mediatorlite-source-generation

name

mediatorlite-source-generation

description

Reference for the MediatorLite.SourceGeneration project -- HandlerDiscoveryGenerator (IIncrementalGenerator) with four discovery pipelines, emission of MediatorLite.Generated.MediatorLiteRegistration + SourceGeneratedMediator, compile-time resolution of notification strategies, unrolled pipelines, and inline logging/tracing with [assembly: DisableMediatorLogging] / [assembly: DisableMediatorTracing] opt-out. Use when modifying code generation, tuning emitted dispatch, adding new generator pipelines, or debugging generated output.

MediatorLite.SourceGeneration

⚠️ Validation note. The validator discovery pipeline now matches FluentValidation AbstractValidator<T> / FluentValidation.IValidator<T> (DataAnnotations discovery was removed) and emits MediatorLite.FluentValidation.FluentValidationBehavior<,> outermost, plus the MEDL1001 missing-package diagnostic. See mediatorlite-validation and .claude/rules/50-validation.md.

Purpose

MediatorLite.SourceGeneration is a Roslyn incremental source generator that transforms every IRequestHandler<,>, INotificationHandler<>, IPipelineBehavior<,>, and IValidator<> in the compilation into a fully unrolled, reflection-free dispatch implementation. It emits two generated files per compilation unit — MediatorLiteRegistration.g.cs (DI wiring + diagnostic counts) and SourceGeneratedMediator.g.cs (O(1) dispatch tables + per-request/per-notification methods) — and resolves every notification execution/error strategy and observability decision at compile time so the runtime Mediator.cs has zero branches.

When to use

• Adding a new attribute, interface, or discovery rule (e.g. a new handler kind).
• Changing the emitted shape of MediatorLiteRegistration or SourceGeneratedMediator.
• Modifying how behaviors compose (e.g. changing ValidationBehavior's ordering, or expanding open generics differently).
• Adjusting compile-time resolution of NotificationExecutionStrategy / NotificationErrorStrategy.
• Adding new observability surfaces (e.g. metrics), or adjusting [assembly: DisableMediatorLogging] / [assembly: DisableMediatorTracing] handling.
• Debugging why a handler / validator / behavior does not appear in generated output.

Project location & entry points

• MediatorLite.SourceGeneration.csproj — targets netstandard2.0 (required for source generators), IsRoslynComponent=true, packages with the analyzers/dotnet/cs pack path.
• HandlerDiscoveryGenerator.cs — the only generator class; everything lives here.
• README.md — public package docs.
• Consumer projects reference it as an analyzer:

  <ProjectReference Include="..\..\src\MediatorLite.SourceGeneration\MediatorLite.SourceGeneration.csproj"
                    OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
  

• Generated output namespace: MediatorLite.Generated. Class names: MediatorLiteRegistration, SourceGeneratedMediator.

Core types / API surface

Generator entry point

[Generator(LanguageNames.CSharp)]
public sealed class HandlerDiscoveryGenerator : IIncrementalGenerator
{

The four discovery pipelines

Initialize builds four IncrementalValuesProvider<...> pipelines (one per artifact type) plus a fifth provider for assembly-level defaults, then combines them into a single source-output callback.

    public void Initialize(IncrementalGeneratorInitializationContext context)
    {
        // Find all class declarations that might be handlers
        var handlerDeclarations = context.SyntaxProvider
            .CreateSyntaxProvider(
                predicate: static (node, _) => IsHandlerCandidate(node),
                transform: static (context, ct) => GetHandlerInfo(context, ct))
            .Where(static info => info is not null);

        // Find all notification types to capture their options
        var notificationDeclarations = context.SyntaxProvider
            .CreateSyntaxProvider(
                predicate: static (node, _) => IsNotificationCandidate(node),
                transform: static (context, ct) => GetNotificationInfo(context, ct))
            .Where(static info => info is not null);

        // Find all behavior declarations
        var behaviorDeclarations = context.SyntaxProvider
            .CreateSyntaxProvider(
                predicate: static (node, _) => IsBehaviorCandidate(node),
                transform: static (context, ct) => GetBehaviorInfo(context, ct))
            .Where(static info => info is not null);

        // Find all validator declarations
        var validatorDeclarations = context.SyntaxProvider
            .CreateSyntaxProvider(
                predicate: static (node, _) => IsValidatorCandidate(node),
                transform: static (context, ct) => GetValidatorInfo(context, ct))
            .Where(static info => info is not null);

        // Assembly-level defaults for notification strategies (compile-time)
        var assemblyDefaults = context.CompilationProvider
            .Select(static (compilation, _) => GetAssemblyDefaults(compilation));

All four Is*Candidate predicates follow the same shape: class/type declaration with a base list and not abstract.

Handler pipeline — GetHandlerInfo

Walks classSymbol.AllInterfaces, matches originalDefinition.ToDisplayString() against the fully qualified open interface name, and captures type arguments. [MediatorGeneration(Skip = true)] short-circuits discovery. [NotificationHandlerOrder] is read here (shared across all notification handler interfaces on the class).

    private static HandlerInfo? GetHandlerInfo(GeneratorSyntaxContext context, CancellationToken ct)
    {
        var classDecl = (ClassDeclarationSyntax)context.Node;
        var semanticModel = context.SemanticModel;
        var classSymbol = semanticModel.GetDeclaredSymbol(classDecl, ct) as INamedTypeSymbol;

        if (classSymbol is null || classSymbol.IsAbstract)
            return null;

        var hasSkipAttribute = classSymbol.GetAttributes()
            .Any(a => a.AttributeClass?.Name == "MediatorGenerationAttribute"
                      && a.NamedArguments.Any(arg => arg.Key == "Skip" && arg.Value.Value is true));

        if (hasSkipAttribute)
            return null;

        int? handlerOrder = null;
        var orderAttr = classSymbol.GetAttributes()
            .FirstOrDefault(a => a.AttributeClass?.Name == "NotificationHandlerOrderAttribute");
        if (orderAttr != null && orderAttr.ConstructorArguments.Length > 0)
        {
            handlerOrder = orderAttr.ConstructorArguments[0].Value as int?;
        }

The HasDataAnnotations flag here is what later drives the automatic DataAnnotationsValidator<TRequest> registration.

Behavior pipeline — GetBehaviorInfo

Captures every IPipelineBehavior<TRequest, TResponse> interface on the class; detects whether the interface arguments are open type parameters and stores that as IsOpenGeneric. Reads [BehaviorOrder(int)] for deterministic ordering during pipeline emission.

        // Extract BehaviorOrderAttribute if present
        int behaviorOrder = 0;
        var orderAttr = classSymbol.GetAttributes()
            .FirstOrDefault(a => a.AttributeClass?.Name == "BehaviorOrderAttribute");
        if (orderAttr != null && orderAttr.ConstructorArguments.Length > 0)
        {
            behaviorOrder = (int)(orderAttr.ConstructorArguments[0].Value ?? 0);
        }

        return new BehaviorInfo(
            ClassName: classSymbol.ToDisplayString(SymbolDisplayFormat.FullyQualifiedFormat),
            Namespace: classSymbol.ContainingNamespace?.ToDisplayString() ?? "",
            BehaviorInterfaces: behaviorInterfaces,
            IsOpenGeneric: isOpenGeneric,
            Order: behaviorOrder);
    }

Validator pipeline — GetValidatorInfo

Only closed validators are discovered — open generic validators (such as the library's DataAnnotationsValidator<T>) are explicitly excluded so they are not double-registered.

    private static ValidatorInfo? GetValidatorInfo(GeneratorSyntaxContext context, CancellationToken ct)
    {
        var classDecl = (ClassDeclarationSyntax)context.Node;
        var semanticModel = context.SemanticModel;
        var classSymbol = semanticModel.GetDeclaredSymbol(classDecl, ct) as INamedTypeSymbol;

        if (classSymbol is null || classSymbol.IsAbstract)
            return null;

        // Skip open generic validators (e.g., DataAnnotationsValidator<T> from the library)
        if (classSymbol.IsGenericType)
            return null;

Notification pipeline — GetNotificationInfo

Unlike the other three pipelines, this one discovers notification type declarations (not handlers) to capture per-type [NotificationExecution] / [NotificationError] attributes. If neither attribute is present, the pipeline returns null and the type falls back to assembly + library defaults.

    private static NotificationTypeInfo? GetNotificationInfo(GeneratorSyntaxContext context, CancellationToken ct)
    {
        var typeDecl = (TypeDeclarationSyntax)context.Node;
        var semanticModel = context.SemanticModel;
        var typeSymbol = semanticModel.GetDeclaredSymbol(typeDecl, ct) as INamedTypeSymbol;

        if (typeSymbol is null)
            return null;

        var implementsNotification = typeSymbol.AllInterfaces
            .Any(i => i.ToDisplayString() == "MediatorLite.INotification");

        if (!implementsNotification)
            return null;

        int? executionStrategy = null;
        int? errorStrategy = null;

        foreach (var attr in typeSymbol.GetAttributes())
        {
            var name = attr.AttributeClass?.Name;
            if (name == "NotificationExecutionAttribute"
                && attr.ConstructorArguments.Length > 0
                && attr.ConstructorArguments[0].Value is int es)
            {
                executionStrategy = es;
            }
            else if (name == "NotificationErrorAttribute"
                && attr.ConstructorArguments.Length > 0
                && attr.ConstructorArguments[0].Value is int ers)
            {
                errorStrategy = ers;
            }
        }

Assembly defaults — GetAssemblyDefaults

Reads the [assembly: ...] attributes in a single pass and captures four signals at once: execution strategy default, error strategy default, logging-disabled flag, and tracing-disabled flag.

    private static AssemblyDefaults GetAssemblyDefaults(Compilation compilation)
    {
        int? execution = null;
        int? error = null;
        bool loggingDisabled = false;
        bool tracingDisabled = false;

        foreach (var attr in compilation.Assembly.GetAttributes())
        {
            var name = attr.AttributeClass?.Name;
            if (name == "DefaultNotificationExecutionAttribute"
                && attr.ConstructorArguments.Length > 0
                && attr.ConstructorArguments[0].Value is int es)
            {
                execution = es;
            }
            else if (name == "DefaultNotificationErrorAttribute"
                && attr.ConstructorArguments.Length > 0
                && attr.ConstructorArguments[0].Value is int ers)
            {
                error = ers;
            }
            else if (name == "DisableMediatorLoggingAttribute")
            {
                loggingDisabled = true;
            }
            else if (name == "DisableMediatorTracingAttribute")
            {
                tracingDisabled = true;
            }
        }

        return new AssemblyDefaults(execution, error, loggingDisabled, tracingDisabled);
    }

Compile-time strategy resolution — ResolveStrategies

Documents the precedence contract: per-notification attribute → assembly default → library default. Library defaults are Sequential for execution and StopOnFirstError for error (enum value 0 for both).

    /// <summary>
    /// Resolves the final (execution, error) strategy tuple for a notification type.
    /// Precedence per strategy: per-notification attribute &gt; assembly default &gt; library default
    /// (Sequential=0 for execution, StopOnFirstError=0 for error).
    /// </summary>
    private static (int Execution, int Error) ResolveStrategies(
        NotificationTypeInfo? perType,
        AssemblyDefaults globals)
    {
        int execution = perType?.ExecutionStrategy ?? globals.ExecutionStrategy ?? 0;
        int error = perType?.ErrorStrategy ?? globals.ErrorStrategy ?? 0;
        return (execution, error);
    }

Execute — glue that builds everything

Determines which requests need validation (from HasDataAnnotations or a discovered custom IValidator), expands open generic behaviors into closed types per request/response pair, and appends ValidationBehavior<TReq, TRes> entries ahead of all other behaviors.

    private static void Execute(
        SourceProductionContext context,
        Compilation compilation,
        ImmutableArray<HandlerInfo?> handlers,
        ImmutableArray<NotificationTypeInfo?> notifications,
        ImmutableArray<BehaviorInfo?> behaviors,
        ImmutableArray<ValidatorInfo?> validators,
        AssemblyDefaults assemblyDefaults)
    {
        var validHandlers = handlers.Where(h => h is not null).Cast<HandlerInfo>().ToList();
        var validNotifications = notifications.Where(n => n is not null).Cast<NotificationTypeInfo>().ToList();
        var validBehaviors = behaviors.Where(b => b is not null).Cast<BehaviorInfo>().ToList();
        var validValidators = validators.Where(v => v is not null).Cast<ValidatorInfo>().ToList();

        if (validHandlers.Count == 0)
        {
            GenerateEmptyRegistration(context);
            return;
        }

        var expandedBehaviors = ExpandBehaviors(validBehaviors, validHandlers);

        // Determine which request types need validation
        var requestTypesWithValidation = DetermineValidationTargets(validHandlers, validValidators);

        // Add ValidationBehavior entries for InvokeBehavior dispatch
        foreach (var (requestType, responseType) in requestTypesWithValidation)
        {
            expandedBehaviors.Add(new ExpandedBehaviorInfo(
                BehaviorTypeName: $"global::MediatorLite.Validation.ValidationBehavior<{requestType}, {responseType}>",
                RequestType: requestType,
                ResponseType: responseType,
                InterfaceType: $"global::MediatorLite.IPipelineBehavior<{requestType}, {responseType}>"));
        }

        GenerateRegistrationCode(context, validHandlers, expandedBehaviors, validValidators, requestTypesWithValidation);
        GenerateSourceGeneratedMediator(context, validHandlers, validNotifications, validBehaviors, expandedBehaviors, assemblyDefaults);
    }

Emitted MediatorLiteRegistration — the public surface

The generated class lives in MediatorLite.Generated and exposes one all-in-one method plus four granular methods, each returning IServiceCollection:

        sb.AppendLine(
            "        public static global::Microsoft.Extensions.DependencyInjection.IServiceCollection AddGeneratedHandlers(");
        sb.AppendLine("            this global::Microsoft.Extensions.DependencyInjection.IServiceCollection services)");
        sb.AppendLine("        {");
        sb.AppendLine("            AddGeneratedRequestHandlers(services);");
        sb.AppendLine("            AddGeneratedNotificationHandlers(services);");
        sb.AppendLine("            AddGeneratedValidators(services);");
        sb.AppendLine("            AddGeneratedBehaviors(services);");
        sb.AppendLine();
        sb.AppendLine("            // Register the source-generated mediator for zero-reflection dispatch.");
        sb.AppendLine("            // Scoped: the mediator captures the resolving scope's IServiceProvider so");
        sb.AppendLine("            // handlers and behaviors resolve with correct scoped lifetimes.");
        sb.AppendLine("            services.AddScoped<global::MediatorLite.IMediator, SourceGeneratedMediator>();");
        sb.AppendLine();
        sb.AppendLine("            return services;");
        sb.AppendLine("        }");
        sb.AppendLine();

SourceGeneratedMediator is registered as the IMediator directly via plain AddScoped (not AddSingleton<ISourceGeneratedMediator, ...>). Because AddMediatorLite() only TryAdds the ThrowingMediator fallback and the container resolves the last IMediator descriptor, the generated mediator wins regardless of call order.

Diagnostic counts (useful for sanity checks in tests and startup logs):

        sb.AppendLine($"        /// <summary>Number of request handlers discovered at compile time.</summary>");
        sb.AppendLine($"        public static int RequestHandlerCount => {requestHandlers.Count};");
        sb.AppendLine();
        sb.AppendLine($"        /// <summary>Number of notification handlers discovered at compile time.</summary>");
        sb.AppendLine($"        public static int NotificationHandlerCount => {notificationHandlers.Count};");
        sb.AppendLine();
        sb.AppendLine($"        /// <summary>Number of pipeline behaviors registered at compile time (including validation behaviors).</summary>");
        sb.AppendLine($"        public static int BehaviorCount => {nonValidationBehaviorCount + requestTypesWithValidation.Count};");
        sb.AppendLine();
        sb.AppendLine($"        /// <summary>Number of validators registered at compile time.</summary>");
        sb.AppendLine($"        public static int ValidatorCount => {totalValidatorCount};");

When no handlers are discovered, GenerateEmptyRegistration emits a stub with all methods returning services unchanged and all counts equal to 0 — consumers can still safely call AddGeneratedHandlers() without an NRE.

    private static void GenerateEmptyRegistration(SourceProductionContext context)
    {
        var source = """
                     // <auto-generated />
                     #nullable enable

                     namespace MediatorLite.Generated
                     {
                         /// <summary>
                         /// Source-generated MediatorLite handler registrations.
                         /// </summary>
                         public static class MediatorLiteRegistration
                         {

Emitted SourceGeneratedMediator — typed switch + unrolled pipelines

SourceGeneratedMediator implements global::MediatorLite.IMediator directly and is registered scoped (it captures the resolving scope's IServiceProvider in a single _sp field). There are no Dictionary<Type, ...> dispatch tables, no RequestDispatcher/NotificationPublisher delegates, and no GetDispatcher/GetPublisher methods — those were deleted. Dispatch is a compile-time C# type-pattern switch over the discovered concrete types (arms emitted most-derived-first):

        sb.AppendLine("    public sealed class SourceGeneratedMediator : global::MediatorLite.IMediator");
        sb.AppendLine("    {");
        sb.AppendLine("        private readonly IServiceProvider _sp;");
        // ... ctor stores _sp ...
        sb.AppendLine("        public ValueTask<TResponse> SendAsync<TResponse>(");
        sb.AppendLine("            global::MediatorLite.IRequest<TResponse> request,");
        sb.AppendLine("            CancellationToken cancellationToken = default)");
        sb.AppendLine("        {");
        sb.AppendLine("            switch (request)");
        sb.AppendLine("            {");
        foreach (var (handler, iface) in dispatchEntries)
        {
            var safeName = GetSafeTypeName(iface.RequestType);
            sb.AppendLine($"                case {iface.RequestType} r_{safeName}:");
            sb.AppendLine("                {");
            sb.AppendLine($"                    var vt = Send_{safeName}(r_{safeName}, cancellationToken);");
            sb.AppendLine($"                    if (typeof(TResponse) == typeof({iface.ResponseType}))");
            sb.AppendLine($"                        return Unsafe.As<ValueTask<{iface.ResponseType}>, ValueTask<TResponse>>(ref vt);");
            sb.AppendLine($"                    return SlowCast<{iface.ResponseType}, TResponse>(vt);");
            sb.AppendLine("                }");
        }
        // case null: throw ArgumentNullException; default: throw InvalidOperationException

Each arm calls a fully typed Send_<SafeRequestName> instance method returning ValueTask<TConcrete>, then converts the exact result to ValueTask<TResponse> via an identity-guarded Unsafe.As (the typeof guard JIT-folds to a constant). Covariant IRequest<out T> dispatch falls back to the SlowCast reference cast. No boxing, no Task<object>.

Each request gets a private Send_<SafeRequestName> method using _sp. When diagnostics are disabled (no logging/tracing emitted) the body returns the pipeline ValueTask directly — no async state machine, no try/catch — and is marked [MethodImpl(AggressiveInlining)]. A zero-behavior request then returns the handler's ValueTask straight through; with behaviors the generator builds a nested delegate chain outside-in. When diagnostics are enabled the method becomes async ValueTask<...> and wraps the chain with the inline LogDebug / Activity emission.

        if (!needsDiagnostics)
        {
            // Fully-disabled fast path — return the pipeline ValueTask directly; no async
            // state machine, no try/catch, no diagnostic locals.
            sb.AppendLine("        [MethodImpl(MethodImplOptions.AggressiveInlining)]");
            sb.AppendLine($"        private ValueTask<{responseType}> Send_{safeName}({requestType} request, CancellationToken ct)");
            sb.AppendLine("        {");
            EmitResolutions("            ");
            sb.AppendLine($"            return {BuildPipelineExpression()};");
            sb.AppendLine("        }");
            sb.AppendLine();
            return;
        }

Where BuildPipelineExpression() yields handler.HandleAsync(request, ct) for zero behaviors, or b1.HandleAsync(request, () => b2.HandleAsync(request, () => handler.HandleAsync(request, ct), ct), ct) for the nested chain.

Inline logging + tracing emission

When observability is enabled (default), the pipeline body is wrapped in try / catch (Exception __ex) with LogDebug / ActivitySource.StartActivity calls inline. Constants for activity names are hardcoded in the generator (since the generator targets netstandard2.0 and cannot reference the runtime):

    // Mirrors constants in src/MediatorLite/Diagnostics/MediatorDiagnostics.cs.
    // Kept in sync manually because the generator project (netstandard2.0) cannot
    // reference the runtime MediatorLite assembly. If the runtime constants change,
    // update these literals to match.
    private const string ActivityNameSendRequest = "MediatorLite.Send";
    private const string ActivityNamePublishNotification = "MediatorLite.Publish";

When diagnostics are enabled the Send_<Type> method is emitted as async ValueTask<...> and the pipeline expression is wrapped in try / catch (Exception __ex) with inline LogDebug / Activity calls resolved through _sp:

        sb.AppendLine($"        private async ValueTask<{responseType}> Send_{safeName}({requestType} request, CancellationToken ct)");
        sb.AppendLine("        {");
        if (loggingEnabled)
        {
            sb.AppendLine("            var __logger = _sp.GetRequiredService<global::Microsoft.Extensions.Logging.ILogger<global::MediatorLite.IMediator>>();");
            sb.AppendLine($"            __logger.LogDebug(\"Sending request {{RequestType}}\", \"{simpleRequest}\");");
        }
        // ... tracing StartActivity + SetTag emission ...
        sb.AppendLine("            try");
        sb.AppendLine("            {");
        EmitResolutions("                ");
        sb.AppendLine($"                var result = await {BuildPipelineExpression()}.ConfigureAwait(false);");
        // ... success LogDebug, return result ...
        sb.AppendLine("            }");
        sb.AppendLine("            catch (global::System.Exception __ex)");
        // ... SetTag error + LogError + throw ...

The diagnostics-disabled path is the [MethodImpl(AggressiveInlining)] direct-return fast path shown above — no async, no try/catch, no diagnostic locals.

Notification publisher emission

GenerateUnrolledNotificationPublisher picks exactly one of GenerateSequentialNotificationExecution, GenerateParallelNotificationExecution, or GenerateStopOnFirstNotificationExecution based on the resolved strategy tuple. The emitted body is therefore branch-free at runtime — no switch on strategy, no runtime enum check. See the ResolveStrategies call:

        var notificationsByType = notifications
            .GroupBy(n => n.TypeName)
            .ToDictionary(g => g.Key, g => g.First());

        foreach (var notifGroup in handlersByNotification)
        {
            var notificationType = notifGroup.Key;
            var safeName = GetSafeTypeName(notificationType);
            var handlersForNotification = notifGroup.Value;

            notificationsByType.TryGetValue(notificationType, out var perTypeOptions);
            var (executionStrategy, errorStrategy) = ResolveStrategies(perTypeOptions, assemblyDefaults);

            GenerateUnrolledNotificationPublisher(
                sb,
                safeName,
                notificationType,
                handlersForNotification,
                executionStrategy,
                errorStrategy,
                loggingEnabled: !assemblyDefaults.LoggingDisabled,
                tracingEnabled: !assemblyDefaults.TracingDisabled);
        }

ValidationBehavior is emitted first, by design

The emission for AddGeneratedBehaviors explicitly registers ValidationBehavior<,> entries before any other behavior so they run before user-ordered behaviors:

        // Register ValidationBehavior FIRST for request types with validators
        // Register by concrete type so unrolled pipeline can resolve each behavior individually
        if (requestTypesWithValidation.Count > 0)
        {
            sb.AppendLine("            // Validation behaviors (registered first to ensure validation runs before other behaviors)");
            foreach (var (requestType, responseType) in requestTypesWithValidation)
            {
                sb.AppendLine($"            services.AddTransient<global::MediatorLite.Validation.ValidationBehavior<{requestType}, {responseType}>>();");
            }
            sb.AppendLine();
        }

DataAnnotationsValidator auto-registration

Request types whose HasDataAnnotations flag is set (any property has a [ValidationAttribute]-derived attribute) are registered with the library's open generic DataAnnotationsValidator<T>:

        // Register DataAnnotationsValidator for request types with DataAnnotation attributes
        var requestTypesWithDataAnnotations = requestHandlers
            .Where(rh => rh.Interface.HasDataAnnotations)
            .Select(rh => rh.Interface.RequestType)
            .Distinct()
            .ToList();

        if (requestTypesWithDataAnnotations.Count > 0)
        {
            foreach (var requestType in requestTypesWithDataAnnotations)
            {
                sb.AppendLine($"            services.AddTransient<global::MediatorLite.Validation.IValidator<{requestType}>, global::MediatorLite.Validation.DataAnnotationsValidator<{requestType}>>();");
            }
        }

HasDataAnnotationAttributes walks all properties and their attributes, climbing the inheritance chain to detect System.ComponentModel.DataAnnotations.ValidationAttribute:

    private static bool HasDataAnnotationAttributes(ITypeSymbol typeSymbol)
    {
        foreach (var member in typeSymbol.GetMembers())
        {
            if (member is IPropertySymbol property)
            {
                foreach (var attr in property.GetAttributes())
                {
                    if (IsValidationAttribute(attr.AttributeClass))
                        return true;
                }
            }
        }
        return false;
    }

    /// <summary>
    /// Checks whether an attribute type inherits from System.ComponentModel.DataAnnotations.ValidationAttribute.
    /// </summary>
    private static bool IsValidationAttribute(INamedTypeSymbol? attributeType)
    {
        var current = attributeType;
        while (current != null)
        {
            if (current.ToDisplayString() == "System.ComponentModel.DataAnnotations.ValidationAttribute")
                return true;
            current = current.BaseType;
        }
        return false;
    }

Emitted file structure summary

Two generated files per compilation with handlers:

• MediatorLiteRegistration.g.cs — namespace MediatorLite.Generated { static class MediatorLiteRegistration { AddGeneratedHandlers, AddGeneratedRequestHandlers, AddGeneratedNotificationHandlers, AddGeneratedValidators, AddGeneratedBehaviors, RequestHandlerCount, NotificationHandlerCount, BehaviorCount, ValidatorCount } }
• SourceGeneratedMediator.g.cs — namespace MediatorLite.Generated { sealed class SourceGeneratedMediator : global::MediatorLite.IMediator { _sp ctor, SendAsync switch, PublishAsync switch, SlowCast, Send_<Type>..., Publish_<Type>... } }

Patterns & invariants

Do:

• Keep any new constant that the generator emits in sync with the runtime equivalent (the generator cannot reference MediatorLite.dll; see the comment above ActivityNameSendRequest).
• Emit the dispatch arms most-derived-first so derived types match before their bases.
• Emit [MethodImpl(MethodImplOptions.AggressiveInlining)] on the diagnostics-disabled fast-path Send_<Type> method (the one that returns the pipeline ValueTask directly without an async state machine).
• Register handlers, behaviors, and validators with AddTransient. SourceGeneratedMediator itself is registered AddScoped<IMediator, SourceGeneratedMediator> because it captures the resolving scope's IServiceProvider.
• Register each behavior by its concrete type (not only by IPipelineBehavior<,>) so the unrolled pipeline can GetRequiredService<T>() it individually.
• Resolve strategies via ResolveStrategies (precedence: per-type → assembly → library default 0).

Don't:

• Don't emit reflection, Activator.CreateInstance, or late-bound dispatch — the whole point is compile-time elimination.
• Don't discover open generic validators — they are explicitly skipped (classSymbol.IsGenericType in GetValidatorInfo).
• Don't add runtime switches on NotificationExecutionStrategy — the emitted body must have exactly one code path.
• Don't reference the MediatorLite runtime from this project (it targets netstandard2.0).

Common tasks

1. Inspect the generated code

   1. In the consumer .csproj add:

      <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
      <CompilerGeneratedFilesOutputPath>$(BaseIntermediateOutputPath)\generated</CompilerGeneratedFilesOutputPath>
      

   2. Build; the generated files land in obj/Debug/netX/generated/MediatorLite.SourceGeneration/MediatorLite.SourceGeneration.HandlerDiscoveryGenerator/.
   3. Alternatively, check MediatorLiteRegistration.RequestHandlerCount at startup — 0 indicates the generator did not discover any handlers in the compilation.

2. Add a new candidate kind (e.g. a new IMyHandler<T>)

   1. Add an Is<Kind>Candidate predicate and Get<Kind>Info transform in the generator, paralleling IsHandlerCandidate / GetHandlerInfo.
   2. Add a new pipeline in Initialize and combine it into the compilationAndData tuple.
   3. Update Execute to call a new Generate<Kind>... emitter.
   4. Update MediatorLiteRegistration to expose an AddGenerated<Kind> method and a <Kind>Count constant.

3. Change how behaviors compose

   1. Edit ExpandBehaviors for how open generic behaviors produce closed types per (request, response) pair.
   2. Edit GenerateUnrolledPipeline.EmitPipelineBody for the actual emission — note the nested delegate shape at lines 1005-1017.

4. Change compile-time strategy precedence

   1. Edit ResolveStrategies only — it is the single source of truth. GenerateUnrolledNotificationPublisher already receives the resolved tuple.

5. Debug "no handler found" in consumer

   1. Confirm the class is public + non-abstract + implements the open interface definition (fully qualified).
   2. Confirm the consumer project adds the analyzer reference with OutputItemType="Analyzer" ReferenceOutputAssembly="false".
   3. Confirm the handler is not decorated with .

Pitfalls & gotchas

• [MediatorGeneration(Skip = true)] is ignored by GetHandlerInfo, GetBehaviorInfo, and GetValidatorInfo — discovery is unconditional. The attribute type stays in Attributes.cs (marked [Obsolete]) for binary compatibility only; remove any usage during migrations.
• Validator open generics are excluded. If you implement MyValidator<T> : IValidator<T>, it will not be auto-registered. Register it manually or make it closed.
• HasDataAnnotationAttributes only scans direct property attributes, not fields or nested object properties. If you need nested validation, wire a custom IValidator<T>.
• Dispatcher keys use typeof(requestType) with the runtime type from Mediator.SendAsync — abstract/base class requests are not registered, only closed concrete types implementing IRequest<TResponse> (for which a handler exists).
• String-based attribute matching: the generator compares attr.AttributeClass?.Name against literals like "NotificationExecutionAttribute". Renaming the attribute types in Attributes.cs breaks the generator.
• Abstract classes are filtered out (classSymbol.IsAbstract). Keep handlers concrete.
• Generator targets netstandard2.0. You cannot use collection expressions, ranges, ArgumentNullException.ThrowIfNull, etc., in generator code — use explicit C# 8-compatible syntax.
• ActivityNameSendRequest / ActivityNamePublishNotification must be kept in sync with MediatorActivitySource.ActivityNames.* in MediatorDiagnostics.cs.
• Logging category hardcoded: emitted code uses ILogger<global::MediatorLite.IMediator> — the filter key is always MediatorLite.IMediator.

Related skills & rules

• mediatorlite-abstractions — source of the interfaces (IRequestHandler<,>, INotificationHandler<>, IPipelineBehavior<,>, IValidator<T>, INotification) and attributes (BehaviorOrderAttribute, NotificationExecutionAttribute, NotificationErrorAttribute, DefaultNotificationExecutionAttribute, DefaultNotificationErrorAttribute, DisableMediatorLoggingAttribute, DisableMediatorTracingAttribute, MediatorGenerationAttribute) that this generator reads.
• mediatorlite-core — hosts the ThrowingMediator diagnostic fallback (the real IMediator is the generated SourceGeneratedMediator); ValidationBehavior and DataAnnotationsValidator are runtime types this generator references by fully-qualified name.
• mediatorlite-tests — tests/MediatorLite.Tests/SourceGeneration/* tests verify the generated dispatch, pipeline composition, notification strategies, and *Count properties.
• mediatorlite-sample-sourcegen — canonical consumer wiring demonstrating AddGeneratedHandlers + AddMediatorLite.
• AGENTS.md: "Source-generation entry point is HandlerDiscoveryGenerator.cs; generated diagnostics surface as MediatorLite.Generated.MediatorLiteRegistration".
• Docs: docs/notifications.md, docs/observability.md, src/MediatorLite.SourceGeneration/README.md.