这是哪错了？

[yjdabc]

System.Text.Encoding encoder = System.Text.Encoding.GetEncoding("gb2312");
System.Text.StringBuilder sb = new System.Text.StringBuilder();
sb.Append("http://www.abc.com/send.php?");
sb.Append ("mycode=1364578xxxx");
sb.Append ("&mypw=xxx");
sb.Append ("&phone=135789256xx");
sb.Append ("&content=oksms");
sb.Append ("&name=");
sb.Append(HttpUtility.UrlEncode("请审批",encoder));
System.Web.HttpRequest request = (System.Web.HttpRequest)WebRequest.Create(sb.ToString());

提示 WebRequest错误！