救命啊~~XmlWriterSettings 设置 XmlWriter的BOM头无效 Encoding设置无效

XmlWriterSettings   settings   =   new   XmlWriterSettings();
settings.Encoding   =   Encoding.UTF8;
StringBuilder   sbXML   =   new   StringBuilder();
using   (XmlWriter   writer   =   XmlWriter.Create(sbXML,   settings))
{
              string   str   =   writer.Settings.Encoding.EncodingName;   //str是Unicode
              writer.WriteStartElement( "ajaxresponse ");
                    ...

              writer.Flush();
              writer.Close();
              Response.AddHeader( "Content-Type ",   "text/xml ");
              Response.Write(sbXML.ToString());

这样我最后输出到页面后的头部是
<?xml   version= "1.0 "   encoding= "utf-16 "?>
而我想要得到
<?xml   version= "1.0 "   encoding= "utf-8 "?>
大哥们救命   为什么设置无效   问题出哪了...

------解决方案--------------------
沙发沙发，帮忙顶

------解决方案--------------------
用replace()替换掉，然后再生成