feat: add IAsyncDiscoveryInitializer for asynchronous initialization during test discovery

TUnit.Core/Interfaces/IAsyncDiscoveryInitializer.cs

@@ -0,0 +1,28 @@
+namespace TUnit.Core.Interfaces;
+
+/// <summary>
+/// Defines a contract for types that require asynchronous initialization during test discovery.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Unlike <see cref="IAsyncInitializer"/> which runs during test execution,
+/// implementations of this interface are initialized during the test discovery phase.
+/// This enables data sources (such as <c>InstanceMethodDataSource</c>) to access
+/// fully-initialized objects when generating test cases.
+/// </para>
+/// <para>
+/// Common use cases include:
+/// <list type="bullet">
+/// <item><description>Starting Docker containers before test case enumeration</description></item>
+/// <item><description>Connecting to databases to discover parameterized test data</description></item>
+/// <item><description>Initializing fixtures that provide data for test case generation</description></item>
+/// </list>
+/// </para>
+/// <para>
+/// This interface extends <see cref="IAsyncInitializer"/>, meaning the same
+/// <see cref="IAsyncInitializer.InitializeAsync"/> method is used. The framework
+/// guarantees exactly-once initialization semantics - objects will not be
+/// re-initialized during test execution.
+/// </para>
+/// </remarks>
+public interface IAsyncDiscoveryInitializer : IAsyncInitializer;

TUnit.Engine/Building/TestBuilder.cs

@@ -66,6 +66,30 @@ private async Task InitializeDeferredClassDataAsync(object?[] classData)
         }
     }
 
+    /// <summary>
+    /// Initializes any IAsyncDiscoveryInitializer objects in class data during test discovery.
+    /// This is called BEFORE method data sources are evaluated, enabling data sources
+    /// to access initialized shared objects (like Docker containers).
+    /// </summary>
+    private static async Task InitializeDiscoveryObjectsAsync(object?[] classData)
+    {
+        if (classData == null || classData.Length == 0)
+        {
+            return;
+        }
+
+        foreach (var data in classData)
+        {
+            if (data is IAsyncDiscoveryInitializer && data is not IDataSourceAttribute)
+            {
+                // Uses ObjectInitializer which handles deduplication.
+                // This also prevents double-init during execution since ObjectInitializer
+                // tracks initialized objects.
+                await ObjectInitializer.InitializeAsync(data);
+            }
+        }
+    }
+
     private async Task<object> CreateInstance(TestMetadata metadata, Type[] resolvedClassGenericArgs, object?[] classData, TestBuilderContext builderContext)
     {
         // Initialize any deferred IAsyncInitializer objects in class data
@@ -206,6 +230,10 @@ public async Task<IEnumerable<AbstractExecutableTest>> BuildTestsFromMetadataAsy
                     var classDataResult = await classDataFactory() ?? [];
                     var classData = DataUnwrapper.Unwrap(classDataResult);
 
+                    // Initialize IAsyncDiscoveryInitializer objects before method data sources are evaluated.
+                    // This enables InstanceMethodDataSource to access initialized shared objects.
+                    await InitializeDiscoveryObjectsAsync(classData);
+
                     var needsInstanceForMethodDataSources = metadata.DataSources.Any(ds => ds is IAccessesInstanceData);
 
                     object? instanceForMethodDataSources = null;
@@ -1386,6 +1414,10 @@ public async IAsyncEnumerable<AbstractExecutableTest> BuildTestsStreamingAsync(
 
                 var classData = DataUnwrapper.Unwrap(await classDataFactory() ?? []);
 
+                // Initialize IAsyncDiscoveryInitializer objects before method data sources are evaluated.
+                // This enables InstanceMethodDataSource to access initialized shared objects.
+                await InitializeDiscoveryObjectsAsync(classData);
+
                 // Handle instance creation for method data sources
                 var needsInstanceForMethodDataSources = metadata.DataSources.Any(ds => ds is IAccessesInstanceData);
                 object? instanceForMethodDataSources = null;

TUnit.PublicAPI/Tests.Core_Library_Has_No_API_Changes.DotNet10_0.verified.txt

@@ -2199,6 +2199,7 @@ namespace .Interfaces
     {
         .<<object?[]>> GenerateDataFactoriesAsync(.DataSourceContext context, .CancellationToken cancellationToken = default);
     }
+    public interface IAsyncDiscoveryInitializer : . { }
     public interface IAsyncInitializer
     {
         . InitializeAsync();

TUnit.PublicAPI/Tests.Core_Library_Has_No_API_Changes.DotNet8_0.verified.txt

@@ -2199,6 +2199,7 @@ namespace .Interfaces
     {
         .<<object?[]>> GenerateDataFactoriesAsync(.DataSourceContext context, .CancellationToken cancellationToken = default);
     }
+    public interface IAsyncDiscoveryInitializer : . { }
     public interface IAsyncInitializer
     {
         . InitializeAsync();

TUnit.PublicAPI/Tests.Core_Library_Has_No_API_Changes.DotNet9_0.verified.txt

@@ -2199,6 +2199,7 @@ namespace .Interfaces
     {
         .<<object?[]>> GenerateDataFactoriesAsync(.DataSourceContext context, .CancellationToken cancellationToken = default);
     }
+    public interface IAsyncDiscoveryInitializer : . { }
     public interface IAsyncInitializer
     {
         . InitializeAsync();

TUnit.PublicAPI/Tests.Core_Library_Has_No_API_Changes.Net4_7.verified.txt

@@ -2131,6 +2131,7 @@ namespace .Interfaces
     {
         .<<object?[]>> GenerateDataFactoriesAsync(.DataSourceContext context, .CancellationToken cancellationToken = default);
     }
+    public interface IAsyncDiscoveryInitializer : . { }
     public interface IAsyncInitializer
     {
         . InitializeAsync();

TUnit.TestProject/Bugs/3997/Tests.cs

@@ -0,0 +1,58 @@
+using TUnit.Core;
+using TUnit.Core.Interfaces;
+using TUnit.TestProject.Attributes;
+
+namespace TUnit.TestProject.Bugs._3997;
+
+/// <summary>
+/// Simulates a data source (like a Docker container) that needs initialization during test discovery
+/// so that InstanceMethodDataSource can access its data when generating test cases.
+/// </summary>
+public class SimulatedContainer : IAsyncDiscoveryInitializer
+{
+    private readonly List<string> _data = [];
+    public bool IsInitialized { get; private set; }
+
+    public IReadOnlyList<string> Data => _data;
+
+    public Task InitializeAsync()
+    {
+        if (IsInitialized)
+        {
+            throw new InvalidOperationException("Container already initialized! InitializeAsync should only be called once.");
+        }
+
+        // Simulate container startup and data population
+        _data.AddRange(["TestCase1", "TestCase2", "TestCase3"]);
+        IsInitialized = true;
+        return Task.CompletedTask;
+    }
+}
+
+/// <summary>
+/// Tests that IAsyncDiscoveryInitializer is called during test discovery,
+/// allowing InstanceMethodDataSource to access initialized data.
+/// </summary>
+[EngineTest(ExpectedResult.Pass)]
+public class DiscoveryInitializerTests
+{
+    [ClassDataSource<SimulatedContainer>(Shared = SharedType.PerClass)]
+    public required SimulatedContainer Container { get; init; }
+
+    /// <summary>
+    /// This property provides test data from the initialized container.
+    /// The container MUST be initialized during discovery before this is evaluated.
+    /// </summary>
+    public IEnumerable<string> TestCases => Container.Data;
+
+    [Test]
+    [InstanceMethodDataSource(nameof(TestCases))]
+    public async Task TestWithContainerData(string testCase)
+    {
+        // Container should be initialized
+        await Assert.That(Container.IsInitialized).IsTrue();
+
+        // testCase should be one of the container's data items
+        await Assert.That(Container.Data).Contains(testCase);
+    }
+}