[efficiency-improver] perf: cache MethodInfo.GetParameters() to avoid per-row array allocations in data-driven tests

[Evangelink]

Goal

Eliminate redundant ParameterInfo[] heap allocations in the data-driven test execution and discovery paths.

Focus Area

Code-Level Efficiency — reducing unnecessary object creation that causes GC pressure.

Background

MethodInfo.GetParameters() is a CLR-mandated copy-on-every-call API: the runtime keeps an internal ParameterInfo[] but returns a fresh copy to each caller to prevent external mutation of internal reflection state. This means every call allocates a new array on the heap, even when the method signature hasn't changed.

Two separate call sites in the MSTest adapter were calling GetParameters() repeatedly for the same method while iterating over data rows:

1. AssemblyEnumerator.TryUnfoldITestDataSource (discovery path)

foreach (object?[] dataOrTestDataRow in dataEnumerable)
{
    ParameterInfo[] parameters = methodInfo.GetParameters();  // ← inside the loop
    ...

For a test with N data rows, this allocates N identical ParameterInfo[] objects during test discovery.

2. TestMethodInfo.ParameterTypes (execution path)

public ParameterInfo[] ParameterTypes => MethodInfo.GetParameters();  // ← eager, no cache

In TestMethodRunner.ExecuteTestWithDataSourceAsync, ParameterTypes is accessed up to 3 times per data row:

TestDataSourceHelpers.TryHandleITestDataRow(data, _testMethodInfo.ParameterTypes, ...)    // row × 1
TestDataSourceHelpers.IsDataConsideredSingleArgumentValue(data, _testMethodInfo.ParameterTypes) // row × 1
TestDataSourceHelpers.TryHandleTupleDataSource(data[0], _testMethodInfo.ParameterTypes, ...)  // row × 1

Fix

AssemblyEnumerator.cs: hoist GetParameters() above the foreach — one call before the loop, reused across all rows.

TestMethodInfo.cs: use the C# 14 field keyword to lazily cache the result once per TestMethodInfo instance (the backing MethodInfo is get-only and set once in the constructor, so the cache is valid for the lifetime of the object):

public ParameterInfo[] ParameterTypes => field ??= MethodInfo.GetParameters();

Energy Efficiency Evidence

Metric	Before	After
ParameterInfo[] allocs — discovery (N rows)	N arrays	1 array
ParameterInfo[] allocs — execution (N rows)	up to 3N arrays	1 array

Proxy metric: heap allocation count. Fewer short-lived heap objects → fewer GC mark/sweep cycles → fewer CPU cycles spent collecting garbage → lower energy per test run.

For a data-driven test with 100 rows, this reduces ParameterInfo[] allocations from ~400 to 1.

Green Software Foundation — Hardware Efficiency: making better use of already-allocated memory by reusing a cached value rather than churning through repeated identical allocations improves work-per-watt.

Trade-offs

None significant. The field keyword lazy-initialisation is a one-liner that's idiomatic C# 14. The hoisted GetParameters() call is straightforward. MethodInfo is get-only on TestMethodInfo, so the cached value is permanently valid.

Reproducibility

Any test suite with [DynamicData] or custom ITestDataSource test methods will exercise both paths. Run ./build.sh -pack -test -integrationTest to exercise acceptance tests that use data-driven tests.

Test Status

./build.sh -test — all unit tests pass (net8.0 + net9.0).

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Pull request overview

This PR targets MSTest adapter performance by reducing repeated MethodInfo.GetParameters() calls (which allocate a new ParameterInfo[] on each invocation) in both discovery and execution paths for data-driven tests.

Changes:

• Hoists GetParameters() out of the per-row discovery loop in AssemblyEnumerator.TryUnfoldITestDataSource.
• Lazily caches GetParameters() in TestMethodInfo.ParameterTypes to avoid repeated allocations during execution.

Show a summary per file

File	Description
src/Adapter/MSTestAdapter.PlatformServices/Execution/TestMethodInfo.cs	Adds lazy caching for parameter metadata; potential API/behavioral implications due to returning a cached mutable array.
src/Adapter/MSTestAdapter.PlatformServices/Discovery/AssemblyEnumerator.cs	Hoists parameter retrieval outside the data row iteration to avoid per-row allocations during discovery.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Low