url传值编码强制转换

怎么对url中传递的值进行强制转换为utf-8，代码如下：

  System.Collections.Specialized.NameValueCollection requestUrl = HttpUtility.ParseQueryString(Request.Url.Query, System.Text.Encoding.GetEncoding("UTF-8"));
  txtQuestion.Text = requestUrl["arg"];

keyword: get url 传值，编码强制转换