How to instantiate Mediatr as part of a Unit Test?

Question

I am trying to build an xUnit Test project for an MVC Core 2.2 Application that is based on the CQRS/ES pattern. I utilize MediatR as part of my CQRS/ES pattern in the MVC application.

In one of my commands which I would like to test, I inject MediatR to publish an event once a customer record has been updated. Kind of like this:

public class UpdateCustomerCommandHandler : IRequestHandler<UpdateCustomerCommand>
{
    public IMediator Mediator { get; set;  }

    public UpdateCustomerCommandHandler(IMediator mediator)
    {
        Mediator = mediator;
    }

    public Task<Unit> Handle(UpdateCustomerCommand request, CancellationToken cancellationToken)
    {
        //do some stuff

        Mediator.Publish(new CustomersChanged());

        return Task.FromResult(new Unit());
    }
}

When writing a unit test for this command, I obviously also must create an instance of MediatR (or a mockup) which I then pass to to the command during test execution.

[Fact]
public async void UpdateCustomerCommand_CustomerDataUpdatedOnDatabase()
{
    //Arange

    var mediator = new Mediator(); // doesn't work that way..

    UpdateCustomerCommand command = new UpdateCustomerCommand();
    UpdateCustomerCommandHandler handler = new UpdateCustomerCommandHandler(mediator);

    //Act
    Unit x = await handler.Handle(command, new System.Threading.CancellationToken());

    //Asert
    //Do the assertion
}

However, instanciating MediatR (outside of the MVC application, where I can utilize the existing dependency injection implementation) seems to be not that simple and frankly speaking I actually do not understand how I can do in my test method.

I understand that I potentially could use a dependency injection framework for which MediatR already provides an implementation (Ninject, etc.), but I actually do not want to use any other third party libraries in my unit tests apart from MediatR, just for the sake of creating an instance.

Is there a simpler way to instantiate MediatR which I might have overseen?

Answer 1

You're on the right lines with or a mockup - you need to mock the IMediator

There's a few mocking libraries out there:

• Moq
• FakeItEasy
• NSubstiutute

Moq is one of the most popular, so, using your test as an example:

[Fact]
public async void UpdateCustomerCommand_CustomerDataUpdatedOnDatabase()
{
    //Arange
    var mediator = new  Mock<IMediator>();

    UpdateCustomerCommand command = new UpdateCustomerCommand();
    UpdateCustomerCommandHandler handler = new UpdateCustomerCommandHandler(mediator.Object);

    //Act
    Unit x = await handler.Handle(command, new System.Threading.CancellationToken());

    //Asert
    //Do the assertion

    //something like:
    mediator.Verify(x=>x.Publish(It.IsAny<CustomersChanged>()));
}

Answer 2

To expand on the accepted answer, if you absolutely need to test interaction between two services using MediatR, you can always moq each publish/handler pair using callbacks:

_mediator = new Mock<IMediator>();
_mediator.Setup(m => m.Publish(It.IsAny<YourNotification>(), It.IsAny<CancellationToken>()))
   .Callback<YourNotification, CancellationToken>((notification, cToken) => 
      _yourHandlerService.Handle(notification, cToken));

The above code, basically says if _mediator gets YourNotification via Publish method, then it will forward it to _yourHandlerService via Handle method. You can repeat that Setup for each type of mediator INotification you want to handle.