asp.net-mvc – 在asp.net mvc 3剃刀中识别html助手的问题

namespace WebApp.WebUI
{
    public static class HtmlExtensions
    {

            public static MvcHtmlString GenerateCaptcha(this HtmlHelper helper,string theme)
            {
                string publicKey = ConfigurationManager.AppSettings["CaptchaKey_Public"];
                string privateKey = ConfigurationManager.AppSettings["CaptchaKey_Private"];
                var captchaControl = new Recaptcha.RecaptchaControl
                        {
                            ID = "recaptcha",Theme = theme,PublicKey = publicKey,PrivateKey = privateKey
                        };

                var htmlWriter = new HtmlTextWriter(new StringWriter());

                captchaControl.RenderControl(htmlWriter);

                return new MvcHtmlString(htmlWriter.InnerWriter.ToString());
            }

     }
}

我在这个视图中尝试使用它：

@{
        ViewBag.Title = "Register";
    }
    @model WebApp.WebUI.ViewModel.RegisterModel

    @using (Html.BeginForm("Register","Auth",FormMethod.Post,new { Id = "ERForm" }))
    {
        @Html.GenerateCaptcha("clean")   
    }

它给我这个错误：

CS1061：’System.Web.Mvc.HtmlHelper< WebApp.WebUI.ViewModel.RegisterModel>‘不包含’GenerateCaptcha’的定义,也没有扩展方法’GenerateCaptcha’接受’System.Web.Mvc.HtmlHelper< WebApp.WebUI.ViewModel.RegisterModel>‘类型的第一个参数.可以找到(您是否缺少一个using指令或程序集引用？)

我究竟做错了什么.我的命名空间是正确的.它不会出现在@Html的智能感知中