From 57f4d2857ed9b718e03896b7fa335fb10115e27b Mon Sep 17 00:00:00 2001
From: Tom Longhurst <30480171+thomhurst@users.noreply.github.com>
Date: Sat, 12 Sep 2026 10:13:04 +0100
Subject: [PATCH] perf: skip receiver registration for ordinary objects

---
 .../Services/EventReceiverOrchestrator.cs     |  9 +-
 .../EventReceiverRegistrationTests.cs         | 84 +++++++++++++++++++
 2 files changed, 92 insertions(+), 1 deletion(-)
 create mode 100644 tests/TUnit.UnitTests/EventReceiverRegistrationTests.cs

diff --git a/src/TUnit.Engine/Services/EventReceiverOrchestrator.cs b/src/TUnit.Engine/Services/EventReceiverOrchestrator.cs
index 47585b0acb..c31ae6eeae 100644
--- a/src/TUnit.Engine/Services/EventReceiverOrchestrator.cs
+++ b/src/TUnit.Engine/Services/EventReceiverOrchestrator.cs
@@ -49,6 +49,13 @@ public void RegisterReceivers(TestContext context)
 
         foreach (var obj in context.GetEligibleEventObjects())
         {
+            // Ordinary attributes, arguments and test instances cannot receive events.
+            // Avoid retaining them in the deduplication set or scanning every receiver interface.
+            if (obj is not IEventReceiver)
+            {
+                continue;
+            }
+
             // Use single TryAdd operation instead of Contains + Add
             if (!_initializedObjects.Add(obj))
             {
@@ -88,7 +95,7 @@ public void RegisterClassInstanceReceiver(TestContext context)
     {
         var classInstance = context.Metadata.TestDetails.ClassInstance;
         Debug.Assert(classInstance is not null, "RegisterClassInstanceReceiver should only be called after ClassInstance is assigned.");
-        if (classInstance is null)
+        if (classInstance is not IEventReceiver)
         {
             return;
         }
diff --git a/tests/TUnit.UnitTests/EventReceiverRegistrationTests.cs b/tests/TUnit.UnitTests/EventReceiverRegistrationTests.cs
new file mode 100644
index 0000000000..a7c3c81b39
--- /dev/null
+++ b/tests/TUnit.UnitTests/EventReceiverRegistrationTests.cs
@@ -0,0 +1,84 @@
+using TUnit.Core.Interfaces;
+using TUnit.Engine.Services;
+
+namespace TUnit.UnitTests;
+
+public class EventReceiverRegistrationTests
+{
+    [Test]
+    public void OrdinaryObjectsDoNotParticipateInReceiverDeduplication()
+    {
+        var context = CreateContext(new OrdinaryObject(), [new OrdinaryAttribute()]);
+        try
+        {
+            var orchestrator = new EventReceiverOrchestrator(null!);
+            orchestrator.RegisterReceivers(context);
+            orchestrator.RegisterClassInstanceReceiver(context);
+        }
+        finally
+        {
+            context.RemoveFromRegistry();
+            context.Dispose();
+        }
+    }
+
+    [Test]
+    public async Task AttributeAndClassReceiversStillReceiveEvents()
+    {
+        var attribute = new ReceiverAttribute();
+        var instance = new ReceiverAttribute();
+        var context = CreateContext(null!, [attribute]);
+        try
+        {
+            var orchestrator = new EventReceiverOrchestrator(null!);
+            orchestrator.RegisterReceivers(context);
+            context.Metadata.TestDetails.ClassInstance = instance;
+            orchestrator.RegisterClassInstanceReceiver(context);
+            orchestrator.RegisterClassInstanceReceiver(context);
+
+            await orchestrator.InvokeTestStartEventReceiversAsync(context, CancellationToken.None);
+            await Assert.That(attribute.Calls).IsEqualTo(1);
+            await Assert.That(instance.Calls).IsEqualTo(1);
+        }
+        finally
+        {
+            context.RemoveFromRegistry();
+            context.Dispose();
+        }
+    }
+
+    private static TestContext CreateContext(object instance, Attribute[] attributes)
+    {
+        var current = TestContext.Current!;
+        var context = new TestContext("Registration", current.ServiceProvider, current.ClassContext,
+            new TestBuilderContext { TestMetadata = current.TestDetails.MethodMetadata }, CancellationToken.None);
+        context.Metadata.TestDetails = new TestDetails(attributes)
+        {
+            TestId = context.Id, TestName = "Registration", ClassType = typeof(EventReceiverRegistrationTests),
+            MethodName = "Registration", ClassInstance = instance, TestMethodArguments = [], TestClassArguments = [],
+            MethodMetadata = current.TestDetails.MethodMetadata, ReturnType = typeof(void),
+            AttributesByType = new Dictionary<Type, IReadOnlyList<Attribute>>()
+        };
+        return context;
+    }
+
+    private sealed class OrdinaryObject
+    {
+        public override int GetHashCode() => throw new InvalidOperationException("Not an event receiver");
+    }
+
+    private sealed class OrdinaryAttribute : Attribute
+    {
+        public override int GetHashCode() => throw new InvalidOperationException("Not an event receiver");
+    }
+
+    private sealed class ReceiverAttribute : Attribute, ITestStartEventReceiver
+    {
+        public int Calls { get; private set; }
+        public ValueTask OnTestStart(TestContext context)
+        {
+            Calls++;
+            return ValueTask.CompletedTask;
+        }
+    }
+}