Xml序列化UTF-8格式错误

我需要得到一个类的Xml序列化后的字符串

using (System.IO.MemoryStream mem = new System.IO.MemoryStream())
{

    XmlTextWriter writer = new XmlTextWriter(mem, Encoding.UTF8);
    XmlSerializer xz = new XmlSerializer(t.GetType());
    xz.Serialize(writer, t);
    writer.Close();
    byte[] bytes = mem.ToArray();
    return System.Text.Encoding.UTF8.GetString(bytes);
}

得到的字符串却被提示格式错误,放到IE会提示如下错误

无法显示 XML 页。 
使用 样式表无法查看 XML 输入。请更正错误然后单击 刷新按钮,或以后重试。 


--------------------------------------------------------------------------------

文档的顶层无效。处理资源 'file:///C:/Documents and Settings/Administrator/桌面/test.txt.xml' 时出错。第 1 行,位置: 1 

<?xml version="1.0" encoding="utf-8"?><LocalUIForm xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd...

 百思不解!查网上,说Trim()一下得到的字符串就可以,试了确实如此。

这提供了另一种方式,利用子类StringWriter
http://*.com/questions/1564718/using-stringwriter-for-xml-serialization

默认的StringWriter得到的序列化的string是

<?xml version="1.0" encoding="utf-16" ?> 
using (StringUTF8Writer sw = new StringUTF8Writer())
{
    XmlSerializer xz = new XmlSerializer(t.GetType());
    xz.Serialize(sw, t);
    return sw.ToString();
}

 我希望得到的xml是utf-8,于是简单重写EnCoding属性

public class StringUTF8Writer : System.IO.StringWriter
{
    public override Encoding Encoding 
    { 
        get { return Encoding.UTF8; }
    }
}