Unit Testing WPF Command

Hi,

I am trying to write unit tests for a command that executes an asynchronous action using tasks (TPL). 

Here is the example code that I am trying to achieve.

class MainPageViewModel
    {
        private IService service_;

        public ICommand StartCommand{get;set;}

        //Bind this method to Start Commmand
        private void StartTask()
        {
            var task = service_.StartServiceAsync();
            task.ContinueWith(AfterService);
        }

        private void AfterService(Task<IResult> result)
        {
            //update UI with result
        }
    }

    class TestClass
    {
        [TestMethod]
        public Test_StartTask()
        {
            MainPageViewModel vm = new MainPageViewModel();
            StartCommand();
            //need to check if UI is updated but since the AfterService is called on a different thread the assert fails

        }
    }

In my Test Method I can't write Assert after the StartTask() call, please help me on how to handle such scenarios for writing test cases? TIA.

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