FastMediatR.Telemetry 0.1.0

FastMediatR

High-performance, AOT-friendly, MIT-licensed in-process mediator for .NET 10 with a public API that is a near drop-in replacement for MediatR.

FastMediatR uses a Roslyn IIncrementalGenerator to emit zero-reflection dispatch code at compile time. The result: faster Send/Publish, fewer allocations, and full Native-AOT/trimming compatibility.

Status: Pre-1.0 — actively under construction. Public APIs may shift until v1.0.0 is tagged.

Features

• Zero-reflection dispatch — a source generator emits a switch-based dispatcher at compile time; no MethodInfo, no dynamic, no runtime type scanning on the hot path.
• Native AOT & trimming friendly — ships IsAotCompatible=true and IsTrimmable=true; the only reflection touchpoint is a one-time startup lookup that has a fully trim-safe alternative.
• Near drop-in for MediatR — IRequest<T>, IRequestHandler<,>, INotification, INotificationHandler<>, ISender, IPublisher, IMediator, and IPipelineBehavior<,> mirror the MediatR surface (see the migration guide).
• Build-time wiring validation — diagnostics FM001 (no handler) and FM002 (duplicate handler) fail the build before you run; FM003 flags non-default handler lifetimes.
• Built-in pipeline behaviors — ValidationBehavior (FluentValidation), LoggingBehavior, and PerformanceBehavior, plus pre-/post-processors.
• Configurable notification publishing — TaskWhenAllPublisher (default, concurrent) and SequentialAwaitPublisher (ordered, fail-fast).
• Optional OpenTelemetry tracing via the FastMediatR.Telemetry package.

Quick start

Define requests and handlers:

using FastMediatR;

public sealed record CreateUserCommand(string Email, string DisplayName) : IRequest<Guid>;

public sealed class CreateUserCommandHandler : IRequestHandler<CreateUserCommand, Guid>
{
    public Task<Guid> Handle(CreateUserCommand request, CancellationToken cancellationToken)
        => Task.FromResult(Guid.NewGuid());
}

Register in Program.cs and dispatch:

using FastMediatR.Extensions.DI;

builder.Services.AddFastMediatR<Program>();

app.MapPost("/users", async (CreateUserCommand command, IMediator mediator, CancellationToken ct) =>
{
    var id = await mediator.Send(command, ct);
    return Results.Created($"/users/{id}", new { id });
});

See the usage guide for DI options, pipeline behaviors, AOT guidance, and MediatR migration.

Project layout

Quick links

• Design specification: docs/260524_1200-fastmediatr-design.md
• Implementation task breakdown: docs/260524_1230-fastmediatr-task-breakdown.md
• Usage guide (living): docs/260524_1300-fastmediatr-usage-guide.md
• Testing guide: docs/260529_1230-fastmediatr-testing-guide.md
• Security & performance review: docs/260529_1200-security-performance-review.md
• Reference sample app: samples/FastMediatR.Sample.Api
• .NET / C# coding standards: docs/DotNet.md

Build

dotnet build FastMediatR.slnx
dotnet test  FastMediatR.slnx

Requires the .NET 10 SDK (see global.json). The canonical solution file is FastMediatR.slnx.

License

MIT