ProductInformationHandler.Tests.cs

public class GivenThereArePurchaseOrdersForTheProduct
{
    [Fact]
    public async Task ShouldCreateValidSuggestion()
    {        
	var productId = {use a dummy id};
	// set up the stub to already have a purchase order stored
        var poStore = new PurchaseOrderStoreStub()
            .WithPurchaseOrder(...); 

        var productStore = new ProductStoreStub();
        var suggestionStore = new SuggestionStoreStub();
        
	// initialise the System Under Test passing in all the stubbed dependencies
        
        var handler = new ProductInformationHandler(
                new ReviewUseCase(
                    suggestionStore,
                    productStore,
                    poStore),
                poStore,
                productStore
            );

        // invoke the handle method (It doesn't matter if its invoked 
        // via an event arriving in a queue or by a test suite like this one)

	// create a test event with canned values
        await handler.Handle(
                CreateTestEvent(...)); 

        // Time to assert on the output of handling the test event.

        var savedProduct = await productStore.GetByProductId(productId);
        var suggestionForProduct = await suggestionStore.GetFor(productId);

        // Assert that incoming product was saved successfully
        savedProduct.Should().NotBeNull();

        // Assert that as a result of an appropriate event, the suggestion was also created
        // and stored in the "database"
        suggestionForProduct.Should().NotBeNull();
        // various other assertions on suggestion attributes.
    }
}