internal static MvcHtmlString LabelHelper(HtmlHelper html, ModelMetadata metadata, string htmlFieldName, string labelText = null, IDictionary<string, object> htmlAttributes = null)
{
            string resolvedLabelText = labelText ?? metadata.DisplayName ?? metadata.PropertyName ?? htmlFieldName.Split('.').Last();
            if (String.IsNullOrEmpty(resolvedLabelText))
            {
                return MvcHtmlString.Empty;
            }

            TagBuilder tag = new TagBuilder("label");
            tag.Attributes.Add("for", TagBuilder.CreateSanitizedId(html.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(htmlFieldName)));
            tag.SetInnerText(resolvedLabelText);
            tag.MergeAttributes(htmlAttributes, replaceExisting: true);
            return tag.ToMvcHtmlString(TagRenderMode.Normal);
}

這是MVC的源碼中對Label()擴展方法的定義，我們可以參考MVC中源碼定義擴展方法的方式自定義一個擴展方法。

下面以span標簽為例進行擴展，擴展方法定義如下：

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

namespace MvcHtmlHelper.Helper
{
    /// <summary>
    /// HTML的擴展類
    /// </summary>
    public static class HtmlHelperExt
    {
        /// <summary>
        /// 用C#代碼自定義一個span標簽的擴展方法
        /// </summary>
        /// <param name="htlper"></param>
        /// <param name="id"></param>
        /// <param name="name"></param>
        /// <param name="style"></param>
        /// <param name="message"></param>
        /// <returns></returns>
        public static MvcHtmlString Messager(this HtmlHelper htlper, string id,string name, string style, object message)
        {
            if (message != null)
            {
                TagBuilder builder = new TagBuilder("span");
                builder.MergeAttribute("style", style); //定義樣式
                builder.MergeAttribute("id", id);     // 定義Id
                builder.MergeAttribute("name", name);  // 定義name
                builder.SetInnerText(message.ToString());
                //ToMvcHtmlString是在TagBuilderExtensions擴展類中定義的
                return builder.ToMvcHtmlString(TagRenderMode.Normal);
            }
            return MvcHtmlString.Empty;
        }
    }
}

TagBuilderExtensions擴展方法定義如下：

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

namespace MvcHtmlHelper.Helper
{
    public static class TagBuilderExtensions
    {
        public static MvcHtmlString ToMvcHtmlString(this TagBuilder tagBuilder, TagRenderMode renderMode)
        {
            System.Diagnostics.Debug.Assert(tagBuilder != null);
            return new MvcHtmlString(tagBuilder.ToString(renderMode));
        }
    }
}

視圖頁面代碼如下：

@using MvcHtmlHelper.Helper;
@{
    ViewBag.Title = "Home Page";
}

<div class="jumbotron">
    <h1>ASP.NET</h1>
    <p class="lead">ASP.NET is a free web framework for building great Web sites and Web applications using HTML, CSS and JavaScript.</p>
    <p><a  rel="external nofollow"  class="btn btn-primary btn-lg">Learn more &raquo;</a></p>
    <p>
        <!--使用自定義的Messager方法-->
        @Html.Messager("lblMessage", "lblMessage", "color:red;font-weight:bold;", "自定義span標簽")
    </p>
</div>

<div class="row">
    <div class="col-md-4">
        <h2>Getting started</h2>
        <p>
            ASP.NET MVC gives you a powerful, patterns-based way to build dynamic websites that
            enables a clean separation of concerns and gives you full control over markup
            for enjoyable, agile development.
        </p>
        <p><a class="btn btn-default"  rel="external nofollow" >Learn more &raquo;</a></p>
    </div>
    <div class="col-md-4">
        <h2>Get more libraries</h2>
        <p>NuGet is a free Visual Studio extension that makes it easy to add, remove, and update libraries and tools in Visual Studio projects.</p>
        <p><a class="btn btn-default"  rel="external nofollow" >Learn more &raquo;</a></p>
    </div>
    <div class="col-md-4">
        <h2>Web Hosting</h2>
        <p>You can easily find a web hosting company that offers the right mix of features and price for your applications.</p>
        <p><a class="btn btn-default"  rel="external nofollow" >Learn more &raquo;</a></p>
    </div>
</div>

運行結(jié)果如下：