如何在使用htmltextwitter渲染时正确添加属性?

我想将href属性添加到链接元素。这是我的代码:

System.Text.StringBuilder stringBuilder = new System.Text.StringBuilder();
using (System.IO.StringWriter stringWriter = new System.IO.StringWriter(stringBuilder))
{
 using (System.Web.UI.HtmlTextWriter htmlTextWriter = new System.Web.UI.HtmlTextWriter(stringWriter))
 {

  htmlTextWriter.RenderBeginTag(HtmlTextWriterTag.Ul);
  htmlTextWriter.RenderBeginTag(HtmlTextWriterTag.Li);
  htmlTextWriter.RenderBeginTag(HtmlTextWriterTag.A);
  htmlTextWriter.AddAttribute(HtmlTextWriterAttribute.Href, "http://www.google.de");
  htmlTextWriter.Write("Google");
  htmlTextWriter.RenderEndTag();
  htmlTextWriter.RenderEndTag();


  htmlTextWriter.RenderEndTag();
 }
}

<ul>
 <li><a>Google</a></li>
</ul>

这是我所期望的。

<ul>
 <li><a href="http://www.google.de">Google</a></li>
</ul>

出了什么问题,我该怎么解决?