How to use MassTransit test harness to test sagas with Activities (Automatonymous.Binders.EventActivityBinder) with constructor dependency injection?

Question

To keep saga code simple I created activities that are executed when a saga event is triggered. Now I'd like to create tests for the saga using MT's test harness. I also need to mock dependencies from these activities. How can this be done in MT 7.0.2?

This question is similar to the one found here: How to use MassTransit test harness to test Consumer with constructor dependency injection?

Below is some pseudo code to exemplify my use case:

public class SomeSaga : MassTransitStateMachine<SomeSagaState>
{
    public Event<InitializeCommand> Initialize { get; set; }
    
    public State Initialized { get; set; }

    public SomeSaga()
    {
        InstanceState(x => x.CurrentState);
        Event(() => Initialize, e => { e.CorrelateById(c => c.Message.CorrelationId); });

        Initially(
            When(Initialize)
                .Activity(x => x.OfType<InitializeActivity>())
                .TransitionTo(Initialized));
    }
}

public class InitializeActivity : Activity<SomeSagaState, InitializeCommand>
{
    private readonly ISomeDependency _dep;

    public InitializeActivity(ISomeDependency dep)
    {
        _dep = dep ?? throw new ArgumentNullException(nameof(dep));
    }
    
    public async Task Execute(...)
    {
        // do something
    }
}

Answer 1

You need at least v7.0.4 of MassTransit, though I'd suggest using the latest version. Container support for the InMemoryTestHarness was added in that version.

There are also extensive examples of how to use it in Sample-Library.