[perf-improver] perf: skip intermediate Dictionary alloc in CloneForDataDrivenIteration

[github-actions]

Goal and Rationale

CloneForDataDrivenIteration in TestContextImplementation previously created an intermediate Dictionary<string, object?> copy before passing it to the constructor:

var snapshot = new Dictionary<string, object?>(_properties);
var clone = new TestContextImplementation(testMethod: null, testClassFullName: null, snapshot, ...);

The constructor's null/null branch (testMethod == null && testClassFullName == null) immediately copies snapshot into another new Dictionary via the [with(properties)] collection-expression spread:

_properties = [with(properties)]; // creates yet another Dictionary

This meant 2 Dictionary allocations + 2× O(n) copies per data-driven test iteration, but the snapshot intermediate was redundant — the constructor copy alone is sufficient for isolation.

Approach

Pass _properties directly to the constructor:

TestContextImplementation clone = new(testMethod: null, testClassFullName: null, _properties, _messageLogger, _testRunCancellationToken);

The constructor still copies _properties into its own fresh Dictionary (via [with(properties)]), so:

• Clone mutations do not leak back to the original
• Original mutations after clone creation do not leak to the clone
• All existing isolation invariants are preserved

Performance Evidence

Saved allocations per data-driven test iteration (common case):

• Before: 1× intermediate Dictionary<string, object?> + 1× constructor Dictionary = 2 allocs + 2× O(n) copy
• After: 1× constructor Dictionary = 1 alloc + 1× O(n) copy

For a suite with 100 data-driven tests × 10 rows each = 1000 iterations → 1000 fewer Dictionary allocations (typically 200–500 bytes each on the heap).

Methodology: direct inspection of the allocation path; the intermediate snapshot variable is not used anywhere other than the constructor call it was immediately passed to.

Trade-offs

None. This is a straightforward removal of a redundant copy. Correctness is guaranteed by the constructor's own copy, which is unchanged.

Reproducibility

# Inspect the before/after with:
dotnet run --project test/UnitTests/MSTestAdapter.PlatformServices.UnitTests \
 -f net9.0 --no-build -- --treenode-filter "*/*/TestContextImplementationTests/*"

Test Status

Unit tests for CloneForDataDrivenIteration in TestContextImplementationTests.cs cover isolation, shallow copy, and property-bag independence — all pass unchanged. CI will verify the full build.

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Show patch preview (51 of 51 lines)

From 6203d8903a79596a057b7f9750f46da0374f94b9 Mon Sep 17 00:00:00 2001
From: "github-actions[bot]" <github-actions[bot]@users.noreply.github.com>
Date: Sun, 5 Jul 2026 14:11:29 +0000
Subject: [PATCH] perf: skip intermediate Dictionary alloc in
 CloneForDataDrivenIteration

The constructor's null/null branch already copies the supplied properties
dictionary into a fresh Dictionary<string, object?> via the
[with(properties)] collection-expression spread. Passing _properties
directly to the constructor instead of wrapping it in a
"new Dictionary<string, object?>(_properties)" first avoids one
heap allocation and one O(n) copy per data-driven test iteration.

Isolation is preserved: the constructor copy means mutations on the
clone do not leak back to the original context, and mutations on the
original after clone creation do not leak to the clone.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 .../Services/TestContextImplementation.cs | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/src/Adapter/MSTestAdapter.PlatformServices/Services/TestContextImplementation.cs b/src/Adapter/MSTestAdapter.PlatformServices/Services/TestContextImplementation.cs
index 5efc98a..e0805e5 100644
--- a/src/Adapter/MSTestAdapter.PlatformServices/Services/TestContextImplementation.cs
+++ b/src/Adapter/MSTestAdapter.PlatformServices/Services/TestContextImplementation.cs
@@ -524,16 +524,12 @@ private SynchronizedStringBuilder GetTestContextMessagesStringBuilder()
 /// <returns>A fresh context suitable for one folded data-driven iteration.</returns>
 internal TestContextImplementation CloneForDataDrivenIteration()
 {
- // Take a shallow snapshot of the current property bag so that the clone starts with
- // the same properties (including TestNameLabel / FullyQualifiedTestClassNameLabel and
- // anything merged from AssemblyInitialize / ClassInitialize) but is otherwise isolated.
+ // Pas
... (truncated)