feat: prevent side effects from non-IAsyncInitializer property getters during discovery

TUnit.Core/Discovery/ObjectGraphDiscoverer.cs

@@ -442,6 +442,17 @@ private static void TraverseInitializerProperties(
         foreach (var property in properties)
         {
             cancellationToken.ThrowIfCancellationRequested();
+
+            // Only access properties whose declared type could be IAsyncInitializer.
+            // This prevents triggering side effects from unrelated property getters
+            // (e.g., WebApplicationFactory.Server which starts the test host when accessed).
+            // Properties typed as object/interfaces not extending IAsyncInitializer won't be
+            // checked - users should properly type their properties or use data source attributes.
+            if (!typeof(IAsyncInitializer).IsAssignableFrom(property.PropertyType))
+            {
+                continue;
+            }
+
             try
             {
                 var value = property.GetValue(obj);

TUnit.TestProject/Bugs/4049/PropertyGetterSideEffectTests.cs

@@ -0,0 +1,132 @@
+using TUnit.Core.Interfaces;
+using TUnit.TestProject.Attributes;
+
+namespace TUnit.TestProject.Bugs._4049;
+
+/// <summary>
+/// Regression test for issue #4049: Property getters should not be called during discovery
+/// for properties that don't return IAsyncInitializer.
+///
+/// This simulates the WebApplicationFactory.Server scenario where accessing a property
+/// causes side effects (like starting a test server) before the object is configured.
+/// </summary>
+[EngineTest(ExpectedResult.Pass)]
+public class PropertyGetterSideEffectTests
+{
+    private static int _sideEffectCount = 0;
+
+    /// <summary>
+    /// Simulates a class like WebApplicationFactory where accessing certain properties
+    /// triggers initialization (like building the test host).
+    /// </summary>
+    public class FixtureWithSideEffects : IAsyncInitializer
+    {
+        private object? _server;
+
+        /// <summary>
+        /// Simulates WebApplicationFactory.Server - accessing this property has side effects.
+        /// This property type is 'object', NOT IAsyncInitializer, so TUnit should NOT access it.
+        /// </summary>
+        public object Server
+        {
+            get
+            {
+                Interlocked.Increment(ref _sideEffectCount);
+                Console.WriteLine($"Server getter called! Side effect count: {_sideEffectCount}");
+                _server ??= new object(); // Side effect: lazy initialization
+                return _server;
+            }
+        }
+
+        public bool IsInitialized { get; private set; }
+
+        public Task InitializeAsync()
+        {
+            IsInitialized = true;
+            Console.WriteLine("FixtureWithSideEffects.InitializeAsync called");
+            return Task.CompletedTask;
+        }
+    }
+
+    [Test]
+    [ClassDataSource<FixtureWithSideEffects>]
+    public async Task PropertyGetters_WithSideEffects_ShouldNotBeCalledDuringDiscovery(FixtureWithSideEffects fixture)
+    {
+        // Verify the fixture was properly initialized
+        await Assert.That(fixture.IsInitialized).IsTrue();
+
+        // The Server property getter should NOT have been called during discovery.
+        // It should only be called if the test explicitly accesses it.
+        // At this point, we haven't accessed Server, so count should be 0.
+        await Assert.That(_sideEffectCount).IsEqualTo(0)
+            .Because("TUnit should not access non-IAsyncInitializer property getters during discovery");
+
+        // Now let's explicitly access the Server to verify it works
+        var server = fixture.Server;
+        await Assert.That(server).IsNotNull();
+        await Assert.That(_sideEffectCount).IsEqualTo(1);
+
+        Console.WriteLine($"Test passed: Server getter was only called when explicitly accessed (count: {_sideEffectCount})");
+    }
+}
+
+/// <summary>
+/// Test to verify that nested IAsyncInitializer properties ARE still discovered
+/// when they are properly typed.
+/// </summary>
+[EngineTest(ExpectedResult.Pass)]
+public class NestedAsyncInitializerDiscoveryTests
+{
+    private static int _parentInitCount = 0;
+    private static int _childInitCount = 0;
+
+    public class ChildInitializer : IAsyncInitializer
+    {
+        public bool IsInitialized { get; private set; }
+
+        public Task InitializeAsync()
+        {
+            Interlocked.Increment(ref _childInitCount);
+            IsInitialized = true;
+            Console.WriteLine($"ChildInitializer.InitializeAsync called (count: {_childInitCount})");
+            return Task.CompletedTask;
+        }
+    }
+
+    public class ParentWithNestedInitializer : IAsyncInitializer
+    {
+        /// <summary>
+        /// This property IS typed as IAsyncInitializer, so TUnit SHOULD access it
+        /// and discover the nested initializer for proper lifecycle management.
+        /// </summary>
+        public IAsyncInitializer NestedInitializer { get; } = new ChildInitializer();
+
+        public bool IsInitialized { get; private set; }
+
+        public Task InitializeAsync()
+        {
+            Interlocked.Increment(ref _parentInitCount);
+            IsInitialized = true;
+            Console.WriteLine($"ParentWithNestedInitializer.InitializeAsync called (count: {_parentInitCount})");
+            return Task.CompletedTask;
+        }
+    }
+
+    [Test]
+    [ClassDataSource<ParentWithNestedInitializer>]
+    public async Task NestedInitializers_WhenProperlyTyped_ShouldBeDiscoveredAndInitialized(ParentWithNestedInitializer parent)
+    {
+        // Verify parent was initialized
+        await Assert.That(parent.IsInitialized).IsTrue();
+        await Assert.That(_parentInitCount).IsGreaterThan(0);
+
+        // Verify child was discovered and initialized
+        var child = parent.NestedInitializer as ChildInitializer;
+        await Assert.That(child).IsNotNull();
+        await Assert.That(child!.IsInitialized).IsTrue()
+            .Because("TUnit should discover and initialize properly-typed IAsyncInitializer properties");
+        await Assert.That(_childInitCount).IsGreaterThan(0);
+
+        Console.WriteLine($"Test passed: Both parent ({_parentInitCount}) and child ({_childInitCount}) were initialized");
+    }
+}