C#写配置文件，在程序运行时查看配置文件有字段，但是退出程序后配置文件该字段又为空了，这个怎么回事？

Question

代码：
               string str = txtMusic.Text;
                //写配置
                System.Configuration.Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
                config.AppSettings.Settings["backMusic"].Value = str;
                //config.Save(ConfigurationSaveMode.Modified);
                //ConfigurationManager.RefreshSection("appSettings");//重新加载新的配置文件 
                config.Save(ConfigurationSaveMode.Modified);配置文件：<?xml version="1.0"?>
<configuration>
<startup><supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/></startup>
  <appSettings>
    <add key="backMusic" value=""/>
  </appSettings>
</configuration>

Answer 1

OpenExeConfiguration不能对配置文件进行修改，换种方法
http://kb.cnblogs.com/a/1274467/

Answer 2

http://hi.baidu.com/591u/blog/item/98c4fcbfa152ee0218d81f1c.html

Answer 3

LZ我的博客里面有关于这个的介绍
传送门还不行+我Q453367672

Answer 4

public void Modify(string key, string strValue)                                                                                         //两个参数：要修改的键值   和   要修改的新值；                               
        {
          string   flagstr = strValue;
            if (strValue == string.Empty)
            {
                MessageBox.Show("连接串不能为空！");
                return;
            }
            //string XPath = "/configuration/userInfo/add[@key='?']";
            try
            {
                string XPath = "/configuration/appSettings/add[@key='?']";
                XmlDocument domWebConfig = new XmlDocument();                //domWebConfig.Load((HttpContext.Current.Server.MapPath("web.config")));
                domWebConfig.Load(filepath);
                XmlNode addKey = domWebConfig.SelectSingleNode((XPath.Replace("?", key)));                if (addKey == null)
                {
                    //Response.Write("<script>alert   (\"没有找到<add   key='" + key + "'   value=.../>的配置节\")</script>");
                    MessageBox.Show("没有找到<add   key='" + key + "'>的配置节");
                    return;
                }
                addKey.Attributes["value"].InnerText = strValue;
                domWebConfig.Save(filepath);
                MessageBox.Show("数据库连接配置成功","信息提示");
                            }
            catch
            {
                               return;
            }        }