ConfigHelper.cs

using System.Configuration;
using System.IO;

/// <summary>
/// 配置文件辅助类
/// </summary>
public class ConfigHelper
{
    /// <summary>
    /// 获得指定配置节点的值
    /// 节点不存在时返回 null
    /// </summary>
    public static string ReadAppConfig(string strKey)
    {
        if (ConfigurationManager.AppSettings[strKey] == null)
            return null;
        return ConfigurationManager.AppSettings[strKey].ToString();
    }

    /// <summary>
    /// 读 ConnectionStrings 节点 ConnectionName 的连接字符串
    /// 节点不存在时返回 null
    /// </summary>
    public static string ReadConnectionStringConfig(string ConnectionName)
    {
        if (ConfigurationManager.ConnectionStrings[ConnectionName] == null)
            return null;
        return ConfigurationManager.ConnectionStrings[ConnectionName].ConnectionString.ToString();
    }

    /// <summary>
    /// 修改指定配置节点的值
    /// 节点不存在则添加
    /// </summary>
    public static void WriteAppConfig(string newKey, string newValue)
    {
        // 打开配置文件
        //Configuration config = ConfigurationManager.OpenExeConfiguration(Application.ExecutablePath);
        Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
        if (ConfigurationManager.AppSettings[newKey] != null)
            config.AppSettings.Settings.Remove(newKey);
        // 添加新的键-值对
        config.AppSettings.Settings.Add(newKey, newValue);
        // 保存对配置文件的更改
        config.Save(ConfigurationSaveMode.Minimal);
        ConfigurationManager.RefreshSection("appSettings");
    }

    /// <summary>
    /// 写 ConnectionStrings 节点 ConnectionName 的连接字符串
    /// 节点不存在则添加
    /// </summary>
    public static void WriteConnectionStringConfig(string newName, string newConString, string newProvideName)
    {
        // 打开配置文件
        //Configuration config = ConfigurationManager.OpenExeConfiguration(Application.ExecutablePath);
        Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
        if (ConfigurationManager.ConnectionStrings[newName] != null)
            config.ConnectionStrings.ConnectionStrings.Remove(newName);
        // 添加新的连接字符串
        config.ConnectionStrings.ConnectionStrings.Add(new ConnectionStringSettings(newName, newConString, newProvideName));
        // 保存对配置文件的更改
        config.Save(ConfigurationSaveMode.Minimal);
        ConfigurationManager.RefreshSection("connectionStrings");
    }

    /// <summary>
    /// 重新读取配置文件
    /// </summary>
    public static void RefreshConfigs()
    {
        ConfigurationManager.RefreshSection("connectionStrings");
        ConfigurationManager.RefreshSection("appSettings");
    }

    // 2016年12月28日17时28分42秒 6 扩展，读其它配置文件
    public static string ReadAppConfig(string filePath, string strKey)
    {
        if (File.Exists(filePath))
        {
            ExeConfigurationFileMap map = new ExeConfigurationFileMap();
            map.ExeConfigFilename = filePath;
            Configuration cfg = ConfigurationManager.OpenMappedExeConfiguration(map, ConfigurationUserLevel.None);
            if (cfg.AppSettings.Settings[strKey] == null)
                return null;
            return cfg.AppSettings.Settings[strKey].Value;
        }
        throw new FileNotFoundException(filePath + " 指定文件不存在！");
    }

    public static string ReadConnectionStringConfig(string filePath, string ConnectionName)
    {
        if (File.Exists(filePath))
        {
            ExeConfigurationFileMap map = new ExeConfigurationFileMap();
            map.ExeConfigFilename = filePath;
            Configuration cfg = ConfigurationManager.OpenMappedExeConfiguration(map, ConfigurationUserLevel.None);
            if (cfg.ConnectionStrings.ConnectionStrings[ConnectionName] == null)
                return null;
            return cfg.ConnectionStrings.ConnectionStrings[ConnectionName].ConnectionString;
        }
        throw new FileNotFoundException(filePath + " 指定文件不存在！");
    }

    public static void WriteAppConfig(string filePath, string newKey, string newValue)
    {
        if (File.Exists(filePath))
        {
            ExeConfigurationFileMap map = new ExeConfigurationFileMap();
            map.ExeConfigFilename = filePath;
            Configuration cfg = ConfigurationManager.OpenMappedExeConfiguration(map, ConfigurationUserLevel.None);
            if (cfg.AppSettings.Settings[newKey] != null)
                cfg.AppSettings.Settings.Remove(newKey);
            cfg.AppSettings.Settings.Add(newKey, newValue);
            // 保存对配置文件的更改
            cfg.Save(ConfigurationSaveMode.Minimal);
            return;
        }
        throw new FileNotFoundException(filePath + " 指定文件不存在！");
    }

    public static void WriteConnectionStringConfig(string filePath, string newName, string newConString, string newProvideName)
    {
        if(File.Exists(filePath))
        {
            ExeConfigurationFileMap map = new ExeConfigurationFileMap();
            map.ExeConfigFilename = filePath;
            Configuration cfg = ConfigurationManager.OpenMappedExeConfiguration(map, ConfigurationUserLevel.None);
            if (cfg.ConnectionStrings.ConnectionStrings[newName] != null)
                cfg.ConnectionStrings.ConnectionStrings.Remove(newName);
            cfg.ConnectionStrings.ConnectionStrings.Add(new ConnectionStringSettings(newName, newConString, newProvideName));
            // 保存对配置文件的更改
            cfg.Save(ConfigurationSaveMode.Minimal);
            return;
        }
        throw new FileNotFoundException(filePath + " 指定文件不存在！");
    }
}

部分内容摘自：http://www.jb51.net/article/34476.htm