perf(engine): address review feedback on per-test cleanups

- Add Debug.Assert guards on the three hot-path receiver getters
  (GetTestStartReceivers, GetTestEndReceivers, GetTestSkippedReceivers)
  to catch invariant violations in Debug/test builds without cost in Release.
- Unify dictionary access in ObjectGraphDiscoverer: both ShouldSkipType and
  GetFlattenedInitializerProperties now use GetOrAdd with a static factory.
- Document the RegisterReceivers -> EnsureEventReceiversCached ordering
  dependency that makes the early skip-path safe on ExecuteTestLifecycleAsync.

TUnit.Core/Discovery/ObjectGraphDiscoverer.cs

@@ -442,39 +442,33 @@ private static void TraverseInitializerProperties(
     /// source-gen registration exists in the type's inheritance chain.
     /// </summary>
     private static InitializerPropertyInfo[]? GetFlattenedInitializerProperties(Type type)
-    {
-        if (FlattenedInitializerPropertiesCache.TryGetValue(type, out var cached))
+        => FlattenedInitializerPropertiesCache.GetOrAdd(type, static t =>
         {
-            return cached;
-        }
+            List<InitializerPropertyInfo>? merged = null;
+            HashSet<string>? seen = null;
 
-        List<InitializerPropertyInfo>? merged = null;
-        HashSet<string>? seen = null;
-
-        for (var currentType = type; currentType != null && currentType != typeof(object); currentType = currentType.BaseType)
-        {
-            var registered = InitializerPropertyRegistry.GetProperties(currentType);
-            if (registered == null)
+            for (var currentType = t; currentType != null && currentType != typeof(object); currentType = currentType.BaseType)
             {
-                continue;
-            }
+                var registered = InitializerPropertyRegistry.GetProperties(currentType);
+                if (registered == null)
+                {
+                    continue;
+                }
 
-            merged ??= new List<InitializerPropertyInfo>(registered.Length);
-            seen ??= new HashSet<string>(StringComparer.Ordinal);
+                merged ??= new List<InitializerPropertyInfo>(registered.Length);
+                seen ??= new HashSet<string>(StringComparer.Ordinal);
 
-            foreach (var p in registered)
-            {
-                if (seen.Add(p.PropertyName))
+                foreach (var p in registered)
                 {
-                    merged.Add(p);
+                    if (seen.Add(p.PropertyName))
+                    {
+                        merged.Add(p);
+                    }
                 }
             }
-        }
 
-        var result = merged?.ToArray();
-        FlattenedInitializerPropertiesCache[type] = result;
-        return result;
-    }
+            return merged?.ToArray();
+        });
 
     /// <summary>
     /// Traverses the pre-flattened source-generated IAsyncInitializer properties.

TUnit.Engine/Extensions/TestContextExtensions.cs

@@ -1,4 +1,5 @@
-using TUnit.Core;
+using System.Diagnostics;
+using TUnit.Core;
 using TUnit.Core.Enums;
 using TUnit.Core.Interfaces;
 using TUnit.Engine.Utilities;
@@ -264,13 +265,15 @@ private static int CountNonNullValues(IDictionary<string, object?> props)
 #if NET
     public static ITestStartEventReceiver[] GetTestStartReceivers(this TestContext testContext, EventReceiverStage stage)
     {
+        Debug.Assert(testContext.EventReceiversBuilt, "EnsureEventReceiversCached must run before GetTestStartReceivers — caller is on the hot path and skips the guard.");
         return stage == EventReceiverStage.Early
             ? testContext.CachedTestStartReceiversEarly!
             : testContext.CachedTestStartReceiversLate!;
     }
 #else
     public static ITestStartEventReceiver[] GetTestStartReceivers(this TestContext testContext)
     {
+        Debug.Assert(testContext.EventReceiversBuilt, "EnsureEventReceiversCached must run before GetTestStartReceivers — caller is on the hot path and skips the guard.");
         return testContext.CachedTestStartReceivers!;
     }
 #endif
@@ -281,13 +284,15 @@ public static ITestStartEventReceiver[] GetTestStartReceivers(this TestContext t
 #if NET
     public static ITestEndEventReceiver[] GetTestEndReceivers(this TestContext testContext, EventReceiverStage stage)
     {
+        Debug.Assert(testContext.EventReceiversBuilt, "EnsureEventReceiversCached must run before GetTestEndReceivers — caller is on the hot path and skips the guard.");
         return stage == EventReceiverStage.Early
             ? testContext.CachedTestEndReceiversEarly!
             : testContext.CachedTestEndReceiversLate!;
     }
 #else
     public static ITestEndEventReceiver[] GetTestEndReceivers(this TestContext testContext)
     {
+        Debug.Assert(testContext.EventReceiversBuilt, "EnsureEventReceiversCached must run before GetTestEndReceivers — caller is on the hot path and skips the guard.");
         return testContext.CachedTestEndReceivers!;
     }
 #endif
@@ -297,6 +302,7 @@ public static ITestEndEventReceiver[] GetTestEndReceivers(this TestContext testC
     /// </summary>
     public static ITestSkippedEventReceiver[] GetTestSkippedReceivers(this TestContext testContext)
     {
+        Debug.Assert(testContext.EventReceiversBuilt, "EnsureEventReceiversCached must run before GetTestSkippedReceivers — caller is on the hot path and skips the guard.");
         return testContext.CachedTestSkippedReceivers!;
     }
 

TUnit.Engine/Services/TestExecution/TestCoordinator.cs

@@ -277,6 +277,15 @@ await _eventReceiverOrchestrator.InvokeLastTestInSessionEventReceiversAsync(
     /// Timeout is passed through to TestExecutor.ExecuteAsync, which applies it only to the test
     /// body — hooks and data source initialization run outside the timeout scope (fixes #4772).
     /// </summary>
+    /// <remarks>
+    /// Ordering invariant: <see cref="ExecuteTestInternalAsync"/> calls
+    /// <c>_eventReceiverOrchestrator.RegisterReceivers</c> (which invokes
+    /// <c>EnsureEventReceiversCached</c>) before entering this method. The hot-path
+    /// receiver getters used by <c>InvokeTest{Skipped,End}EventReceiversAsync</c> therefore
+    /// skip their own guards and rely on that upstream call to have populated the caches —
+    /// the skip-path branch below is safe because it cannot be reached without that
+    /// prerequisite having run.
+    /// </remarks>
     private async ValueTask ExecuteTestLifecycleAsync(AbstractExecutableTest test, CancellationToken cancellationToken)
     {
         // Check if this test should be skipped before creating the class instance.
@@ -287,6 +296,7 @@ private async ValueTask ExecuteTestLifecycleAsync(AbstractExecutableTest test, C
         {
             _stateManager.MarkSkipped(test, test.Context.SkipReason!);
 
+            // Receiver caches are populated upstream (see <remarks> on this method).
             await _eventReceiverOrchestrator.InvokeTestSkippedEventReceiversAsync(test.Context, cancellationToken).ConfigureAwait(false);
 
             await _eventReceiverOrchestrator.InvokeTestEndEventReceiversAsync(test.Context, cancellationToken).ConfigureAwait(false);