I am trying to unit test / verify that a method is being called on a dependency, by the system under test.

• The depenedency is IFoo.
• The dependent class is IBar.
• IBar is implemented as Bar.
• Bar will call Start() on IFoo in a new (System.Threading.Tasks.)Task, when Start() is called on Bar instance.

Unit Test (Moq):

    [Test]
    public void StartBar_ShouldCallStartOnAllFoo_WhenFoosExist()
    {
        //ARRANGE

        //Create a foo, and setup expectation
        var mockFoo0 = new Mock<IFoo>();
        mockFoo0.Setup(foo => foo.Start());

        var mockFoo1 = new Mock<IFoo>();
        mockFoo1.Setup(foo => foo.Start());


        //Add mockobjects to a collection
        var foos = new List<IFoo>
                       {
                           mockFoo0.Object,
                           mockFoo1.Object
                       };

        IBar sutBar = new Bar(foos);

        //ACT
        sutBar.Start(); //Should call mockFoo.Start()

        //ASSERT
        mockFoo0.VerifyAll();
        mockFoo1.VerifyAll();
    }

Implementation of IBar as Bar:

    class Bar : IBar
    {
        private IEnumerable<IFoo> Foos { get; set; }

        public Bar(IEnumerable<IFoo> foos)
        {
            Foos = foos;
        }

        public void Start()
        {
            foreach(var foo in Foos)
            {
                Task.Factory.StartNew(
                    () =>
                        {
                            foo.Start();
                        });
            }
        }
    }

I appears that the issue is obviously due to the fact that the call to "foo.Start()" is taking place on another thread (/task), and the mock (Moq framework) can't detect it. But I could be wrong.

Thanks