用C#写得一个登陆界面,要求使用xml记录用户名密码!.....

 private void button4_Click(object sender, EventArgs e)
        {
            string userbl = "",passbl ="";
            string fileName = ("login.xml");
            XmlTextReader reader = new XmlTextReader(fileName);
             while (reader.Read())
            {
                if (reader.NodeType == XmlNodeType.Element)
                {
                    if (reader.LocalName.Equals("username"))
                    {
                        if (reader.ReadString() == "blu")
                            xie();
                     }
                   
                  }
              }
        }
        public void xie()
        {            XmlDocument xmlDoc = new XmlDocument();
            xmlDoc.Load("login.xml");
            XmlNodeList nodeList = xmlDoc.SelectSingleNode("users//user").ChildNodes;
            foreach (XmlNode xn in nodeList)
            {
                XmlElement xe = (XmlElement)xn;
                if (xe.Name == "username")
                {
                    xe.InnerText = "blu";
                }
                if (xe.Name == "username")
                {
                    xe.InnerText = "1111";
                }
            }
            xmlDoc.Save("login.xml");
        }
这是其中一段,是添加新用户按钮的,为啥老说文件被另一程序使用?还有写得不太好,求xml文件与textbox控件绑定方法~~