[Perf Improver] perf: eliminate LINQ allocations in IsIgnored hot path

[Evangelink]

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Goal and Rationale

Reduce heap allocations in AttributeExtensions.IsIgnored, which is called twice per test execution — once for the test class (ClassType.IsIgnored) and once for the test method (MethodInfo.IsIgnored). This means the allocations scale linearly with test count.

Approach

Before:

IEnumerable<ConditionBaseAttribute> attributes = ReflectHelper.Instance.GetAttributes<ConditionBaseAttribute>(type);
IEnumerable<IGrouping<string, ConditionBaseAttribute>> groups = attributes.GroupBy(attr => attr.GroupName);
foreach (IGrouping<string, ConditionBaseAttribute>? group in groups)
{
    // ...
}

Every call allocates:

1. A yield iterator state machine from GetAttributes<ConditionBaseAttribute> (even when there are no ConditionBaseAttribute present)
2. A LINQ GroupBy operator object

After:

Attribute[] attributes = ReflectHelper.Instance.GetCustomAttributesCached(type);
Dictionary<string, (bool Satisfied, string? FirstIgnoreMessage)>? groups = null;

foreach (Attribute attr in attributes)
{
    if (attr is not ConditionBaseAttribute condAttr) { continue; }
    // ... manual grouping, groups allocated lazily only if ConditionBaseAttribute found
}

Direct iteration of the cached Attribute[] — no iterator, no LINQ operator. The groups dictionary is allocated only when a ConditionBaseAttribute is present (the uncommon case). For most tests (no [Ignore], no OSCondition, etc.), zero allocations.

Performance Evidence

Methodology: Allocation count per IsIgnored call.

Scenario	Before (objects)	After
No ConditionBaseAttribute (common)	1 yield iterator + 1 GroupBy operator	0
[Ignore] present on class/method	1 yield iterator + LINQ Lookup internals	1 Dictionary
Per test execution (class + method)	~4 iterator/LINQ objects	0 (common case)

For a test suite with 1,000 tests: eliminates ~4,000 short-lived LINQ objects per run in the typical case.

Trade-offs

• Manual grouping logic is slightly more verbose, but follows the established allocation-free pattern used by GetTestPropertiesAsTraits and GetTestCategories in the same codebase.
• The Dictionary allocated in the uncommon case ([Ignore] present) replaces a LINQ Lookup<> — comparable or better allocation profile.
• Semantics are identical: OR within group, AND across groups, first non-null ignore message reported.

Reproducibility

export PATH="$PWD/.dotnet:$PATH"
dotnet restore src/Adapter/MSTestAdapter.PlatformServices/MSTestAdapter.PlatformServices.csproj -p:TargetFramework=net8.0
dotnet build src/Adapter/MSTestAdapter.PlatformServices/MSTestAdapter.PlatformServices.csproj -f net8.0 -warnaserror

Test Status

✅ MSTestAdapter.PlatformServices builds with 0 warnings, 0 errors on net8.0.

Note: MSTestAdapter.PlatformServices.UnitTests has a pre-existing build failure in this environment due to AwesomeAssertions 9.3.0 only shipping netstandard2.1 (no net8.0 folder). This is unrelated to these changes. CI will run the full test suite.

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Note

This was originally intended as a pull request, but GitHub Actions is not permitted to create or approve pull requests in this repository.
The changes have been pushed to branch perf-assist/eliminate-linq-allocations-isignored.

Click here to create the pull request

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