我想出了一种方法来进行单元测试:

I figured out a way to do it for unit testing:

[TestMethod]
public void TestDoStuff()
{
    //+ Arrange
    myViewModel.IsSearchShowing = true;

    // container is my Unity container and it setup in the init method.
    container.Resolve<IOrderService>().Returns(orderService);
    orderService = Substitute.For<IOrderService>();

    var lookupTask = Task<IOrder>.Factory.StartNew(() =>
                                  {
                                      return new Order();
                                  });

    orderService.LookUpIdAsync(Arg.Any<long>()).Returns(lookupTask);

    //+ Act
    myViewModel.DoLookupCommand.Execute(0);
    lookupTask.Wait();

    //+ Assert
    myViewModel.IsSearchShowing.Should().BeFalse();
}

这里的关键是，因为我正在进行单元测试，所以我可以替换我想让异步调用(在我的异步无效内)返回的任务.然后，在继续之前，我只需确保任务已完成.

The key here is that because I am unit testing I can substitute in the task I want to have my async call (inside my async void) to return. I then just make sure the task has completed before I move on.