fix(mocks): preserve nullable annotations on generated event implementations

TUnit.Mocks.SourceGenerator.Tests/MockGeneratorTests.cs

@@ -105,6 +105,34 @@ void M()
         return VerifyGeneratorOutput(source);
     }
 
+    [Test]
+    public Task Interface_With_Nullable_Event()
+    {
+        // Regression test for #5424: nullable event handler types must
+        // preserve nullability in generated explicit interface implementation,
+        // otherwise CS8615 (nullability mismatch) is emitted.
+        var source = """
+            #nullable enable
+            using System;
+            using TUnit.Mocks;
+
+            public interface IFoo
+            {
+                event EventHandler<string>? Something;
+            }
+
+            public class TestUsage
+            {
+                void M()
+                {
+                    var mock = Mock.Of<IFoo>();
+                }
+            }
+            """;
+
+        return VerifyGeneratorOutput(source);
+    }
+
     [Test]
     public Task Interface_With_Multiple_Multi_Parameter_Events()
     {

TUnit.Mocks.SourceGenerator.Tests/Snapshots/Interface_With_Nullable_Event.verified.txt

@@ -0,0 +1,160 @@
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks.Generated
+{
+    public sealed class IFooMock : global::TUnit.Mocks.Mock<global::IFoo>, global::IFoo
+    {
+        [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+        internal IFooMock(global::IFoo mockObject, global::TUnit.Mocks.MockEngine<global::IFoo> engine)
+            : base(mockObject, engine) { }
+
+        event global::System.EventHandler<string>? global::IFoo.Something { add => Object.Something += value; remove => Object.Something -= value; }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks.Generated
+{
+    [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+    public readonly struct IFoo_MockEvents
+    {
+        internal readonly global::TUnit.Mocks.MockEngine<global::IFoo> Engine;
+
+        internal IFoo_MockEvents(global::TUnit.Mocks.MockEngine<global::IFoo> engine) => Engine = engine;
+    }
+
+    public static class IFoo_MockEventsExtensions
+    {
+        extension(global::TUnit.Mocks.Mock<global::IFoo> mock)
+        {
+            public IFoo_MockEvents Events => new(global::TUnit.Mocks.MockRegistry.GetEngine(mock));
+        }
+
+        extension(IFoo_MockEvents events)
+        {
+            public global::TUnit.Mocks.EventSubscriptionAccessor Something
+                => new(events.Engine, "Something");
+        }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks.Generated
+{
+    file sealed class IFooMockImpl : global::IFoo, global::TUnit.Mocks.IRaisable, global::TUnit.Mocks.IMockObject
+    {
+        private readonly global::TUnit.Mocks.MockEngine<global::IFoo> _engine;
+
+        [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+        global::TUnit.Mocks.IMock? global::TUnit.Mocks.IMockObject.MockWrapper { get; set; }
+
+        internal IFooMockImpl(global::TUnit.Mocks.MockEngine<global::IFoo> engine)
+        {
+            _engine = engine;
+        }
+
+        private global::System.EventHandler<string>? _backing_Something;
+
+        public event global::System.EventHandler<string>? Something
+        {
+            add { _backing_Something += value; _engine.RecordEventSubscription("Something", true); }
+            remove { _backing_Something -= value; _engine.RecordEventSubscription("Something", false); }
+        }
+
+        [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+        internal void Raise_Something(string e)
+        {
+            _backing_Something?.Invoke(this, e);
+        }
+
+        [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
+        public void RaiseEvent(string eventName, object? args)
+        {
+            switch (eventName)
+            {
+                case "Something":
+                    {
+                        Raise_Something((string)args!);
+                        break;
+                    }
+                default:
+                    throw new global::System.InvalidOperationException($"No event named '{eventName}' exists on this mock.");
+            }
+        }
+    }
+
+    file static class IFooMockFactory
+    {
+        [global::System.Runtime.CompilerServices.ModuleInitializer]
+        internal static void Register()
+        {
+            global::TUnit.Mocks.MockRegistry.RegisterFactory<global::IFoo>(Create);
+        }
+
+        internal static global::TUnit.Mocks.Mock<global::IFoo> Create(global::TUnit.Mocks.MockBehavior behavior, object[] constructorArgs)
+        {
+            if (constructorArgs.Length > 0) throw new global::System.ArgumentException($"Interface mock 'global::IFoo' does not support constructor arguments, but {constructorArgs.Length} were provided.");
+            var engine = new global::TUnit.Mocks.MockEngine<global::IFoo>(behavior);
+            var impl = new IFooMockImpl(engine);
+            engine.Raisable = impl;
+            var mock = new IFooMock(impl, engine);
+            return mock;
+        }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks.Generated
+{
+    public static class IFoo_MockMemberExtensions
+    {
+        public static void RaiseSomething(this global::TUnit.Mocks.Mock<global::IFoo> mock, string e)
+        {
+            ((global::TUnit.Mocks.IRaisable)global::TUnit.Mocks.MockRegistry.GetEngine(mock).Raisable!).RaiseEvent("Something", (object?)e);
+        }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks
+{
+    public static class IFoo_MockStaticExtension
+    {
+        extension(global::IFoo)
+        {
+            public static global::TUnit.Mocks.Generated.IFooMock Mock(global::TUnit.Mocks.MockBehavior behavior = global::TUnit.Mocks.MockBehavior.Loose)
+            {
+                return (global::TUnit.Mocks.Generated.IFooMock)global::TUnit.Mocks.Mock.Of<global::IFoo>(behavior);
+            }
+        }
+    }
+}
+
+
+// ===== FILE SEPARATOR =====
+
+// <auto-generated/>
+#nullable enable
+
+namespace TUnit.Mocks.Generated;

TUnit.Mocks.SourceGenerator/Builders/MockImplBuilder.cs

@@ -946,11 +946,11 @@ private static void GeneratePartialProperty(CodeWriter writer, MockMemberModel p
     private static void GenerateEvent(CodeWriter writer, MockEventModel evt)
     {
         // Backing delegate field
-        writer.AppendLine($"private {evt.EventHandlerType}? _backing_{evt.Name};");
+        writer.AppendLine($"private {evt.EventHandlerTypeNonNullable}? _backing_{evt.Name};");
         writer.AppendLine();
 
         // Event add/remove accessors
-        writer.AppendLine($"public event {evt.EventHandlerType}? {evt.Name}");
+        writer.AppendLine($"public event {evt.EventHandlerTypeNonNullable}? {evt.Name}");
         writer.OpenBrace();
         writer.AppendLine($"add {{ _backing_{evt.Name} += value; _engine.RecordEventSubscription(\"{evt.Name}\", true); }}");
         writer.AppendLine($"remove {{ _backing_{evt.Name} -= value; _engine.RecordEventSubscription(\"{evt.Name}\", false); }}");
@@ -979,11 +979,11 @@ private static void GenerateEvent(CodeWriter writer, MockEventModel evt)
     private static void GeneratePartialEvent(CodeWriter writer, MockEventModel evt)
     {
         // Backing delegate field
-        writer.AppendLine($"private {evt.EventHandlerType}? _backing_{evt.Name};");
+        writer.AppendLine($"private {evt.EventHandlerTypeNonNullable}? _backing_{evt.Name};");
         writer.AppendLine();
 
         // Event add/remove accessors with override
-        writer.AppendLine($"public override event {evt.EventHandlerType}? {evt.Name}");
+        writer.AppendLine($"public override event {evt.EventHandlerTypeNonNullable}? {evt.Name}");
         writer.OpenBrace();
         writer.AppendLine($"add {{ _backing_{evt.Name} += value; _engine.RecordEventSubscription(\"{evt.Name}\", true); }}");
         writer.AppendLine($"remove {{ _backing_{evt.Name} -= value; _engine.RecordEventSubscription(\"{evt.Name}\", false); }}");

TUnit.Mocks.SourceGenerator/Discovery/MemberDiscovery.cs

@@ -649,7 +649,7 @@ private static MockMemberModel CreateIndexerModel(IPropertySymbol indexer, ref i
 
     private static MockEventModel CreateEventModel(IEventSymbol evt, string? explicitInterfaceName, string? declaringInterfaceName = null)
     {
-        var eventHandlerType = evt.Type.GetFullyQualifiedName();
+        var eventHandlerType = evt.Type.GetFullyQualifiedNameWithNullability();
 
         // Determine if this is an EventHandler pattern (sender + args)
         var isEventHandlerPattern = IsEventHandlerType(evt.Type);

TUnit.Mocks.SourceGenerator/Models/MockEventModel.cs

@@ -5,8 +5,19 @@ namespace TUnit.Mocks.SourceGenerator.Models;
 internal sealed record MockEventModel : IEquatable<MockEventModel>
 {
     public string Name { get; init; } = "";
+    /// <summary>
+    /// The fully qualified event handler type, with nullable annotations
+    /// preserved from the declaring interface. Used when emitting explicit
+    /// interface event implementations so that nullability matches the
+    /// interface declaration (otherwise CS8615 is emitted, see issue #5424).
+    /// For the always-nullable backing delegate field, use
+    /// <see cref="EventHandlerTypeNonNullable"/> and append <c>?</c>.
+    /// </summary>
     public string EventHandlerType { get; init; } = "";
 
+    /// <summary>The handler type with any trailing nullable annotation removed.</summary>
+    public string EventHandlerTypeNonNullable => EventHandlerType.TrimEnd('?');
+
     /// <summary>
     /// The argument expression for invoking the backing delegate.
     /// E.g., "this, args" for EventHandler&lt;string&gt;, or "arg1, arg2" for Action&lt;string, int&gt;.