Rig.TUnit.Mediator 0.1.0-beta.2

Rig.TUnit.Mediator

Pipeline-inspection helpers and an in-memory IMediator fake for Martin Othamar's Mediator library.

What this package is

A thin layer over the Mediator / Mediator.Abstractions packages that lets you (a) assert the order and composition of pipeline behaviours executed for a given request, (b) replace individual handlers with NSubstitute-backed fakes without rewiring DI, and (c) capture every handled request/response pair for after-the-fact assertions.

It does not wrap MediatR — the two libraries have incompatible source generators. See ADR-005 — family-level contracts for why contract tests live at the family base.

When to use it

• Testing a CQRS slice where a command/query goes through MediatR pipeline behaviours (validation, logging, retry).
• Asserting the behaviour-chain composition is what you expect.
• Stubbing a single handler while leaving the rest real.
• Not for: testing the generated source code of Mediator.SourceGenerator — that is the library's job.

Prerequisites

• .NET 10 SDK
• Project under test references Mediator + Mediator.SourceGenerator.

Quick start

using Rig.TUnit.Mediator.Helpers;
using Mediator;

var probe = new MediatorPipelineProbe();
var mediator = probe.BuildMediator(services =>
{
    services.AddSingleton<IRequestHandler<Ping, Pong>>(
        _ => new PingHandler());
});

var pong = await mediator.Send(new Ping("hi"));
await Assert.That(probe.RequestsHandled).HasCount(1);

Options

Fixture + helper APIs

• Rig.TUnit.Mediator.Helpers.MediatorPipelineProbe — inspection
• Rig.TUnit.Mediator.Helpers.RequestSpy<TRequest, TResponse> — record calls
• Rig.TUnit.Mediator.Helpers.BehaviourOrderAssert — pipeline-order assertion

Per-test isolation

MediatorPipelineProbe is scoped — each test instantiates its own; handlers and behaviours register per-probe. No static state.

Parallelism + performance

• Probe construction: ~5 ms (one DI container per probe).
• Handler dispatch: zero overhead vs bare Mediator — the probe intercepts via a pipeline behaviour registered at position 0.
• Safe under full test parallelism.

Troubleshooting

• NoHandlerForRequest — the source generator did not discover your handler. Ensure it is public (or internal with [InternalsVisibleTo] for the test project).
• Pipeline behaviour skipped — Mediator composes behaviours in DI registration order; check services.AddTransient order.

See docs/troubleshooting.md.

Provider quirks + edge cases

• Mediator.SourceGenerator writes the dispatcher at compile time — a handler added at runtime without source-gen re-run will not be found.
• The probe's order assertion ignores framework-internal behaviours (exception interception, logging).

Benchmarks

See MediatorPipelineBenchmarks.cs; tracked in benchmarks/baseline-005.json.

Related docs

• ADR-005 — family-level contracts
• Architecture diagram
• Glossary

License

MIT. See LICENSE.