Sample Unit test looking at a Controller returning the WebAPI 2 pattern IHttpActionResult for reference.

UnitTestIHttpActionResult.cs

[TestMethod]
public void Study_DatetimeOffsets_in_DayViewModel()
{
  // http://www.asp.net/web-api/overview/testing-and-debugging/unit-testing-controllers-in-web-api
  // The controller is returning IHttpActionResult, so look at the sample in the bottom of the ms article. 
  // Using OkNegotiatedContentResult safecast resulted in a null in my case. 
  
  var controller = new DayController();
  controller.Request = new HttpRequestMessage();
  controller.Configuration = new HttpConfiguration();
  
  IHttpActionResult actionResult = controller.Get("15D8F055-F8A7-40B0-82ED-540B6EF0CC03", new DateTime(2015, 5, 1));
  Assert.IsNotNull(actionResult);
  
  //var contentResult = actionResult as OkNegotiatedContentResult<IEnumerable<DayViewModel>>;
  //Assert.IsNotNull(contentResult); // resulted in null
  
  // initializing the httpResponseMessage Task and using TryGetContentValue solved it... 
  var httpResponseMessage = httpActionResult.ExecuteAsync(new CancellationToken()).GetAwaiter().GetResult(); //To properly re-throw an exception: http://stackoverflow.com/questions/20170527/how-to-correctly-rethrow-an-exception-of-task-already-in-faulted-state
  Assert.IsNotNull(httpResponseMessage);
  Assert.IsTrue(httpResponseMessage.IsSuccessStatusCode);

  IEnumerable<DayViewModel> dayViewModels;
  Assert.IsTrue(httpResponseMessage.TryGetContentValue<IEnumerable<DayViewModel>>(out dayViewModels));
  Assert.IsNotNull(dayViewModels);

  var result = dayViewModels.FirstOrDefault();
  Assert.IsNotNull(result);
  Assert.IsTrue(result.response.Success);

  Assert.IsNotNull(result.production.Count > 0);
}