double-lookup (ContainsKey then indexer assignment). Since this PR targets dictionary lookup reductions, consider switching to TryAdd (warn+overwrite if it returns false) or TryGetValue to keep the warning behavior while avoiding an extra hash lookup on the common add path.

…locations - Replace ContainsKey + indexer with TryGetValue across 10+ locations (ParallelRunDataAggregator, TestSessionPool, ProxyTestSessionManager, InProcDataCollectionSink, PortableSymbolReader, FullSymbolReader, TestSessionStartArgs, SessionEvents, SimpleJSON) - Change Collection<InvokedDataCollector> to HashSet<InvokedDataCollector> in ParallelRunDataAggregator for O(1) dedup instead of O(n) - Extract static HashSets for blame parameter names instead of allocating new[] per LINQ predicate evaluation - Consolidate string += into string interpolation in TestRunnerConnectionInfoExtensions

Address review comments from PR #15533: - Revert HashSet<InvokedDataCollector> back to Collection with Contains, as this code runs once at end, not in parallel (nohwnd feedback) - Add clarifying comment to Aggregate method about non-parallel usage - Revert barrier-based parallel test back to simpler sequential version (nohwnd feedback - was reverted before) - Revert test assertions to index-based access (order is deterministic with Collection) - Change BlameParameterNames from string[] to HashSet<string> with OrdinalIgnoreCase comparer for O(1) Contains lookups (Copilot/Youssef) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- Remove lock and non-parallel note from GetAggregatedRunStats (called once at end, not in parallel - lock was reverted before) - Keep TryGetValue and kvp iteration optimizations - Restore non-nullable Dictionary<string, IDiaSymbol> in FullSymbolReader Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Copilot · 2026-03-25T15:47:09Z

            _runDataAggregator.GetAggregatedException(),
            _runDataAggregator.RunContextAttachments,
-            _runDataAggregator.InvokedDataCollectors,
+            new Collection<InvokedDataCollector>(_runDataAggregator.InvokedDataCollectors.ToList()),


This materializes a new List<T> via ToList() and then wraps it in a new Collection<T>, adding allocations on the completion path. If the downstream API can be adjusted, prefer accepting IReadOnlyCollection<T> / IEnumerable<T> to avoid forcing a copy. If the signature cannot change, consider passing a one-time snapshot that is created earlier (or reusing a cached snapshot) so this call site doesn't allocate on every completion.

Suggested change

new Collection<InvokedDataCollector>(_runDataAggregator.InvokedDataCollectors.ToList()),

_runDataAggregator.InvokedDataCollectors.ToList(),

Jakub Jareš (nohwnd) · 2026-03-26T16:29:01Z

blocked on flakiness from main

This PR contains the following updates: | Package | Change | [Age](https://docs.renovatebot.com/merge-confidence/) | [Confidence](https://docs.renovatebot.com/merge-confidence/) | |---|---|---|---| | [Microsoft.NET.Test.Sdk](https://github.com/microsoft/vstest) | `18.5.1` → `18.6.0` | ![age](https://developer.mend.io/api/mc/badges/age/nuget/Microsoft.NET.Test.Sdk/18.6.0?slim=true) | ![confidence](https://developer.mend.io/api/mc/badges/confidence/nuget/Microsoft.NET.Test.Sdk/18.5.1/18.6.0?slim=true) | --- ### Release Notes <details> <summary>microsoft/vstest (Microsoft.NET.Test.Sdk)</summary> ### [`v18.6.0`](https://github.com/microsoft/vstest/releases/tag/v18.6.0) #### What's Changed - Revert removal of Video Recorder by [@nohwnd](https://github.com/nohwnd) in [#15336](microsoft/vstest#15336) - Speed up blame by filtering non-.NET processes from dump collection by [@nohwnd](https://github.com/nohwnd) in [#15518](microsoft/vstest#15518) - Add README.md to NuGet packages by [@nohwnd](https://github.com/nohwnd) in [#15550](microsoft/vstest#15550) - Report child process info on connection timeout by [@nohwnd](https://github.com/nohwnd) in [#15603](microsoft/vstest#15603) ##### Changes to tests and infra - Brand as 18.6 by [@nohwnd](https://github.com/nohwnd) in [#15423](microsoft/vstest#15423) - Upgrading code coverage version to 18.5.1, by [@fhnaseer](https://github.com/fhnaseer) in [#15422](microsoft/vstest#15422) - Updating System.Collections.Immutable to 9.0.11 by [@MSLukeWest](https://github.com/MSLukeWest) in [#15425](microsoft/vstest#15425) - Fix attachVS when used for debugging integration tests by [@nohwnd](https://github.com/nohwnd) in [#15451](microsoft/vstest#15451) - Replace dotnet.config, with global.json by [@nohwnd](https://github.com/nohwnd) in [#15449](microsoft/vstest#15449) - Document debugging integration tests with AttachVS by [@Copilot](https://github.com/Copilot) in [#15452](microsoft/vstest#15452) - Fix stack overflow tests by [@nohwnd](https://github.com/nohwnd) in [#15461](microsoft/vstest#15461) - Make TestAssets.sln buildable locally by [@Youssef1313](https://github.com/Youssef1313) in [#15466](microsoft/vstest#15466) - 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Copilot AI review requested due to automatic review settings March 20, 2026 10:11

Amaury Levé (Evangelink) enabled auto-merge (squash) March 20, 2026 10:12

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Jakub Jareš (nohwnd) mentioned this pull request Mar 24, 2026

Apply review feedback to perf dictionary double-lookups PR #15563

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Copilot AI review requested due to automatic review settings March 24, 2026 07:49

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Amaury Levé (Evangelink) and others added 5 commits March 25, 2026 16:35

Address review comments

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Move non-parallel note to GetAggregatedRunStats

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Copilot AI review requested due to automatic review settings March 25, 2026 15:40

Jakub Jareš (nohwnd) force-pushed the dev/amauryleve/perf-fix-dictionary-double-lookups branch from bc0b5f6 to 038b6c5 Compare March 25, 2026 15:40

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Jakub Jareš (nohwnd) approved these changes Mar 26, 2026

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Jakub Jareš (nohwnd) disabled auto-merge March 26, 2026 16:29

Jakub Jareš (nohwnd) merged commit f427180 into main Mar 26, 2026
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Jakub Jareš (nohwnd) deleted the dev/amauryleve/perf-fix-dictionary-double-lookups branch March 26, 2026 16:29

This was referenced Mar 27, 2026

[main] Source code updates from microsoft/vstest dotnet/dotnet#5511

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[release/11.0.1xx-preview3] Source code updates from microsoft/vstest dotnet/dotnet#5743

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dotnet-maestro Bot mentioned this pull request Apr 27, 2026

[release/10.0.4xx] Source code updates from microsoft/vstest dotnet/dotnet#6290

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	Dictionary<string, IDiaSymbol>? methodSymbolsForType;
	Dictionary<string, IDiaSymbol> methodSymbolsForType;

		// Note: This method is called once at the end of the test run to aggregate results.
		// It is NOT called in parallel, so thread-safety optimizations here would be misleading.

	new Collection<InvokedDataCollector>(_runDataAggregator.InvokedDataCollectors.ToList()),
	_runDataAggregator.InvokedDataCollectors.ToList(),

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Amaury Levé (Evangelink) commented Mar 20, 2026

Summary

Changes

1. Dictionary ContainsKey + indexer → TryGetValue

2. Collection<InvokedDataCollector> → HashSet<InvokedDataCollector>

3. new[] allocation inside LINQ predicate → static HashSet

4. String += → string interpolation

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Jakub Jareš (nohwnd) commented Mar 20, 2026

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Jakub Jareš (nohwnd) commented Mar 26, 2026

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1. Dictionary `ContainsKey` + indexer → `TryGetValue`

2. `Collection<InvokedDataCollector>` → `HashSet<InvokedDataCollector>`

3. `new[]` allocation inside LINQ predicate → static `HashSet`

4. String `+=` → string interpolation