string username = comboBox1.SelectedItem.ToString();//用于检索
                XElement rootNode = XElement.Load("User.xml");//加载xml文件
                var query = from serch in rootNode.Elements("user") where serch.Descendants("id").Select(sel => sel.Value).Contains("" + username + "") select serch;
                foreach (XElement node in query)
                {
                    serstr = node.Element("id").Value.ToString();//把查到的id值放到Serstr变量中
                }
                    //登陆界面checkbox控件，打上勾就是保存密码
                if (checkBox1.Checked == true&&serstr!=comboBox1.Text)//如果xml没有这个用户并且打上勾了，就保存到xml文件中
                {
                  

                  

                    try
                    {
                     

                        XmlDocument xmldoc = new XmlDocument();
                        xmldoc.Load("User.xml");
                        XmlNode root = xmldoc.SelectSingleNode("UserLogin");//查找根节点
                        XmlElement xesub1 = xmldoc.CreateElement("user");//获得子节点
                        XmlElement useid = xmldoc.CreateElement("id");//用户名
                        useid.InnerText = comboBox1.Text.Trim();//用户名添加到xml文件id元素中
                        xesub1.AppendChild(useid);//添加到user节点中
                        XmlElement usepwd = xmldoc.CreateElement("pwd");//创建密码pwd属性
                        usepwd.InnerText = textBox1.Text.Trim();//添加密码到xml文件pwd中
                        xesub1.AppendChild(usepwd);//pwd添加到user节点中
                        root.AppendChild(xesub1);//user节点添加到根节点userlogin中
                        xmldoc.Save("User.xml");    //保存xml文件

                    }
                    catch (Exception ex)
                    {
                        MessageBox.Show(ex.Message);
                    }
                }
              

                //==================
                this.DialogResult = DialogResult.OK;
                this.Close();