4.2 更新connectionStrings配置节
///<summary>
///更新连接字符串
///</summary>
///<param name="newName">连接字符串名称</param>
///<param name="newConString">连接字符串内容</param>
///<param name="newProviderName">数据提供程序名称</param>
private static void UpdateConnectionStringsConfig(string newName,
    string newConString,
    string newProviderName)
{
    bool isModified = false;    //记录该连接串是否已经存在
    //如果要更改的连接串已经存在
    if (ConfigurationManager.ConnectionStrings[newName] != null)
    {
        isModified = true;
    }
    //新建一个连接字符串实例
    ConnectionStringSettings mySettings = 
        new ConnectionStringSettings(newName, newConString, newProviderName);
    // 打开可执行的配置文件*.exe.config
    Configuration config = 
        ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
    // 如果连接串已存在，首先删除它
    if (isModified)
    {
        config.ConnectionStrings.ConnectionStrings.Remove(newName);
    }
    // 将新的连接串添加到配置文件中.
    config.ConnectionStrings.ConnectionStrings.Add(mySettings);
    // 保存对配置文件所作的更改
    config.Save(ConfigurationSaveMode.Modified);
    // 强制重新载入配置文件的ConnectionStrings配置节
    ConfigurationManager.RefreshSection("ConnectionStrings");
}