Cannot subscribe to events from an ASP.NET Core 6.0 API NServiceBus endpoint

Question

I want to make my API both publish events (outside any incoming message handlers, e.g. from an action method) and subscribe to events. Publishing events works. I used the older versions of Program and Startup to integrate using GenericHost and an IMessageSession is sucessfully injected into the constructor.

However, when the commented-out code is activated to make the API subscribe to and handle events, suddenly there is an exception about injecting IMessageSession being thrown when initializing the host and running the application.

TestController.cs

namespace TestContext
{
    [Route("api/[controller]")]
    [ApiController]
    public class TestController : ControllerBase/*, IHandleMessages<ReceivedEvent>*/
    {
        private IMessageSession _session;

        public TestController(IMessageSession session)
        {
            _session = session;
        }
        /*
        [NonAction]
        public Task Handle(ReceivedEvent message, IMessageHandlerContext context)
        {
            throw new NotImplementedException();
        }*/

        [HttpPost]
        public async Task<ActionResult> Test()
        {
            await _session.Publish(new PublishedEvent());
            return Ok();
        }
    }
}

Program.cs

public class Program
{
    public static void Main(string[] args)
    {
        CreateHostBuilder(args).Build().Run();
    }
    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .UseNServiceBus(context =>
            {
                var endpointConfiguration = new EndpointConfiguration("Test");
                var transport = endpointConfiguration.UseTransport<LearningTransport>();
                
                return endpointConfiguration;
            })
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseStartup<Startup>();
            });
}

Exception

Why is the exception being thrown and how to make the API subscribe to and handle events?

EDIT

When handling the event from a separate service class, the exception is not thrown.

public class HandlerService : IHandleMessages<ReceivedEvent>
{
    public Task Handle(ReceivedEvent message, IMessageHandlerContext context)
    {
        Console.WriteLine(message);
        return Task.CompletedTask;
    }
}

Answer 1

The exception message is actually pretty good here. Most of the NServiceBus exception messages are.

Interfaces IMessageSession or IEndpointInstance should not be resolved from the container to enable sending or publishing messages from within sagas or message handlers. Instead, use the context parameter on the TestController.Handle method to send or publish messages.

So first, don't mix a handler in a controller. It needs to be a separate class.

Then, don't inject IMessageSession. That interface is explicitly for things like controller actions where you don't have an existing "a message is being handled right now" and you want to send a message.

Instead, if you want to send or publish within the handler, use the context:

Don't do this

public class MyHandler : IHandleMessages<ReceivedEvent>
{
    private IMessageSession session;

    public MyHandler(IMessageSession session)
    {
        // Won't work! Don't inject IMessageSession!
        this.session = session;
    }

    public async Task Handle(ReceivedEvent message, IMessageHandlerContext context)
    {
        // NOPE!
        session
    }
}

Do this instead

public class MyHandler : IHandleMessages<ReceivedEvent>
{
    public async Task Handle(ReceivedEvent message, IMessageHandlerContext context)
    {
        context.Send(new CommandToSend());
        context.Publish(new EventToPublish());
    }
}