C# html-helper extend existing method without overriding?

Question

I have already searched for the question and found possible answer, but I still need some help.

I am trying to write an html-helper to extend functionality of already ex

Answer 1

Here's a complete example that does what your asking

Accessing the model attributes in a helper extension class

public static class LabelExtensions
{
    public static MvcHtmlString LabelFor<TModel, TValue>(this HtmlHelper<TModel> html,
        Expression<Func<TModel, TValue>> expression, IDictionary<String, Object> htmlAttributes,
        String requiredMarker = "*")
    {
        return LabelHelper(html, ModelMetadata.FromLambdaExpression(expression, html.ViewData),
            ExpressionHelper.GetExpressionText(expression), null, htmlAttributes, requiredMarker);
    }

    public static MvcHtmlString LabelFor<TModel, TValue>(this HtmlHelper<TModel> html,
        Expression<Func<TModel, TValue>> expression, Object htmlAttributes, String requiredMarker)
    {
        return LabelFor(html, expression, HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes), requiredMarker);
    }

    internal static MvcHtmlString LabelHelper(HtmlHelper html, ModelMetadata metadata, String htmlFieldName,
        String labelText = null, IDictionary<String, Object> htmlAttributes = null, String requiredMarker = null)
    {
        var resolvedLabelText = labelText ??
                                metadata.DisplayName ?? metadata.PropertyName ?? htmlFieldName.Split('.').Last();

        var tag = new TagBuilder("label");
        tag.Attributes.Add("for",
            TagBuilder.CreateSanitizedId(html.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(htmlFieldName)));
        tag.SetInnerText(resolvedLabelText);
        tag.MergeAttributes(htmlAttributes, true);

        if (metadata.IsRequired && !String.IsNullOrWhiteSpace(requiredMarker))
        {
            var requiredSpan = new TagBuilder("span") {InnerHtml = requiredMarker};
            requiredSpan.AddCssClass("required");

            tag.InnerHtml += requiredSpan;
        }

        var result = tag.ToString(TagRenderMode.Normal);

        return new MvcHtmlString(result);
    }
}

and here's the unit tests

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);
    }
}