在XML中添加前缀字符串all标签

Question

 XDocument xdoc = XDocument.Load(@"C:\Users\xmlformatı.txt");
        XmlDocument doc = new XmlDocument();           
        doc.Load(@"C:\Users\xmlformatı.txt");

    XmlNodeList xmlList = doc.SelectNodes("//*");
        foreach (XElement element in xdoc.Descendants())
        {

            foreach (XmlNode xn in xmlList)
            {
                string sa = xn.Name;
                string neww = "fat:" + xn.Name;
                string last = sa.Replace(sa, neww);
                element.Name = last;
                \\file writing things


                doc.Save(@"C:\Users\denemexml.txt");

            }
        }

但是我得到了例外：名称中不能包含'：'字符，十六进制值0x3A。'