string[] data1 = File.ReadAllLines("E:\\HtmlTxt.txt");//参考文件
            string[] data2 = File.ReadAllLines("E:\\sqlTxt.txt");//数据文件
            IEnumerable<string> data5 = data1.Except(data2);
           
            StringBuilder xmlResult = new StringBuilder("<?xml version=\"1.0\" encoding=\"utf-8\"?>");
            foreach (string m in data5)
            {
                string[] mm = m.Split(',');
                xmlResult.AppendFormat("<cityconfig><city>{0}</city><jc>{1}</jc><nbf>s</nbf><viewname>view_news_title_index_{1}</viewname></cityconfig>",mm[0],mm[1]);
              
            }

            //写入文件  
            try
            {
                //1.创建文件流    
                FileStream fileStream = new FileStream("E:\\diffHXml.xml",FileMode.Create);
                //2.创建写入器    
                StreamWriter streamWriter = new StreamWriter(fileStream);
                //3.将内容写入文件    
                streamWriter.WriteLine(xmlResult);
                //4.关闭写入器    
                streamWriter.Close();
                //5.关闭文件流    
                fileStream.Close();
            }
            catch 
            { }  

使用StringBuilder生成XML