[perf-improver] perf: skip allocation in CaptureLifecycleProperties when no user properties set

[Evangelink]

Goal and rationale

CaptureLifecycleProperties() is called once per [AssemblyInitialize] / [ClassInitialize] execution to snapshot any TestContext.Properties entries set during init, so they can be propagated to each test's context. In the common case — where neither init method sets any properties — the method unconditionally allocated two objects:

1. new Dictionary<string, object?>(capacity) — the snapshot buffer
2. new ReadOnlyDictionary<string, object?>(snapshot) — the immutable wrapper

Those two objects are then stored in PostAssemblyInitProperties / PostClassInitProperties and passed to every test via MergeProperties. MergeProperties already returned early for null, but not for an empty dictionary, so it also acquired a lock per call before doing nothing.

Approach

• CaptureLifecycleProperties: change to lazy allocation. The Dictionary is allocated only on the first non-label entry. If the loop finds nothing to snapshot, the method returns null (return type changed from IReadOnlyDictionary<string,object?> to IReadOnlyDictionary<string,object?>?). This is a single-pass implementation — no extra iteration.
• MergeProperties: extend the null guard to null or { Count: 0 }, so an empty dict is also short-circuited cheaply (defensive, and matches the new null-return convention).
• No caller changes: PostAssemblyInitProperties and PostClassInitProperties are already declared as nullable; MergeProperties(null) already short-circuits.

Performance evidence

Every test class that has [AssemblyInitialize] or [ClassInitialize] (but doesn't write to TestContext.Properties in it) benefits:

Call site	Before	After
CaptureLifecycleProperties (empty)	2 heap allocs	0 allocs
MergeProperties(emptySnapshot) per test	lock acquire + empty loop	1 null-check, return
MergeProperties(null) per test	1 null-check, return	1 null/count-check, return

For a test suite with an [AssemblyInitialize] / [ClassInitialize] that sets no properties:

• −2 heap allocations at init time
• −2 lock acquisitions per test (one for PostAssemblyInitProperties, one for PostClassInitProperties merge in RunSingleTestAsync)

In a 1000-test suite, that's ~2000 lock acquisitions avoided.

Trade-offs

None significant. The change is backward-compatible: callers that store the snapshot in nullable fields already handle null; MergeProperties(null) was already a no-op.

Reproducibility

# Build and run unit tests for MSTestAdapter.PlatformServices
./build.sh -test -projects test/UnitTests/MSTestAdapter.PlatformServices.UnitTests/MSTestAdapter.PlatformServices.UnitTests.csproj

# Or for just the new tests, after build:
dotnet run --project test/UnitTests/MSTestAdapter.PlatformServices.UnitTests -f net8.0 --no-build -- \
  --treenode-filter "/*/*/*/TestContextImplementationTests/CaptureLifecyclePropertiesShouldReturnNullWhenNoNonLabelPropertiesExist"

Test status

No SDK is available in this CI environment, so local build/test could not be run. The change is confined to the MSTestAdapter.PlatformServices project and its unit tests; all callers already handle null snapshots.

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Pull request overview

Optimizes MSTestAdapter.PlatformServices’ TestContextImplementation lifecycle property flow by avoiding allocations and locking when no user properties are set during AssemblyInitialize / ClassInitialize.

Changes:

• Make CaptureLifecycleProperties() lazily allocate and return null when there are no non-label properties to snapshot.
• Make MergeProperties() return early for null or empty dictionaries to avoid unnecessary locking.
• Add/adjust unit tests to validate null snapshot behavior and empty-merge short-circuit behavior.

Show a summary per file

File	Description
src/Adapter/MSTestAdapter.PlatformServices/Services/TestContextImplementation.cs	Implements lazy snapshot allocation + nullable return, and adds empty-dictionary fast path in MergeProperties.
test/UnitTests/MSTestAdapter.PlatformServices.UnitTests/Services/TestContextImplementationTests.cs	Updates snapshot tests for nullable return and adds a test for merging an empty dictionary.
test/UnitTests/MSTestAdapter.PlatformServices.UnitTests/Execution/TestClassInfoTests.cs	Adds coverage that PostClassInitProperties stays null when class init sets no properties.
test/UnitTests/MSTestAdapter.PlatformServices.UnitTests/Execution/TestAssemblyInfoTests.cs	Adds coverage that PostAssemblyInitProperties stays null when assembly init sets no properties.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 1
• Review effort level: Low