1. In MVC3 Razor we have HTML helpers to render different controls. 
2. Instead of using these helpers we can create our own helper classes and methods and use them as we want.
3. The creation of custom helpers provides flexibility to change the attributes and style of the control that renders.
4. In this article we will create Custom HTML helper for DropdownList to render DropdownList i.e select element with options.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Text;
using System.Web.Routing;

namespace CustomHtmlHelpers.CustomHelpers
{
    public static class CustomDropdDownList
    {
        //This overload is extension method that accepts two parameters i.e. name and Ienumerable list of values to populate.
        public static MvcHtmlString Custom_DropdownList(this HtmlHelper helper ,string name, IEnumerable<SelectListItem> list)
        {
            //This method in turns calls below overload.
            return Custom_DropdownList(helper, name, list, null);
        }

        //This overload is extension method accepts name, list and htmlAttributes as parameters.
        public static MvcHtmlString Custom_DropdownList(this HtmlHelper helper, string name, IEnumerable<SelectListItem> list, object htmlAttributes)
        {
            //Creating a select element using TagBuilder class which will create a dropdown.
            TagBuilder dropdown = new TagBuilder("select");
            //Setting the name and id attribute with name parameter passed to this method.
            dropdown.Attributes.Add("name", name);
            dropdown.Attributes.Add("id", name);

            //Created StringBuilder object to store option data fetched oen by one from list.
            StringBuilder options = new StringBuilder();
            //Iterated over the IEnumerable list.
            foreach (var item in list)
            {
                //Each option represents a value in dropdown. For each element in the list, option element is created and appended to the stringBuilder object.
                options = options.Append("<option value='" + item.Value + "'>" + item.Text + "</option>");
            }
            //assigned all the options to the dropdown using innerHTML property.
            dropdown.InnerHtml = options.ToString();
            //Assigning the attributes passed as a htmlAttributes object.
            dropdown.MergeAttributes(new RouteValueDictionary(htmlAttributes));
            //Returning the entire select or dropdown control in HTMLString format.
            return MvcHtmlString.Create(dropdown.ToString(TagRenderMode.Normal));
        }
    }
}

@model CustomHtmlHelpers.Models.Register

@{
    ViewBag.Title = "About Us";
    List<SelectListItem> list = new List<SelectListItem>();
    list.Add(new SelectListItem { Text = "Jack Wilshere", Value = "10" });
    list.Add(new SelectListItem { Text = "Thierry Henry", Value = "14" });
    list.Add(new SelectListItem { Text = "Cesc Fabregas", Value = "04" });
    list.Add(new SelectListItem { Text = "Santi Cazorla", Value = "19" });
    list.Add(new SelectListItem { Text = "Tomas Rosicky", Value = "08" });
}

<h2>About</h2>
<p>
@Html.Custom_DropdownList("Players", list)
@Html.Custom_DropdownList("Players", list, new { multiple = "multiple"})
</p>

<select id="Players" name="Players"><option value='10'>Jack Wilshere</option><option value='14'>Thierry Henry</option><option value='04'>Cesc Fabregas</option><option value='19'>Santi Cazorla</option><option value='08'>Tomas Rosicky</option></select>
<select id="Players" multiple="multiple" name="Players"><option value='10'>Jack Wilshere</option><option value='14'>Thierry Henry</option><option value='04'>Cesc Fabregas</option><option value='19'>Santi Cazorla</option><option value='08'>Tomas Rosicky</option></select>