ASP.MVC: How to unit test html tag helper

I answered this question which shows my unit tests for testing HtmlHelpers.

Here's the code for the testing

public static class LabelExtensionFixtures
{
    [TestFixture]
    public class should_return_label_with_required_info : MvcExtensionFixtureBase
    {
        private class TestClass
        {
            [Required]
            public Guid Id { get; set; }
        }

        private MvcHtmlString _expectedResult;
        private HtmlHelper<TestClass> _sut;
        private MvcHtmlString _result;

        [SetUp]
        public void Given()
        {
            //arrange
            _expectedResult =
                MvcHtmlString.Create(
                    "<label class=\"control-label col-md-2\" for=\"Id\">Id<span class=\"required\">*</span></label>");
            _sut = CreateHtmlHelper(new TestClass {Id = Guid.NewGuid()});

            //act
            _result = _sut.LabelFor(model => model.Id, new { @class = "control-label col-md-2" }, "*");
        }

        [Test]
        public void Test()
        {
            //asert
            Assert.That(_result.ToHtmlString(), Is.EqualTo(_expectedResult.ToHtmlString()));
        }
    }
}

    public abstract class MvcExtensionFixtureBase
    {
        protected HtmlHelper<T> CreateHtmlHelper<T>(T instance)
        {
            var viewDataDictionary = new ViewDataDictionary<T>(instance);
            var viewContext = A.Fake<ViewContext>();
            A.CallTo(() => viewContext.ViewData).Returns(viewDataDictionary);

            var viewDataContainer = A.Fake<IViewDataContainer>();
            A.CallTo(() => viewDataContainer.ViewData).Returns(viewDataDictionary);

            return new HtmlHelper<T>(viewContext, viewDataContainer);
        }
    }

I'm using FakeItEasy for the fakes.

What issues are you seeing when you testing? Maybe post your code?

One way to test it is to create a transformation in the other way too, i.e. from html to some C# representation of your control. This allows you to re-parse the generated HTML and verify things against the C# objects - and then, of course, attribute orders etc doesn't matter anymore.

A sample test might look like this:

[Fact]
public void ControlIsGeneratedCorrectly()
{
    var expected = new CustomControl { Foo = "Foo", Bar = 16 };

    var parser = new CustomControlParser();
    var model = new SomeViewModel { Foo = "Foo" };

    var actual = parser.Parse(Html.InputFor(model));

    Assert.AreEqual(expected.Foo, actual.Foo);
    Assert.AreEqual(expected.Bar, actual.Bar);
}

Of course, this adds complication to your tests since the parser is probably non-trivial. But that one is easier to test; just give it a couple of HTML samples and make sure the returned C# objects show the correct properties. If you find a case where the parser breaks the tests of your html helper, then you probably need to add a few test cases to the parser and fix those too.