De-duping notification handlers before dispatching

[jbogard]

Fix: Duplicate Notification Handler Invocation (Issue #1118)

Context

When a derived notification type (E2 : E1) exists alongside a handler for the base type (C1 : INotificationHandler<E1>) and a handler for the derived type (C2 : INotificationHandler<E2>), publishing E2 incorrectly calls C1 twice with DI containers that support variance (e.g. DryIoc).

Root cause: INotificationHandler<in TNotification> is contravariant. MediatR's ServiceRegistrar registers C1 for both INotificationHandler<E1> AND INotificationHandler<E2> (via CanBeCastTo / IsAssignableFrom). This is intentional — it allows MSDI (which does not support variance) to dispatch C1 when publishing E2. However, DI containers that do support variance (like DryIoc) additionally return C1 from their own contravariant resolution of INotificationHandler<E1>, yielding C1 twice.

Why a registration-only fix doesn't work: Removing C1's INotificationHandler<E2> registration breaks MSDI users — C1 would never be called when publishing E2 with MSDI.

Fix

Deduplicate by handler type in NotificationHandlerWrapperImpl<TNotification>.Handle() before building the executor list. Each handler class is then invoked at most once, regardless of how many times the DI container returns it.

src/MediatR/Wrappers/NotificationHandlerWrapper.cs:

var handlers = serviceFactory
    .GetServices<INotificationHandler<TNotification>>()
    .GroupBy(static x => x.GetType())   // deduplicate
    .Select(static g => g.First())
    .Select(static x => new NotificationHandlerExecutor(...));

ServiceRegistrar.cs is unchanged.

Tests

test/MediatR.Tests/MicrosoftExtensionsDI/Issue1118Tests.cs (new file):

• Publishing_BaseEvent_Should_Call_BaseEventHandler_Once
• Publishing_DerivedEvent_Should_Call_BaseEventHandler_ExactlyOnce
• Publishing_DerivedEvent_Should_Call_DerivedEventHandler_Once

Handlers use IServiceProvider (always injectable) to soft-resolve CallLog, so PipelineTests (which uses ValidateOnBuild=true on the same assembly) does not fail.

Verification

dotnet test test/MediatR.Tests/MediatR.Tests.csproj
# 169 passed, 0 failed

[Copilot]

Pull request overview

This PR fixes issue #1118 where notification handlers were being called multiple times when using DI containers that support variance (e.g., DryIoc). When a derived notification type (E2 : E1) exists alongside handlers for both base (C1 : INotificationHandler) and derived types (C2 : INotificationHandler), publishing E2 would incorrectly invoke C1 twice due to the contravariant interface combined with MediatR's intentional dual registration for MSDI compatibility.

Changes:

• Added deduplication logic in NotificationHandlerWrapper.cs using GroupBy(x => x.GetType()) pattern to ensure each handler type is invoked only once
• Added comprehensive regression tests in Issue1118Tests.cs to verify handlers are called exactly once for both base and derived events
• Minor whitespace cleanup in ServiceRegistrar.cs

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
src/MediatR/Wrappers/NotificationHandlerWrapper.cs	Added deduplication logic using GroupBy/Select pattern to prevent duplicate handler invocations
test/MediatR.Tests/MicrosoftExtensionsDI/Issue1118Tests.cs	New regression tests verifying handlers are called exactly once for base and derived notification events
src/MediatR/Registration/ServiceRegistrar.cs	Removed trailing whitespace (formatting only)

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