Fix: detect handler methods inherited from base interfaces

XrmPluginCore.SourceGenerator.Tests/DiagnosticTests/DiagnosticReportingTests.cs

@@ -160,6 +160,69 @@ public void Process() { }
 		errorDiagnostics.Should().OnlyContain(d => d.Severity == DiagnosticSeverity.Error);
 	}
 
+	[Fact]
+	public async Task Should_Not_Report_XPC4001_When_Handler_Method_Is_Inherited_From_Base_Interface()
+	{
+		// Arrange - the handler method is declared on a grandparent interface in a
+		// multi-level inheritance chain (ITestService : IMidService : IBaseService).
+		// The method must still be detected via the transitive AllInterfaces walk.
+		const string pluginSource = """
+
+			using XrmPluginCore;
+			using XrmPluginCore.Enums;
+			using Microsoft.Extensions.DependencyInjection;
+			using TestNamespace;
+
+			namespace TestNamespace
+			{
+			    public class TestPlugin : Plugin
+			    {
+			        public TestPlugin()
+			        {
+			            RegisterStep<Account, ITestService>(EventOperation.Update, ExecutionStage.PostOperation,
+			                service => service.Process)
+			                .WithPreImage(x => x.Name);
+			        }
+
+			        protected override IServiceCollection OnBeforeBuildServiceProvider(IServiceCollection services)
+			        {
+			            return services.AddScoped<ITestService, TestService>();
+			        }
+			    }
+
+			    public interface IBaseService
+			    {
+			        void Process();
+			    }
+
+			    public interface IMidService : IBaseService
+			    {
+			    }
+
+			    public interface ITestService : IMidService
+			    {
+			    }
+
+			    public class TestService : ITestService
+			    {
+			        public void Process() { }
+			    }
+			}
+			""";
+
+		var source = TestFixtures.GetCompleteSource(pluginSource);
+
+		// Act - Run analyzer instead of generator
+		var diagnostics = await GetAnalyzerDiagnosticsAsync(source, new HandlerMethodNotFoundAnalyzer());
+
+		// Assert
+		var errorDiagnostics = diagnostics
+			.Where(d => d.Id == "XPC4001")
+			.ToArray();
+
+		errorDiagnostics.Should().BeEmpty("XPC4001 should not be reported when the handler method is inherited from a base interface");
+	}
+
 	[Fact]
 	public async Task Should_Report_XPC4002_When_Handler_Missing_PreImage_Parameter()
 	{

XrmPluginCore.SourceGenerator/Helpers/TypeHelper.cs

@@ -11,24 +11,44 @@ internal static class TypeHelper
 {
 	/// <summary>
 	/// Gets all methods with the specified name, including inherited methods.
+	/// For classes this walks the base type chain; for interfaces it also includes
+	/// members inherited from base interfaces (which are exposed via AllInterfaces, not BaseType).
 	/// </summary>
 	public static IMethodSymbol[] GetAllMethodsIncludingInherited(ITypeSymbol type, string methodName)
 	{
 		var methods = new List<IMethodSymbol>();
-		var currentType = type;
-		while (currentType != null)
+
+		void AddMethods(ITypeSymbol from)
 		{
-			foreach (var member in currentType.GetMembers(methodName))
+			foreach (var member in from.GetMembers(methodName))
 			{
 				if (member is IMethodSymbol method)
 				{
 					methods.Add(method);
 				}
 			}
+		}
 
+		var currentType = type;
+		while (currentType != null)
+		{
+			AddMethods(currentType);
 			currentType = currentType.BaseType;
 		}
 
+		// Interfaces don't expose inherited members via BaseType; their base
+		// interfaces (transitively) are available through AllInterfaces. We only
+		// do this for interface inputs - for classes the implementing methods live
+		// on the class/base chain, and walking AllInterfaces would surface
+		// (unimplemented) interface declaration methods.
+		if (type.TypeKind == TypeKind.Interface)
+		{
+			foreach (var inheritedInterface in type.AllInterfaces)
+			{
+				AddMethods(inheritedInterface);
+			}
+		}
+
 		return methods.ToArray();
 	}