Unit Testing Html Helpers for ASP.NET MVC

Html helpers for ASP.NET MVC are static extension methods, which frequently reference the ViewContext and  HttpContext. Combined, this can make unit testing a bit tricky. Let's write a new Html helper using a test-first methodology. Let's start with a prototype function:

        public static MvcHtmlString MyTable(this HtmlHelper helper, MyModel model, IDictionary<string, object> htmlAttributes)

        {

            return MvcHtmlString.Empty;

        }

I've added just enough code here to get the prototype to compile. Now let's write some unit tests. First we need to validate the arguments. I'm using MSTest here:

        [TestMethod]

        [ExpectedException(typeof(ArgumentNullException))]

        public void MyTable_should_throw_when_helper_argument_null()

        {

            var model = new MyModel();



            MyHelpers.MyTable(null, model, null);

        }



        [TestMethod]

        [ExpectedException(typeof(ArgumentNullException))]

        public void MyTable_should_throw_when_model_argument_null()

        {

            var hh = new HtmlHelper(null, null);



            MyHelpers.MyTable(hh, null, null);

        }

Note that I have elected to call this method as a regular static method rather than as an extension method. Either one is valid, but since I am unit testing the case where the HtmlHelper argument is null, it makes more sense to call the method under test as a regular static method.

Those tests will fail when run, so I'll write some code to fix that:

        public static MvcHtmlString MyTable(this HtmlHelper helper, MyModel model, IDictionary<string, object> htmlAttributes)

        {

            if (helper == null)

            {

                throw new ArgumentNullException("helper");

            }

            if (model == null)

            {

                throw new ArgumentNullException("model");

            }

            return MvcHtmlString.Empty;

        }

Now let's presume that the requirements of the new method specify that the table should get its HTML id from the HttpContext.Items. This is of course a very stupid place to store an HTML id, but I want to demonstrate how to fake the HttpContext, since that is something that people often find difficult. So let's make a fake HttpContext. I'll need a lightweight implementation of IViewDataContainer, as well, for reasons which will be clear later on:

        private class FakeHttpContext : HttpContextBase

        {

            private Dictionary<object, object> _items = new Dictionary<object, object>();

            public override IDictionary Items { get { return _items; } }

        }



        private class FakeViewDataContainer : IViewDataContainer

        {

            private ViewDataDictionary _viewData = new ViewDataDictionary();

            public ViewDataDictionary ViewData { get { return _viewData; } set { _viewData = value; } }

        }

I elected to only implement the Items property of HttpContextBase, since that is the only one I'll need to reference in the implementation of my helper. Were I to reference other HttpContextBase properties which I did not override, I would expect to see a NotImplementedException at runtime.

With these two, small classes, I can write a test for the helper which will execute without throwing any exceptions. Well, except for the fact that the assertion fails:

        [TestMethod]

        public void MyTable_should_render_HTML_table_with_id_from_http_context()

        {

            var vc = new ViewContext();

            vc.HttpContext = new FakeHttpContext();

            vc.HttpContext.Items.Add(MyHelpers.IdKey, "foo");

            var hh = new HtmlHelper(vc, new FakeViewDataContainer());

            var model = new MyModel();



            var result = MyHelpers.MyTable(hh, model, null).ToString();



            Assert.AreEqual("<table id = \"foo\"></table>", result);

        }

I needed the IViewDataContainer because the two argument constructor for HtmlHelper will throw ArgumentNullException if you pass a null for the second argument.

Finally, I can implement the helper to make this unit test pass:

        public static MvcHtmlString MyTable(this HtmlHelper helper, MyModel model, IDictionary<string, object> htmlAttributes)

        {

            if (helper == null)

            {

                throw new ArgumentNullException("helper");

            }

            if (model == null)

            {

                throw new ArgumentNullException("model");

            }

            TagBuilder tagBuilder = new TagBuilder("table");

            tagBuilder.MergeAttributes(htmlAttributes);

            tagBuilder.GenerateId(helper.ViewContext.HttpContext.Items[MyHelpers.IdKey]);

            return MvcHtmlString.Create(tagBuilder.ToString(TagRenderMode.Normal));

        }