Offside

MediatR integration

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Offside.MediatR connects failed Result values to MediatR notifications without adding MediatR to the Offside core package. A notification always carries one Error; it is not a domain event describing a state change.

Install and register

dotnet add package Offside
dotnet add package Offside.MediatR

Configure MediatR first, then the integration:

using Offside.MediatR;

builder.Services.AddMediatR(configuration =>
    configuration.RegisterServicesFromAssemblyContaining<Program>());
builder.Services.AddOffsideMediatR();

AddOffsideMediatR is idempotent. It registers a scoped collector and its notification handler, but it does not call AddMediatR, register IPublisher, or configure a MediatR license.

You do not need to scan the Offside.MediatR assembly. If it is scanned, the collector protects a single notification instance from duplicate handler registration.

Publish a result

Inject IPublisher, then publish at the application boundary:

public async Task<Result> Cancel(string id, CancellationToken cancellationToken)
{
    var result = _orders.Cancel(id);
    return await result.PublishDomainNotificationsAsync(_publisher, cancellationToken);
}

The generic overload returns the same Result<T>:

Result<Order> result = _orders.Get(id);
return await result.PublishDomainNotificationsAsync(_publisher, cancellationToken);

Success publishes nothing. Failure publishes one DomainNotification per error, sequentially and in result order. With E errors and H handlers, the upper-bound dispatch work is E × H handler executions.

Read the collector

Inject IDomainNotificationCollector in the same dependency-injection scope:

if (collector.HasNotifications)
    return collector.ToResult();

return collector.ToResult(order);

Errors returns an independent snapshot. ToResult() returns success when empty and a failure with the collected errors otherwise. ToResult<T>(value) returns the supplied value only when the collector is empty.

Reads never remove notifications; there is no Clear. Create one scope per logical operation:

await using var scope = scopeFactory.CreateAsyncScope();
var worker = scope.ServiceProvider.GetRequiredService<OrderWorker>();
await worker.Process(message, stoppingToken);

ASP.NET Core already creates one scope per request. Workers must create a new scope per message or job; reusing a long-lived scope also retains old errors.

Ordering, concurrency, and failures

• The Result extensions guarantee error order because they await each publish before starting the next.
• Concurrent external publishes are thread-safe, but their relative order is unspecified.
• Handler exceptions and cancellation stop publication and propagate to the caller.
• Handlers for earlier errors — and handlers earlier in the current MediatR strategy — may already have run. There is no rollback.
• Retrying the extension creates new notifications and can repeat side effects. Make handlers idempotent when retries are possible.

Every handler receives the complete Error, including Arguments and diagnostic data on Unexpected. HTTP sanitization does not run in this path. Never place secrets in an error.

MediatR versions and licensing

The package supports MediatR 12.0.1 through 14.x and is tested against 12.0.1, 13.1.0, and 14.2.0. The NuGet dependency range is [12.0.1,15.0.0).

MediatR 13 introduced a license key and changed its upstream licensing model. Hosts using 13 or 14 must configure logging and evaluate the applicable upstream license. Configure a key through MediatR itself; Offside does not accept a key or suppress license warnings. See the MediatR 13 release and official licensing page.

Deliberate omissions

• No automatic pipeline behavior: publication is visible at the call site.
• No Clear: the scope owns the collector lifetime.
• No retry or transaction abstraction: those policies belong to the application.

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