第一种方法：采用MS现有的ConfigurationManager来进行读写

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using System.Configuration;
 
 
namespace Zwj.TEMS.Common
{
    public abstract class ConfigHelper
    {
        private ConfigHelper()
        { }
 
        /// <summary>
        /// 获取配置值
        /// </summary>
        /// <param name="key"></param>
        /// <returns></returns>
        public static string GetAppSettingValue(string key)
        {
            return ConfigurationManager.AppSettings[key];
        }
 
        /// <summary>
        /// 设置配置值（存在则更新，不存在则新增）
        /// </summary>
        /// <param name="key"></param>
        /// <param name="value"></param>
        public static void SetAppSettingValue(string key, string value)
        {
            var config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
            var setting = config.AppSettings.Settings[key];
            if (setting==null)
            {
                config.AppSettings.Settings.Add(key, value);
            }
            else
            {
                setting.Value = value;
            }
 
            config.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection("appSettings");
        }
 
        /// <summary>
        /// 删除配置值
        /// </summary>
        /// <param name="key"></param>
        public static void RemoveAppSetting(string key)
        {
            var config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
            config.AppSettings.Settings.Remove(key);
            config.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection("appSettings");
        }
 
 
        /// <summary>
        /// 获取连接字符串
        /// </summary>
        /// <param name="name"></param>
        /// <returns></returns>
        public static string GetConnectionString(string name)
        {
          return  ConfigurationManager.ConnectionStrings[name].ConnectionString;
        }
 
        /// <summary>
        /// 设置连接字符串的值（存在则更新，不存在则新增）
        /// </summary>
        /// <param name="name"></param>
        /// <param name="connstr"></param>
        /// <param name="provider"></param>
        public static void SetConnectionString(string name,string connstr, string provider)
        {
            var config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
            ConnectionStringSettings connStrSettings = config.ConnectionStrings.ConnectionStrings[name];
            if (connStrSettings != null)
            {
                connStrSettings.ConnectionString = connstr;
                connStrSettings.ProviderName = provider;
            }
            else
            {
                connStrSettings = new ConnectionStringSettings(name, connstr, provider);
                config.ConnectionStrings.ConnectionStrings.Add(connStrSettings);
            }
 
            config.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection("connectionStrings");
        }
 
        /// <summary>
        /// 删除连接字符串配置项
        /// </summary>
        /// <param name="name"></param>
        public static void RemoveConnectionString(string name)
        {
            var config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
            config.ConnectionStrings.ConnectionStrings.Remove(name);
            config.Save(ConfigurationSaveMode.Modified);
            ConfigurationManager.RefreshSection("connectionStrings");
        }
 
    }
}

　注意：不能直接使用ConfigurationManager.AppSettings及ConfigurationManager.ConnectionStrings进行写操作（即：Add,Remove）,因为这两个属性是只读的，本人之前也疑惑，明明能够调用Add,Remove方法，但使用时却报错。

第二种方法：采用原生的XML+XPATH来读写（来源于网络）

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//==============================================
// FileName: ConfigManager
// Description: 静态方法业务类，用于对C#、ASP.NET中的WinForm & WebForm 项目程序配置文件
// app.config和web.config的[appSettings]和[connectionStrings]节点进行新增、修改、删除和读取相关的操作。
//==============================================
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Collections.Generic;
using System.Text;
using System.Xml;
 
public enum ConfigurationFile
{
    AppConfig = 1,
    WebConfig = 2
}
/// <summary>
/// ConfigManager 应用程序配置文件管理器
/// </summary>
public class ConfigManager
{
    public ConfigManager()
    {
        //
        // TODO: 在此处添加构造函数逻辑
        //
    }
 
    /// <summary>
    /// 对[appSettings]节点依据Key值读取到Value值，返回字符串
    /// </summary>
    /// <param name="configurationFile">要操作的配置文件名称,枚举常量</param>
    /// <param name="key">要读取的Key值</param>
    /// <returns>返回Value值的字符串</returns>
    public static string ReadValueByKey(ConfigurationFile configurationFile, string key)
    {
        string value = string.Empty;
        string filename = string.Empty;
        if (configurationFile.ToString() == ConfigurationFile.AppConfig.ToString())
        {
            filename = System.Windows.Forms.Application.ExecutablePath + ".config";
        }
        else
        {
            filename = System.AppDomain.CurrentDomain.BaseDirectory + "web.config";
        }
 
        XmlDocument doc = new XmlDocument();
        doc.Load(filename); //加载配置文件
        XmlNode node = doc.SelectSingleNode("//appSettings"); //得到[appSettings]节点
        ////得到[appSettings]节点中关于Key的子节点
        XmlElement element = (XmlElement)node.SelectSingleNode("//add[@key='" + key + "']");
        if (element != null)
        {
            value = element.GetAttribute("value");
        }
        return value;
    }
    /// <summary>
    /// 对[connectionStrings]节点依据name值读取到connectionString值，返回字符串
    /// </summary>
    /// <param name="configurationFile">要操作的配置文件名称,枚举常量</param>
    /// <param name="name">要读取的name值</param>
    /// <returns>返回connectionString值的字符串</returns>
    public static string ReadConnectionStringByName(ConfigurationFile configurationFile, string name)
    {
        string connectionString = string.Empty;
        string filename = string.Empty;
        if (configurationFile.ToString() == ConfigurationFile.AppConfig.ToString())
        {
            filename = System.Windows.Forms.Application.ExecutablePath + ".config";
        }
        else
        {
            filename = System.AppDomain.CurrentDomain.BaseDirectory + "web.config";
        }
        XmlDocument doc = new XmlDocument();
        doc.Load(filename); //加载配置文件
        XmlNode node = doc.SelectSingleNode("//connectionStrings"); //得到[appSettings]节点
        ////得到[connectionString]节点中关于name的子节点
        XmlElement element = (XmlElement)node.SelectSingleNode("//add[@name='" + name + "']");
        if (element != null)
        {
            connectionString = element.GetAttribute("connectionString");
        }
        return connectionString;
    }
    /// <summary>
    /// 更新或新增[appSettings]节点的子节点值，存在则更新子节点Value,不存在则新增子节点，返回成功与否布尔值
    /// </summary>
    /// <param name="configurationFile">要操作的配置文件名称,枚举常量</param>
    /// <param name="key">子节点Key值</param>
    /// <param name="value">子节点value值</param>
    /// <returns>返回成功与否布尔值</returns>
    public static bool UpdateOrCreateAppSetting(ConfigurationFile configurationFile, string key, string value)
    {
        bool isSuccess = false;
        string filename = string.Empty;
        if (configurationFile.ToString() == ConfigurationFile.AppConfig.ToString())
        {
            filename = System.Windows.Forms.Application.ExecutablePath + ".config";
        }
        else
        {
            filename = System.AppDomain.CurrentDomain.BaseDirectory + "web.config";
        }
        XmlDocument doc = new XmlDocument();
        doc.Load(filename); //加载配置文件
        XmlNode node = doc.SelectSingleNode("//appSettings"); //得到[appSettings]节点
        try
        {
            ////得到[appSettings]节点中关于Key的子节点
            XmlElement element = (XmlElement)node.SelectSingleNode("//add[@key='" + key + "']");
            if (element != null)
            {
                //存在则更新子节点Value
                element.SetAttribute("value", value);
            }
            else
            {
                //不存在则新增子节点
                XmlElement subElement = doc.CreateElement("add");
                subElement.SetAttribute("key", key);
                subElement.SetAttribute("value", value);
                node.AppendChild(subElement);
            }
            //保存至配置文件(方式一)
            using (XmlTextWriter xmlwriter = new XmlTextWriter(filename, null))
            {
                xmlwriter.Formatting = Formatting.Indented;
                doc.WriteTo(xmlwriter);
                xmlwriter.Flush();
            }
            isSuccess = true;
        }
        catch (Exception ex)
        {
            isSuccess = false;
            throw ex;
        }
        return isSuccess;
    }
    /// <summary>
    /// 更新或新增[connectionStrings]节点的子节点值，存在则更新子节点,不存在则新增子节点，返回成功与否布尔值
    /// </summary>
    /// <param name="configurationFile">要操作的配置文件名称,枚举常量</param>
    /// <param name="name">子节点name值</param>
    /// <param name="connectionString">子节点connectionString值</param>
    /// <param name="providerName">子节点providerName值</param>
    /// <returns>返回成功与否布尔值</returns>
    public static bool UpdateOrCreateConnectionString(ConfigurationFile configurationFile, string name, string connectionString, string providerName)
    {
        bool isSuccess = false;
        string filename = string.Empty;
        if (configurationFile.ToString() == ConfigurationFile.AppConfig.ToString())
        {
            filename = System.Windows.Forms.Application.ExecutablePath + ".config";
        }
        else
        {
            filename = System.AppDomain.CurrentDomain.BaseDirectory + "web.config";
        }
        XmlDocument doc = new XmlDocument();
        doc.Load(filename); //加载配置文件
        XmlNode node = doc.SelectSingleNode("//connectionStrings"); //得到[connectionStrings]节点
        try
        {
            ////得到[connectionStrings]节点中关于Name的子节点
            XmlElement element = (XmlElement)node.SelectSingleNode("//add[@name='" + name + "']");
            if (element != null)
            {
                //存在则更新子节点
                element.SetAttribute("connectionString", connectionString);
                element.SetAttribute("providerName", providerName);
            }
            else
            {
                //不存在则新增子节点
                XmlElement subElement = doc.CreateElement("add");
                subElement.SetAttribute("name", name);
                subElement.SetAttribute("connectionString", connectionString);
                subElement.SetAttribute("providerName", providerName);
                node.AppendChild(subElement);
            }
            //保存至配置文件(方式二)
            doc.Save(filename);
            isSuccess = true;
        }
        catch (Exception ex)
        {
            isSuccess = false;
            throw ex;
        }
        return isSuccess;
    }
    /// <summary>
    /// 删除[appSettings]节点中包含Key值的子节点，返回成功与否布尔值
    /// </summary>
    /// <param name="configurationFile">要操作的配置文件名称,枚举常量</param>
    /// <param name="key">要删除的子节点Key值</param>
    /// <returns>返回成功与否布尔值</returns>
    public static bool DeleteByKey(ConfigurationFile configurationFile, string key)
    {
        bool isSuccess = false;
        string filename = string.Empty;
        if (configurationFile.ToString() == ConfigurationFile.AppConfig.ToString())
        {
            filename = System.Windows.Forms.Application.ExecutablePath + ".config";
        }
        else
        {
            filename = System.AppDomain.CurrentDomain.BaseDirectory + "web.config";
        }
        XmlDocument doc = new XmlDocument();
        doc.Load(filename); //加载配置文件
        XmlNode node = doc.SelectSingleNode("//appSettings"); //得到[appSettings]节点
        ////得到[appSettings]节点中关于Key的子节点
        XmlElement element = (XmlElement)node.SelectSingleNode("//add[@key='" + key + "']");
        if (element != null)
        {
            //存在则删除子节点
            element.ParentNode.RemoveChild(element);
        }
        else
        {
            //不存在
        }
        try
        {
            //保存至配置文件(方式一)
            using (XmlTextWriter xmlwriter = new XmlTextWriter(filename, null))
            {
                xmlwriter.Formatting = Formatting.Indented;
                doc.WriteTo(xmlwriter);
                xmlwriter.Flush();
            }
            isSuccess = true;
        }
        catch (Exception ex)
        {
            isSuccess = false;
        }
        return isSuccess;
    }
    /// <summary>
    /// 删除[connectionStrings]节点中包含name值的子节点，返回成功与否布尔值
    /// </summary>
    /// <param name="configurationFile">要操作的配置文件名称,枚举常量</param>
    /// <param name="name">要删除的子节点name值</param>
    /// <returns>返回成功与否布尔值</returns>
    public static bool DeleteByName(ConfigurationFile configurationFile, string name)
    {
        bool isSuccess = false;
        string filename = string.Empty;
        if (configurationFile.ToString() == ConfigurationFile.AppConfig.ToString())
        {
            filename = System.Windows.Forms.Application.ExecutablePath + ".config";
        }
        else
        {
            filename = System.AppDomain.CurrentDomain.BaseDirectory + "web.config";
        }
        XmlDocument doc = new XmlDocument();
        doc.Load(filename); //加载配置文件
        XmlNode node = doc.SelectSingleNode("//connectionStrings"); //得到[connectionStrings]节点
        ////得到[connectionStrings]节点中关于Name的子节点
        XmlElement element = (XmlElement)node.SelectSingleNode("//add[@name='" + name + "']");
        if (element != null)
        {
            //存在则删除子节点
            node.RemoveChild(element);
        }
        else
        {
            //不存在
        }
        try
        {
            //保存至配置文件(方式二)
            doc.Save(filename);
            isSuccess = true;
        }
        catch (Exception ex)
        {
            isSuccess = false;
        }
        return isSuccess;
    }
}