[Efficiency Improver] perf: eliminate yield-iterator state machine in PropertyBag.OfType(T)()

[Evangelink]

Replace the yield-based iterator from Property.OfType() with a direct linked-list walk inside PropertyBag.OfType().

Before: _property.OfType() (yield return) allocates one state- machine heap object per call, even when no matching property is found.

After: direct while-loop walk; no state-machine allocated; returns [] for the common case (no matching property) with zero heap allocations beyond the empty array constant.

Fixes #7914

[Copilot]

Pull request overview

This PR optimizes a hot-path in Microsoft.Testing.Platform by removing the yield return-based iterator allocation from PropertyBag.OfType<TProperty>() and replacing it with a direct linked-list traversal.

Changes:

• Replace [.. _property.OfType<TProperty>()] (yield iterator) with a manual while loop over the Property linked list.
• Preserve existing fast-path behavior for TestNodeStateProperty lookups and the empty/null property-list cases.

Show a summary per file

File	Description
src/Platform/Microsoft.Testing.Platform/Messages/PropertyBag.cs	Reworks OfType<TProperty>() to avoid iterator state-machine allocations by directly walking the linked list.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0

[github-actions]

Summary

Workflow: Test Expert Reviewer 🧪
Date: 2026-04-29
Repository: microsoft/testfx

Key Findings

The PR replaces the yield-based OfType<TProperty>() iterator with a direct linked-list walk, introducing three distinct return paths:

1. _testNodeStateProperty is TProperty → [testNodeStateProperty] (unchanged)
2. New combined early-return: typeof(TestNodeStateProperty).IsAssignableFrom(typeof(TProperty)) || _property is null → []
3. New while-loop: !foundAny → [], single match → [first!], multiple → [.. overflow]

Paths 2 and 3 (both new) have gaps in PropertyBagTests:

Gap	Missing path	Risk
_property is null early-return	OfType<DummyProperty>() on a bag with only TestNodeStateProperty	Silent regression if early-return is dropped
Single-item fast path	OfType<DummyProperty>() with exactly 1 match	[first!] returns wrong result/throws undetected
No-match path in walk	OfType<DummyProperty2>() on a non-empty bag	Manual loop doesn't cover LINQ's implicitly empty-on-no-match

The production logic itself looks correct — the overflow initialization overflow ??= [first!] correctly seeds the list with the first match before adding the second. No functional bugs detected.

Recommendations

1. Add an assertion Assert.IsEmpty(property.OfType<DummyProperty>()) for a bag containing only a TestNodeStateProperty.
2. Add OfType<DummyProperty>() with exactly 1 DummyProperty in the bag.
3. Add Assert.IsEmpty(property.OfType<DummyProperty2>()) to the existing OfType_Should_Return_CorrectObject test (one line, costs nothing).

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Generated by Test Expert Reviewer

🧪 Test quality reviewed by Test Expert Reviewer 🧪

[github-actions]

[Coverage] The single-item fast path ([first!] when overflow is null) is not exercised by any existing test. OfType_Should_Return_CorrectObject only tests with 2 DummyProperty instances, which goes through the overflow branch. The [first!] path needs its own case.

Impact: A regression where a single-result bag returns [] or throws would not be caught.

Suggestion: Extend OfType_Should_Return_CorrectObject or add a dedicated test:

PropertyBag bag = new();
DummyProperty single = new();
bag.Add(single);
bag.Add(PassedTestNodeStateProperty.CachedInstance);
TProperty[] result = bag.OfType<DummyProperty>();
Assert.HasCount(1, result);
Assert.AreSame(single, result[0]);

[github-actions]

[Coverage] The || _property is null branch is new (the original ternary had _property is null ? [] as its own arm). There's currently no test for calling OfType<SomeDummyProperty>() on a PropertyBag that contains only a TestNodeStateProperty (i.e., _testNodeStateProperty is set but _property is null).

Impact: The early-return path added here goes unverified. If a future refactor accidentally removes it, a silent linear walk over a null reference would produce incorrect results.

Suggestion: Add a test case to PropertyBagTests:

[TestMethod]
public void OfType_WithOnlyTestNodeStateProperty_ReturnsEmpty()
{
    PropertyBag property = new();
    property.Add(PassedTestNodeStateProperty.CachedInstance);
    // _property is null; _testNodeStateProperty is set — exercises the new early-return path
    Assert.IsEmpty(property.OfType<DummyProperty>());
}

[github-actions]

[Coverage] The !foundAny ? [] return at the end of the while-loop (no matching property found in a non-empty bag) is not tested. DummyProperty2 is used in Single / SingleOrDefault tests, but OfType<DummyProperty2>() is never called on a bag that has other properties.

Impact: The new manual traversal diverges from the old LINQ-based _property.OfType<TProperty>(). A bug that incorrectly returns null or throws instead of [] when no match is found would go undetected.

Suggestion: Add one assertion to the existing OfType_Should_Return_CorrectObject test:

// No DummyProperty2 in the bag — exercises the "no match found" path in the while-loop
Assert.IsEmpty(property.OfType<DummyProperty2>());

[Evangelink]

@copilot address review comments

[Copilot]

@copilot address review comments

Added the three missing test cases in commits 911b697 and 813c705:

• OfType_Should_Return_CorrectObject — extended with Assert.IsEmpty(property.OfType<DummyProperty2>()) to cover the "no match found" path
• OfType_WithSingleMatch_ReturnsSingleItemArray — new test covering the single-item fast path ([first!] when overflow is null)
• OfType_WithOnlyTestNodeStateProperty_ReturnsEmpty — new test covering the _property is null early return when only a TestNodeStateProperty is present