Fix test design flaw in IAsyncInitializer discovery test

TUnit.Core/Helpers/DataSourceHelpers.cs

@@ -2,6 +2,7 @@
 using System.Collections.Concurrent;
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
+using TUnit.Core.Interfaces;
 
 namespace TUnit.Core.Helpers;
 
@@ -177,8 +178,12 @@ public static T InvokeIfFunc<T>(object? value)
         // If it's a Func<TResult>, invoke it first
         var actualData = InvokeIfFunc(data);
 
-        // Initialize the object if it implements IAsyncInitializer
-        await ObjectInitializer.InitializeAsync(actualData);
+        // Only initialize during discovery if explicitly opted-in via IAsyncDiscoveryInitializer
+        // Regular IAsyncInitializer objects are initialized during test execution by ObjectLifecycleService
+        if (actualData is IAsyncDiscoveryInitializer)
+        {
+            await ObjectInitializer.InitializeAsync(actualData);
+        }
 
         return actualData;
     }
@@ -197,7 +202,11 @@ public static T InvokeIfFunc<T>(object? value)
         if (enumerator.MoveNext())
         {
             var value = enumerator.Current;
-            await ObjectInitializer.InitializeAsync(value);
+            // Only initialize during discovery if explicitly opted-in via IAsyncDiscoveryInitializer
+            if (value is IAsyncDiscoveryInitializer)
+            {
+                await ObjectInitializer.InitializeAsync(value);
+            }
             return value;
         }
 
@@ -224,14 +233,22 @@ public static T InvokeIfFunc<T>(object? value)
             if (enumerator.MoveNext())
             {
                 var value = enumerator.Current;
-                await ObjectInitializer.InitializeAsync(value);
+                // Only initialize during discovery if explicitly opted-in via IAsyncDiscoveryInitializer
+                if (value is IAsyncDiscoveryInitializer)
+                {
+                    await ObjectInitializer.InitializeAsync(value);
+                }
                 return value;
             }
             return null;
         }
 
-        // For non-enumerable types, just initialize and return
-        await ObjectInitializer.InitializeAsync(actualData);
+        // Only initialize during discovery if explicitly opted-in via IAsyncDiscoveryInitializer
+        // Regular IAsyncInitializer objects are initialized during test execution by ObjectLifecycleService
+        if (actualData is IAsyncDiscoveryInitializer)
+        {
+            await ObjectInitializer.InitializeAsync(actualData);
+        }
         return actualData;
     }
 
@@ -579,8 +596,12 @@ public static void RegisterTypeCreator<T>(Func<MethodMetadata, string, Task<T>>
             {
                 var value = args[0];
 
-                // Initialize the value if it implements IAsyncInitializer
-                await ObjectInitializer.InitializeAsync(value);
+                // Only initialize during discovery if explicitly opted-in via IAsyncDiscoveryInitializer
+                // Regular IAsyncInitializer objects are initialized during test execution by ObjectLifecycleService
+                if (value is IAsyncDiscoveryInitializer)
+                {
+                    await ObjectInitializer.InitializeAsync(value);
+                }
 
                 return value;
             }

TUnit.TestProject/Bugs/3992/InstanceMethodDataSourceWithAsyncInitializerTests.cs

@@ -0,0 +1,128 @@
+using System.Collections.Concurrent;
+using TUnit.Core.Interfaces;
+using TUnit.TestProject.Attributes;
+
+namespace TUnit.TestProject.Bugs._3992;
+
+/// <summary>
+/// Regression test for issue #3992: IAsyncInitializer should not run during test discovery
+/// when using InstanceMethodDataSource with ClassDataSource.
+///
+/// This test replicates the user's scenario where:
+/// 1. A ClassDataSource fixture implements IAsyncInitializer (e.g., starts Docker containers)
+/// 2. An InstanceMethodDataSource returns predefined test case identifiers
+/// 3. The fixture should NOT be initialized during discovery - only during execution
+///
+/// The key insight is that test case IDENTIFIERS are known ahead of time (predefined),
+/// but the actual fixture initialization (Docker containers, DB connections, etc.)
+/// should only happen when tests actually execute.
+///
+/// The bug caused Docker containers to start during test discovery (e.g., in IDE or --list-tests),
+/// which was unexpected and resource-intensive.
+/// </summary>
+[EngineTest(ExpectedResult.Pass)]
+public class InstanceMethodDataSourceWithAsyncInitializerTests
+{
+    private static int _initializationCount;
+    private static int _testExecutionCount;
+    private static readonly ConcurrentBag<Guid> _observedInstanceIds = [];
+
+    /// <summary>
+    /// Simulates a fixture like ClientServiceFixture that starts Docker containers.
+    /// Implements IAsyncInitializer (NOT IAsyncDiscoveryInitializer) because initialization
+    /// should only happen during test execution, not during discovery.
+    /// 
+    /// With the engine fix in DataSourceHelpers.cs, fixtures implementing IAsyncInitializer
+    /// are NOT initialized during discovery - only during test execution.
+    /// </summary>
+    public class SimulatedContainerFixture : IAsyncInitializer
+    {
+        /// <summary>
+        /// Unique identifier for this instance to verify sharing behavior.
+        /// </summary>
+        public Guid InstanceId { get; } = Guid.NewGuid();
+
+        public bool IsInitialized { get; private set; }
+
+        public Task InitializeAsync()
+        {
+            Interlocked.Increment(ref _initializationCount);
+            Console.WriteLine($"[SimulatedContainerFixture] InitializeAsync called on instance {InstanceId} (count: {_initializationCount})");
+
+            // Simulate expensive container startup - this should NOT happen during discovery
+            IsInitialized = true;
+
+            return Task.CompletedTask;
+        }
+    }
+
+    [ClassDataSource<SimulatedContainerFixture>(Shared = SharedType.PerClass)]
+    public required SimulatedContainerFixture Fixture { get; init; }
+
+    /// <summary>
+    /// This property is accessed by InstanceMethodDataSource during discovery (compile-time).
+    /// It MUST return test case identifiers WITHOUT accessing the fixture, because:
+    /// 1. Source generation happens at compile-time, not runtime
+    /// 2. The fixture is created at runtime via ClassDataSource
+    /// 3. Accessing runtime properties during compilation is impossible
+    /// 
+    /// Test case identifiers are predefined here, and the test verifies that the fixture
+    /// (which implements IAsyncInitializer) is only initialized during test execution,
+    /// not during discovery.
+    /// </summary>
+    public IEnumerable<string> TestExecutions => ["TestCase1", "TestCase2", "TestCase3"];
+
+    [Test]
+    [InstanceMethodDataSource(nameof(TestExecutions))]
+    public async Task Test_WithInstanceMethodDataSource_DoesNotInitializeDuringDiscovery(string testCase)
+    {
+        Interlocked.Increment(ref _testExecutionCount);
+
+        // Track this instance to verify sharing
+        _observedInstanceIds.Add(Fixture.InstanceId);
+
+        // The fixture should be initialized by the time the test runs
+        await Assert.That(Fixture.IsInitialized)
+            .IsTrue()
+            .Because("the fixture should be initialized before test execution");
+
+        await Assert.That(testCase)
+            .IsNotNullOrEmpty()
+            .Because("the test case data should be available");
+
+        Console.WriteLine($"[Test] Executed with testCase='{testCase}', instanceId={Fixture.InstanceId}, " +
+                          $"initCount={_initializationCount}, execCount={_testExecutionCount}");
+    }
+
+    [After(Class)]
+    public static async Task VerifyInitializationAndSharing()
+    {
+        // With SharedType.PerClass, the fixture should be initialized exactly ONCE
+        // during test execution, NOT during discovery.
+        //
+        // Before the fix: _initializationCount would be 2+ (discovery + execution)
+        // After the fix: _initializationCount should be exactly 1 (execution only)
+
+        Console.WriteLine($"[After(Class)] Final counts - init: {_initializationCount}, exec: {_testExecutionCount}");
+        Console.WriteLine($"[After(Class)] Unique instance IDs observed: {_observedInstanceIds.Distinct().Count()}");
+
+        await Assert.That(_initializationCount)
+            .IsEqualTo(1)
+            .Because("IAsyncInitializer should only be called once during execution, not during discovery");
+
+        await Assert.That(_testExecutionCount)
+            .IsEqualTo(3)
+            .Because("there should be 3 test executions (one per test case)");
+
+        // Verify that all tests used the SAME fixture instance (SharedType.PerClass)
+        var uniqueInstanceIds = _observedInstanceIds.Distinct().ToList();
+        await Assert.That(uniqueInstanceIds)
+            .HasCount().EqualTo(1)
+            .Because("with SharedType.PerClass, all tests should share the same fixture instance");
+
+        // Reset for next run
+        _initializationCount = 0;
+        _testExecutionCount = 0;
+        _observedInstanceIds.Clear();
+    }
+}