From 811dab0d57c615b98bf18a1a789740059511ff4c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Amaury=20Lev=C3=A9?= <amauryleve@microsoft.com>
Date: Sun, 28 Jun 2026 00:17:46 +0200
Subject: [PATCH] perf: eliminate LINQ iterator allocations in
 SourceGeneratedReflectionOperations attribute helpers
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Replace OfType<T>().Any(), OfType<T>().FirstOrDefault(), and
OfType<T>().Take(2) LINQ chains in IsAttributeDefined, GetFirstAttributeOrDefault,
and GetSingleAttributeOrDefault with direct foreach loops over the cached
Attribute[] array.

Each LINQ operator (OfType, Any/FirstOrDefault/Take) allocates a heap-based
iterator state machine. With direct foreach, the array's struct enumerator is
used — zero heap allocations.

These methods are called per test method and per test class during attribute
discovery (e.g. GetSingleAttributeOrDefault<TestMethodAttribute> in
TestMethodInfo, IsAttributeDefined<DoNotParallelizeAttribute> in TypeEnumerator).
In a suite with source-generated reflection enabled, eliminating these allocations
reduces Gen-0 GC pressure across the entire test run.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 .../SourceGeneratedReflectionOperations.cs    | 48 +++++++++++++++----
 1 file changed, 38 insertions(+), 10 deletions(-)

diff --git a/src/Adapter/MSTestAdapter.PlatformServices/SourceGeneration/SourceGeneratedReflectionOperations.cs b/src/Adapter/MSTestAdapter.PlatformServices/SourceGeneration/SourceGeneratedReflectionOperations.cs
index a925434152..cf6ed80411 100644
--- a/src/Adapter/MSTestAdapter.PlatformServices/SourceGeneration/SourceGeneratedReflectionOperations.cs
+++ b/src/Adapter/MSTestAdapter.PlatformServices/SourceGeneration/SourceGeneratedReflectionOperations.cs
@@ -287,24 +287,52 @@ private static bool TryInvokeMatchingConstructor(
 
     public bool IsAttributeDefined<TAttribute>(ICustomAttributeProvider attributeProvider)
         where TAttribute : Attribute
-        => attributeProvider is null
-            ? throw new ArgumentNullException(nameof(attributeProvider))
-            : GetCustomAttributesCached(attributeProvider).OfType<TAttribute>().Any();
+    {
+        foreach (Attribute attr in GetCustomAttributesCached(attributeProvider))
+        {
+            if (attr is TAttribute)
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
 
     public TAttribute? GetFirstAttributeOrDefault<TAttribute>(ICustomAttributeProvider attributeProvider)
         where TAttribute : Attribute
-        => GetCustomAttributesCached(attributeProvider).OfType<TAttribute>().FirstOrDefault();
+    {
+        foreach (Attribute attr in GetCustomAttributesCached(attributeProvider))
+        {
+            if (attr is TAttribute match)
+            {
+                return match;
+            }
+        }
+
+        return null;
+    }
 
     public TAttribute? GetSingleAttributeOrDefault<TAttribute>(ICustomAttributeProvider attributeProvider)
         where TAttribute : Attribute
     {
-        TAttribute[] matches = [.. GetCustomAttributesCached(attributeProvider).OfType<TAttribute>().Take(2)];
-        return matches.Length switch
+        TAttribute? first = null;
+        foreach (Attribute attr in GetCustomAttributesCached(attributeProvider))
         {
-            0 => null,
-            1 => matches[0],
-            _ => throw new InvalidOperationException(string.Format(CultureInfo.InvariantCulture, "Found multiple attributes of type '{0}' when only one was expected.", typeof(TAttribute))),
-        };
+            if (attr is not TAttribute match)
+            {
+                continue;
+            }
+
+            if (first is not null)
+            {
+                throw new InvalidOperationException(string.Format(CultureInfo.InvariantCulture, "Found multiple attributes of type '{0}' when only one was expected.", typeof(TAttribute)));
+            }
+
+            first = match;
+        }
+
+        return first;
     }
 
     public IEnumerable<TAttributeType> GetAttributes<TAttributeType>(ICustomAttributeProvider attributeProvider)