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Test Publisher

The Hermodr.TestPublisher package provides an in-memory publish channel (TestEventPublishChannel) that can be used in unit and integration tests to assert on published events without requiring a real messaging transport.

Installation​

dotnet add package Hermodr.TestPublisher

Registration​

Register a test channel using one of the AddTestChannel overloads on EventPublisherBuilder.

With a Func<CloudEvent, Task> callback​

using CloudNative.CloudEvents;
using Hermodr;
using Microsoft.Extensions.DependencyInjection;

var publishedEvents = new List<CloudEvent>();

var services = new ServiceCollection();
services.AddEventPublisher()
        .AddTestChannel(async @event =>
        {
            publishedEvents.Add(@event);
            await Task.CompletedTask;
        });

var provider = services.BuildServiceProvider();

With an Action<CloudEvent> callback (synchronous)​

var publishedEvents = new List<CloudEvent>();

services.AddEventPublisher()
        .AddTestChannel(@event => publishedEvents.Add(@event));

With a custom IEventPublishCallback​

public class MyTestCallback : IEventPublishCallback
{
    public List<CloudEvent> Events { get; } = new();

    public Task OnEventPublishedAsync(CloudEvent @event)
    {
        Events.Add(@event);
        return Task.CompletedTask;
    }
}

var callback = new MyTestCallback();

services.AddEventPublisher()
        .AddTestChannel(callback);

Usage in xUnit​

using CloudNative.CloudEvents;
using Hermodr;
using Microsoft.Extensions.DependencyInjection;
using Xunit;

public class OrderServiceTests
{
    private readonly EventPublisher _publisher;
    private readonly List<CloudEvent> _published = new();

    public OrderServiceTests()
    {
        var services = new ServiceCollection();
        services.AddEventPublisher()
                .AddTestChannel(@event => _published.Add(@event));
        services.AddTransient<OrderService>();

        var provider = services.BuildServiceProvider();
        _publisher = provider.GetRequiredService<EventPublisher>();
    }

    [Fact]
    public async Task PlaceOrder_PublishesOrderPlacedEvent()
    {
        var service = new OrderService(_publisher);

        await service.PlaceOrderAsync(Guid.NewGuid(), 99.95m, "USD");

        Assert.Single(_published);
        var @event = _published[0];
        Assert.Equal("order.placed", @event.Type);
        Assert.Equal("USD", ((OrderPlacedData)@event.Data!).Currency);
    }
}

Named test channels​

When the code under test publishes to a named channel, register the test channel with the same name by passing the optional channelName parameter:

var ordersEvents       = new List<CloudEvent>();
var notificationEvents = new List<CloudEvent>();

services.AddEventPublisher()
        .AddTestChannel(@ev => ordersEvents.Add(@ev),       channelName: "rabbit-orders")
        .AddTestChannel(@ev => notificationEvents.Add(@ev), channelName: "rabbit-notifications");

Each named test channel fires only for events whose per-call options carry the matching ChannelName. A test channel registered without a name (or with channelName: null) is treated as anonymous and receives every event, just like any unnamed production channel.

Typed test channels​

Use AddTestChannel<TEvent>(...) when you want the callback to run only for one annotated event type:

var orders = new List<CloudEvent>();

services.AddEventPublisher()
        .AddTestChannel<OrderPlacedData>(@event => orders.Add(@event));

Typed test channels behave like any other typed channel in the framework: when typed channels are registered for an event type, the publisher routes that event to the typed channels for that type.

IEventPublishCallback​

public interface IEventPublishCallback
{
    Task OnEventPublishedAsync(CloudEvent @event);
}

Implement this interface when you need richer callback logic (e.g. recording timestamps, simulating failures, asserting within the callback).

Combining with IEventSystemTime​

Replace the system time to control event timestamps in tests:

services.AddEventPublisher()
        .UseSystemTime<FrozenSystemTime>()
        .AddTestChannel(@event => _published.Add(@event));
public class FrozenSystemTime : IEventSystemTime
{
    public DateTimeOffset UtcNow { get; } = new DateTimeOffset(2025, 6, 1, 12, 0, 0, TimeSpan.Zero);
}