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

[github-actions]

Goal and Rationale

CloneForDataDrivenIteration is called once per data-driven test row (for both DataRow-folded and ITestDataSource-folded paths). It was allocating two Dictionary objects per iteration:

1. An intermediate snapshot Dictionary copied from _properties
2. The constructor's own copy of the incoming dictionary (via [with(properties)])

The second copy is unavoidable — the clone's property bag must be independent. The first copy is redundant.

Approach

The constructor's null/null branch (testMethod: null, testClassFullName: null) already does _properties = [with(properties)], which is a full shallow copy. Passing _properties directly instead of a pre-made snapshot eliminates the intermediate allocation while preserving identical observable behavior: the clone's property bag is fully isolated from the original.

Performance Evidence

Before: 2 Dictionary allocations per CloneForDataDrivenIteration call:

• new Dictionary<string, object?>(_properties) — the snapshot
• [with(properties)] inside the constructor — the actual clone

After: 1 Dictionary allocation per call:

• [with(properties)] inside the constructor — a direct copy of _properties

Estimated saving: 1 Dictionary (≈ 104 B on .NET 9 for a small property bag) + O(n) copy per data-driven test iteration. For a test with 100 data rows, this removes 100 Dictionary allocations per test run.

Trade-offs

None. The change is purely mechanical — same API, same isolation guarantees, same outcome.

Reproducibility

To verify the allocation reduction, run a data-driven test suite with a profiler (e.g. PerfView / dotnet-trace) and observe the Dictionary..ctor callcount on CloneForDataDrivenIteration's stack.

Test Status

All existing CloneForDataDrivenIteration* unit tests cover: shallow copy semantics, isolation from original, fresh output state, TestRunCount propagation. CI will verify build and test results.

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Show patch preview (56 of 56 lines)

From b704b9f6744c18e87aa68a756a517db88be233d2 Mon Sep 17 00:00:00 2001
From: "github-actions[bot]" <github-actions[bot]@users.noreply.github.com>
Date: Sat, 4 Jul 2026 14:08:29 +0000
Subject: [PATCH] perf: skip intermediate Dictionary alloc in
 CloneForDataDrivenIteration
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

The constructor's null/null branch already copies the incoming dictionary
via a collection-expression ([with(properties)]). The previous code first
created a snapshot Dictionary and then passed it to the ctor, which copied
it again — two Dictionary allocations per data-driven test iteration.

Pass _properties directly to the ctor so it copies once, saving one
Dictionary + O(n) copy per data-driven test iteration.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 .../Services/TestContextImplementation.cs | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/src/Adapter/MSTestAdapter.PlatformServices/Services/TestContextImplementation.cs b/src/Adapter/MSTestAdapter.PlatformServices/Services/TestContextImplementation.cs
index 5efc98a..f2d0a4e 100644
--- a/src/Adapter/MSTestAdapter.PlatformServices/Services/TestContextImplementation.cs
+++ b/src/Adapter/MSTestAdapter.PlatformServices/Services/TestContextImplementation.cs
@@ -524,16 +524,15 @@ 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.
- // Per-iteration mutations to the clone's property bag won't leak back to this ins
... (truncated)