mediatorlite-abstractions

name	mediatorlite-abstractions
description	Reference for the MediatorLite.Abstractions project -- IMediator (ValueTask dispatch), IRequest, IRequestHandler, INotification, INotificationHandler, IPipelineBehavior, Unit, IValidator, validation models, and every attribute in Attributes.cs (NotificationHandlerOrder, NotificationExecution, NotificationError, DefaultNotificationExecution, DefaultNotificationError, BehaviorOrder, MediatorGeneration [obsolete], DisableMediatorLogging, DisableMediatorTracing). Use when editing contracts, adding new abstractions, or understanding the public surface consumers implement.
triggers	IMediator, IRequest, IRequestHandler, INotification, INotificationHandler, IPipelineBehavior, Unit, mediator abstractions, notification attributes, behavior order, NotificationExecutionAttribute, NotificationErrorAttribute, DisableMediatorLogging, DisableMediatorTracing, ValidationException, IValidator, ValueTask dispatch, SourceGeneratedMediator, typed switch dispatch

MediatorLite.Abstractions

⚠️ Partially STALE. The in-house IValidator<T> and ValidationResult were removed; validation is now FluentValidation-based. Only ValidationException and ValidationError remain in the abstractions as the error contract. See mediatorlite-validation and docs/validation.md.

Purpose

MediatorLite.Abstractions is the public contract layer of MediatorLite. It contains every interface, attribute, delegate, and validation model that consumers implement or that the source generator and runtime depend on. The project deliberately has no runtime code, no DI dependencies, and no reflection — only attributes, interfaces, and small value types. This means the abstractions can be referenced from analyzers, source generators, and consumer libraries without pulling in Microsoft.Extensions.*.

When to use

• Adding or modifying a core contract (IMediator, IRequest<T>, IRequestHandler<,>, INotification, INotificationHandler<>, IPipelineBehavior<,>).
• Adding, deprecating, or tuning a compile-time attribute consumed by HandlerDiscoveryGenerator.
• Understanding how the generated SourceGeneratedMediator implements IMediator directly via a compile-time typed switch.
• Evolving the Unit struct, the validation surface (IValidator<T>, ValidationResult, ValidationError, ValidationException), or strategy enums.

Project location & entry points

• MediatorLite.Abstractions.csproj
• Core interfaces folder: src/MediatorLite.Abstractions/Abstractions/
  • IMediator.cs
  • IRequest.cs
  • IRequestHandler.cs
  • INotification.cs
  • INotificationHandler.cs
  • IPipelineBehavior.cs
  • Unit.cs
  • Attributes.cs
• Validation folder: src/MediatorLite.Abstractions/Validation/
  • IValidator.cs
  • ValidationException.cs
  • Models/ValidationResult.cs
  • Models/ValidationError.cs

Core types / API surface

IMediator — the public mediator contract

The public mediator surface is ValueTask-based end-to-end (ValueTask<TResponse> for SendAsync, ValueTask for PublishAsync) so a synchronously completing handler with no behaviors allocates nothing. A ValueTask must be consumed exactly once — await it directly; for Task.WhenAll, fan-out, or storing the result, convert it first with .AsTask(). The generated SourceGeneratedMediator (namespace MediatorLite.Generated) implements this interface directly.

public interface IMediator
{
    /// <summary>
    /// Sends a request to a single handler and returns the response.
    /// </summary>
    /// <returns>A <see cref="ValueTask{TResponse}"/> representing the response from the handler.</returns>
    /// <exception cref="InvalidOperationException">Thrown when no handler is registered for the request type.</exception>
    ValueTask<TResponse> SendAsync<TResponse>(
        IRequest<TResponse> request,
        CancellationToken cancellationToken = default);

    /// <summary>
    /// Publishes a notification to all registered handlers.
    /// </summary>
    ValueTask PublishAsync<TNotification>(
        TNotification notification,
        CancellationToken cancellationToken = default)
        where TNotification : INotification;
}

Requests and the IRequest / IRequest<TResponse> split

IRequest is a convenience interface equivalent to IRequest<Unit> — use it for commands with no meaningful return value.

public interface IRequest<out TResponse>;

/// <summary>
/// Marker interface for requests that don't return a meaningful response.
/// </summary>
public interface IRequest : IRequest<Unit>;

Request handlers (IRequestHandler<,> + void convenience overload)

Handlers return ValueTask<TResponse>. The IRequestHandler<TRequest> overload lets a command handler return ValueTask directly; explicit interface implementation adapts it to Unit.

public interface IRequestHandler<in TRequest, TResponse>
    where TRequest : IRequest<TResponse>
{
    /// <summary>
    /// Handles a request asynchronously.
    /// </summary>
    /// <param name="request">The request to handle.</param>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    /// <returns>A <see cref="ValueTask{TResponse}"/> representing the asynchronous operation with the response.</returns>
    ValueTask<TResponse> HandleAsync(TRequest request, CancellationToken cancellationToken = default);
}

public interface IRequestHandler<in TRequest> : IRequestHandler<TRequest, Unit>
    where TRequest : IRequest<Unit>
{
    new ValueTask HandleAsync(TRequest request, CancellationToken cancellationToken = default);

    /// <summary>
    /// Explicit implementation that wraps the void HandleAsync to return Unit.
    /// </summary>
    async ValueTask<Unit> IRequestHandler<TRequest, Unit>.HandleAsync(TRequest request, CancellationToken cancellationToken)
    {
        await HandleAsync(request, cancellationToken);
        return Unit.Value;
    }
}

Notifications

Notifications are fire-and-forget pub/sub: zero or more handlers, no return value.

public interface INotification;

public interface INotificationHandler<in TNotification>
    where TNotification : INotification
{
    /// <summary>
    /// Handles a notification asynchronously.
    /// </summary>
    /// <param name="notification">The notification to handle.</param>
    /// <param name="cancellationToken">Cancellation token for the operation.</param>
    /// <returns>A <see cref="ValueTask"/> representing the asynchronous operation.</returns>
    ValueTask HandleAsync(TNotification notification, CancellationToken cancellationToken = default);
}

Pipeline behavior + RequestHandlerDelegate<TResponse>

namespace MediatorLite;

/// <summary>
/// Delegate representing the next handler or behavior in the request pipeline.
/// </summary>
/// <typeparam name="TResponse">The type of response from the handler.</typeparam>
/// <returns>A <see cref="ValueTask{TResponse}"/> representing the response.</returns>
public delegate ValueTask<TResponse> RequestHandlerDelegate<TResponse>();

public interface IPipelineBehavior<in TRequest, TResponse>
    where TRequest : IRequest<TResponse>
{
    /// <summary>
    /// Handles the request by optionally performing work before/after invoking the next handler.
    /// </summary>
    ValueTask<TResponse> HandleAsync(
        TRequest request,
        RequestHandlerDelegate<TResponse> next,
        CancellationToken cancellationToken = default);
}

A behavior short-circuits the pipeline by not calling next(). Order is controlled with [BehaviorOrder(int)] — lower values run first.

Dispatch is the generated SourceGeneratedMediator — no separate contract

There is no ISourceGeneratedMediator interface, RequestDispatcher / NotificationPublisher delegate, or Dictionary<Type, ...> dispatch table in v2 — they were all deleted. The source generator emits a single SourceGeneratedMediator : global::MediatorLite.IMediator (namespace MediatorLite.Generated) that implements IMediator directly.

Dispatch is a compile-time C# type-pattern switch over the concrete request/notification types (arms emitted most-derived-first), each arm calling a fully typed per-request method:

// Emitted shape (namespace MediatorLite.Generated)
public sealed class SourceGeneratedMediator : global::MediatorLite.IMediator
{
    private readonly IServiceProvider _sp;
    public SourceGeneratedMediator(IServiceProvider serviceProvider) => _sp = serviceProvider;

    public ValueTask<TResponse> SendAsync<TResponse>(IRequest<TResponse> request, CancellationToken ct = default)
    {
        switch (request)
        {
            case MyQuery r:
            {
                var vt = Send_MyQuery(r, ct);            // ValueTask<MyResult>
                if (typeof(TResponse) == typeof(MyResult))
                    return Unsafe.As<ValueTask<MyResult>, ValueTask<TResponse>>(ref vt);
                return SlowCast<MyResult, TResponse>(vt); // covariant IRequest<out T> fallback
            }
            case null: throw new ArgumentNullException(nameof(request));
            default:   throw new InvalidOperationException(/* no handler */);
        }
    }
    // ...PublishAsync switch, Send_<Type>/Publish_<Type> methods using _sp...
}

Key properties of the generated dispatch:

• No boxing. The exact-type ValueTask<TConcrete> result is converted to ValueTask<TResponse> via an identity-guarded System.Runtime.CompilerServices.Unsafe.As (the typeof guard JIT-folds to a constant, so the reinterpret is free). Value-type responses stay typed — there is no Task<object>. v1 boxed every value-type response through Task<object>; v2 eliminated that entirely.
• SlowCast fallback. Covariant IRequest<out T> dispatch (where TResponse is a wider reference type than the concrete response) falls back to a SlowCast reference cast — still no value-type boxing.
• Per-request methods are Send_<SafeType> (instance methods returning ValueTask<TResponse>, using the _sp IServiceProvider field). A zero-behavior request with diagnostics disabled returns the handler's ValueTask directly — no async state machine.
• Publishers are Publish_<SafeType> returning ValueTask. The notification switch matches the runtime type, so base/interface-typed publishes dispatch correctly (v1's typeof(TNotification) dictionary lookup silently no-oped for those).

Unit

public readonly record struct Unit : IEquatable<Unit>, IComparable<Unit>
{
    /// <summary>
    /// Gets the singleton <see cref="Unit"/> value.
    /// </summary>
    public static readonly Unit Value = default;

    /// <summary>
    /// Returns a completed <see cref="ValueTask{Unit}"/> with the default <see cref="Unit"/> value.
    /// </summary>
    public static ValueTask<Unit> CompletedTask { get; } = ValueTask.FromResult(Value);

    /// <summary>
    /// Compares this instance with another <see cref="Unit"/> instance.
    /// All <see cref="Unit"/> instances are considered equal.
    /// </summary>
    public int CompareTo(Unit other) => 0;

    /// <summary>
    /// Returns a string representation of this <see cref="Unit"/> instance.
    /// </summary>
    /// <returns>The string "()".</returns>
    public override string ToString() => "()";
}

Validation surface

IValidator<in TRequest> is the pluggable contract; every discovered implementation is registered automatically by the source generator.

public interface IValidator<in TRequest>
{
    /// <summary>
    /// Validates the specified request.
    /// </summary>
    ValueTask<ValidationResult> ValidateAsync(TRequest request, CancellationToken cancellationToken = default);
}

ValidationResult is immutable (private init setters) with a cached Success instance and two Failure factories:

public sealed class ValidationResult
{
    public static ValidationResult Success { get; } = new() { IsValid = true };
    public bool IsValid { get; private init; }
    public IReadOnlyList<ValidationError> Errors { get; private init; } = [];
    public static ValidationResult Failure(params ValidationError[] errors)
    {
        return new ValidationResult
        {
            IsValid = false,
            Errors = errors
        };
    }
    public static ValidationResult Failure(IEnumerable<ValidationError> errors)
    {
        return new ValidationResult
        {
            IsValid = false,
            Errors = errors.ToList()
        };
    }
}

ValidationError is a record with a nullable AttemptedValue:

public sealed record ValidationError(
    string PropertyName,
    string ErrorMessage,
    object? AttemptedValue = null);

ValidationException builds a human-readable message from the errors and exposes Errors:

public sealed class ValidationException : Exception
{
    public IReadOnlyList<ValidationError> Errors { get; }

    public ValidationException(IEnumerable<ValidationError> errors)
        : this([.. errors])
    {
    }

    private ValidationException(IReadOnlyList<ValidationError> errors)
        : base(BuildMessage(errors))
    {
        Errors = errors;
    }

    public ValidationException(string message, IEnumerable<ValidationError> errors)
        : base(message)
    {
        Errors = errors.ToList();
    }

Attributes (all in Attributes.cs)

Notification strategies (enums)

public enum NotificationExecutionStrategy
{
    /// <summary>
    /// Execute handlers one after another in order.
    /// This is the default strategy.
    /// </summary>
    Sequential = 0,

    /// <summary>
    /// Execute all handlers concurrently using Task.WhenAll.
    /// </summary>
    Parallel = 1,

    /// <summary>
    /// Stop execution after the first handler completes successfully.
    /// </summary>
    ...
    StopOnFirst = 2
}

public enum NotificationErrorStrategy
{
    /// <summary>
    /// Stop execution immediately when a handler throws an exception.
    /// </summary>
    StopOnFirstError = 0,

    /// <summary>
    /// Continue executing all handlers even if some throw exceptions.
    /// All exceptions are collected and thrown as an <see cref="AggregateException"/>.
    /// </summary>
    ...
    ContinueAndAggregate = 1
}

NotificationHandlerOrderAttribute — handler execution order (class target)

[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
public sealed class NotificationHandlerOrderAttribute(int order) : Attribute
{
    /// <summary>
    /// Gets the execution order. Lower values execute first.
    /// </summary>
    public int Order { get; } = order;
}

NotificationExecutionAttribute / NotificationErrorAttribute — per-type compile-time strategy

[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
public sealed class NotificationExecutionAttribute(NotificationExecutionStrategy strategy) : Attribute
{
    /// <summary>
    /// Gets the execution strategy for this notification type.
    /// </summary>
    public NotificationExecutionStrategy Strategy { get; } = strategy;
}

[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
public sealed class NotificationErrorAttribute(NotificationErrorStrategy strategy) : Attribute
{
    /// <summary>
    /// Gets the error handling strategy for this notification type.
    /// </summary>
    public NotificationErrorStrategy Strategy { get; } = strategy;
}

Both attributes are compile-time only; the source generator inlines the resolved strategy into each generated Publish_* method.

DefaultNotificationExecutionAttribute / DefaultNotificationErrorAttribute — assembly-wide defaults

[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = false, Inherited = false)]
public sealed class DefaultNotificationExecutionAttribute(NotificationExecutionStrategy strategy) : Attribute
{
    /// <summary>
    /// Gets the assembly-wide default execution strategy.
    /// </summary>
    public NotificationExecutionStrategy Strategy { get; } = strategy;
}

[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = false, Inherited = false)]
public sealed class DefaultNotificationErrorAttribute(NotificationErrorStrategy strategy) : Attribute
{
    /// <summary>
    /// Gets the assembly-wide default error handling strategy.
    /// </summary>
    public NotificationErrorStrategy Strategy { get; } = strategy;
}

Resolution order (see HandlerDiscoveryGenerator.cs ResolveStrategies): per-notification attribute → assembly default → library default (Sequential for execution, StopOnFirstError for error).

BehaviorOrderAttribute — pipeline order

[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
public sealed class BehaviorOrderAttribute(int order) : Attribute
{
    /// <summary>
    /// Gets the execution order. Lower values execute first.
    /// </summary>
    public int Order { get; } = order;
}

Validation behaviors are emitted before any other ordered behavior for validated request types (see HandlerDiscoveryGenerator.Execute).

Obsolete: MediatorGenerationAttribute

[Obsolete("This attribute is no longer valid with the complete source generator implementation.")]
[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
public sealed class MediatorGenerationAttribute : Attribute
{
    /// <summary>
    /// Gets or sets whether to skip source generation for this type.
    /// Default is false.
    /// </summary>
    public bool Skip { get; set; }
}

The generator ignores [MediatorGeneration(Skip = true)]; discovery in GetHandlerInfo, GetBehaviorInfo, and GetValidatorInfo is unconditional. The attribute type is kept for binary compatibility but has no effect — do not use it.

Observability opt-outs (assembly-level, no-arg)

[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = false, Inherited = false)]
public sealed class DisableMediatorLoggingAttribute : Attribute { }

[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = false, Inherited = false)]
public sealed class DisableMediatorTracingAttribute : Attribute { }

When either attribute is applied at assembly level, the generator omits the corresponding ILogger<IMediator>.LogDebug(...) calls or ActivitySource.StartActivity(...) calls from every emitted Send_* / Publish_* method. These are compile-time switches; there is no runtime toggle.

Patterns & invariants

Do:

• Implement IRequest<T> / IRequest on records or classes; use records for value-semantics equality in CQRS.
• Use ValueTask in handlers and behaviors. Return ValueTask.CompletedTask or Unit.CompletedTask for synchronous paths.
• Mark handlers public and concrete (no abstract) — the generator skips abstract classes (see GetHandlerInfo).
• Put [NotificationHandlerOrder] on the handler class, not the notification class.
• Put [NotificationExecution] / [NotificationError] on the notification class, not the handler.
• Apply [assembly: DisableMediatorLogging] / [assembly: DisableMediatorTracing] in benchmark / perf-sensitive projects (see tests/MediatorLite.Benchmarks/AssemblyInfo.cs and tests/MediatorLite.RestApiBenchmarks/AssemblyInfo.cs).

Don't:

• Don't keep the MediatorGenerationAttribute — it is obsolete.
• Don't change IMediator's public signature without also updating the generator emission in HandlerDiscoveryGenerator.cs.
• Don't add runtime logic to MediatorLite.Abstractions. It must stay dependency-free; behaviors and validation runtime live in src/MediatorLite/.
• Don't expect a reflection fallback — there has never been one in v2. Without AddGeneratedHandlers() the only IMediator in the container is the ThrowingMediator diagnostic fallback, which throws on every dispatch.

Common tasks

1. Add a new request contract

   1. Create public sealed record MyQuery(...) : IRequest<MyResponse>; in the consumer project.
   2. Create a handler implementing IRequestHandler<MyQuery, MyResponse> with a ValueTask<MyResponse> HandleAsync(...) method.
   3. No attribute needed — the generator discovers it via AllInterfaces matching MediatorLite.IRequestHandler<TRequest, TResponse>.

2. Add a compile-time notification strategy

   1. Put [NotificationExecution(NotificationExecutionStrategy.Parallel)] and optionally [NotificationError(NotificationErrorStrategy.ContinueAndAggregate)] on the INotification record.
   2. The generator inlines the chosen strategy into the emitted Publish_* method — no registration call needed.

3. Set an assembly-wide notification default

   1. In any source file (typically AssemblyInfo.cs), add [assembly: DefaultNotificationExecution(NotificationExecutionStrategy.Sequential)] and/or [assembly: DefaultNotificationError(NotificationErrorStrategy.ContinueAndAggregate)].

4. Add a new attribute that the generator should read

   1. Define it in Attributes.cs.
   2. Teach the generator to read it via attr.AttributeClass?.Name == "MyAttribute" inside GetHandlerInfo / GetNotificationInfo / GetBehaviorInfo / GetValidatorInfo / GetAssemblyDefaults.
   3. Add a unit test to MediatorLite.Tests/UnitTests/AttributeTests.cs.

5. Add a custom validator

   1. Implement IValidator<MyCommand> in the consumer project (non-generic — the generator filters open generics out of validator discovery).
   2. The generator registers it via AddGeneratedValidators and inserts ValidationBehavior<MyCommand, TResponse> first in the emitted pipeline.

Pitfalls & gotchas

• No response boxing: responses stay typed (ValueTask<TResponse>) end-to-end through the generated typed switch; there is no Task<object> and no value-type boxing. (v1 boxed; v2 does not.) See docs/benchmarks.md.
• IRequest : IRequest<Unit> double-dispatch: When you implement IRequestHandler<TRequest> (void overload), the explicit interface method must not be overridden — the auto-generated IRequestHandler<TRequest, Unit>.HandleAsync calls your void overload and returns Unit.Value.
• [MediatorGeneration(Skip = true)] silently skips everything: the generator drops the handler/behavior/validator from all outputs, not just DI registration. Don't use it.
• Assembly-level opt-out attributes affect the whole compilation unit, not just a specific file. If your library is consumed downstream, the consumer may want logging back on — consider exposing a compile-time configuration alternative.
• [NotificationExecution] / [NotificationError] on a handler has no effect — they target the notification class. The generator's GetNotificationInfo only reads them off INotification implementations.
• DefaultNotificationExecutionAttribute must match the notification assembly. The generator reads compilation.Assembly.GetAttributes(), so the default must live in the same assembly as the notification types (or any assembly participating in the same compilation).
• Null guarding: the generated SendAsync / PublishAsync switches have a case null: arm that throws ArgumentNullException before any handler resolution (see the generated SourceGeneratedMediator.g.cs).

Related skills & rules

• mediatorlite-core — how ServiceCollectionExtensions, the ThrowingMediator diagnostic fallback, and validation runtime consume these abstractions (the IMediator implementation itself is generated, not hand-written).
• mediatorlite-source-generation — how HandlerDiscoveryGenerator reads every attribute and interface defined here and emits the SourceGeneratedMediator (implementing IMediator) in the MediatorLite.Generated namespace.
• mediatorlite-tests — tests/MediatorLite.Tests/UnitTests/AttributeTests.cs and UnitTests/ValidationTests.cs give direct coverage of the types in this project.
• Workspace rule: AGENTS.md — always use IRequest<Unit> for commands and keep changes aligned with the generated SourceGeneratedMediator.
• Relevant docs: docs/notifications.md, docs/validation.md, docs/pipeline-behaviors.md, docs/observability.md.