This is Part 2 in a (very) mini series on HtmlHelper methods. Here is the link for Part 1, Using HtmlHelpers to generate HTML elements

This article is about creating a HtmlHelper for a TextArea although the same principles apply for other input controls as well. For this example I have created a simple Employee model class.

namespace HtmlHelpersDemo.Models
{
    public class Employee
    {
        public string Title { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
    }
}

Nothing special about the controller. An Employee object is instantiated and passed to the view. Following MVC conventions the Index view from the Home folder is rendered.

using HtmlHelpersDemo.Models;
using System.Web.Mvc;

namespace HtmlHelpersDemo.Controllers
{
    public class HomeController : Controller
    {
        public ActionResult Index()
        {
            return View(new Employee());
        }
    }
}

Before looking at the new TextAreaFor HtmlHelper method we will look at how it is called. The method is written as an extension to the Mvc HtmlHelper class (System.Web.Mvc) which allows it to be called the same way the built in TextAreaFor methods are called using @Html..
The first example passes values in a htmlattributes parameter in the form of a class and a data_ref attribute which has an underscore rather than hyphen.

@model HtmlHelpersDemo.Models.Employee
@using HtmlHelpersDemo.Code;
@{
    ViewBag.Title = "Index";
}
<h2>Index</h2>

@Html.TextAreaFor(m => m.FirstName, new { @class = "input" , data_ref="123"}, false)

@Html.TextAreaFor(m => m.FirstName,  false)

Here is the code that does all the work. As mentioned this is written as a HtmlHelper extension method so the first parameter is not actually passed in from the calling code. Rather it defines the class the method is associated with and provides us with a local reference to manipulate, in this instance the local name of HtmlHelper is htmlHelper.

using System;
using System.Linq.Expressions;
using System.Web.Mvc;
using System.Web.Routing;

namespace HtmlHelpersDemo.Code
{
    public static class TextAreaExtensions
    {
        public static MvcHtmlString TextAreaFor<TModel, TValue>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TValue>> expression, object htmlAttributes, bool IsReadonly)
        {
            var attributes = HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes);
			attributes["class"] = "form-control" + " " + attributes["class"];

            if (IsReadonly)
            {
                attributes.Add("readonly", IsReadonly);
            }

            MvcHtmlString html = default(MvcHtmlString);
            RouteValueDictionary routeValues = new RouteValueDictionary(attributes);
            html = System.Web.Mvc.Html.TextAreaExtensions.TextAreaFor(htmlHelper, expression, routeValues);
			TextAreaFor()
            return html;
        }

        public static MvcHtmlString TextAreaFor<TModel, TValue>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TValue>> expression, bool IsReadonly)
        {
            return htmlHelper.TextAreaFor(expression, null, IsReadonly);
        }
    }
}

Lets disect what the method is doing. The first line calls a built in method passing in the htmlAttributes parameter. This method replaces underscore characters with hyphens which is how the data_ref is translated to data-ref when rendered.

var attributes = HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes);

As all text area controls is this application requires the form-control css class, it is being added to the attributes in this method along with any other css class passed in throught the htmlattributes parameter.

attributes["class"] = "form-control" + 
    (!string.IsNullOrEmpty( attributes["class"] as string) ? " "+ attributes["class"] : "");

If the IsReadOnly parameter is set, the readonly state of the control is assigned to the attribues object.

if (IsReadonly)
{
	attributes.Add("readonly", IsReadonly);
}

The final four rows create a TextArea input control from the attributes and route values if there are any.

MvcHtmlString html = default(MvcHtmlString);
RouteValueDictionary routeValues = new RouteValueDictionary(attributes);
html = System.Web.Mvc.Html.TextAreaExtensions.TextAreaFor(htmlHelper, expression, routeValues);
return html;

There rendered html for the two examples above looks like this.

<textarea class="form-control input" cols="20" data-ref="123" id="FirstName" name="FirstName" rows="2">
</textarea>

<textarea class="form-control" cols="20" id="FirstName" name="FirstName" rows="2">
</textarea>