<appSettings>

    <add key="domainExist" value="false"></add>

  </appSettings>

string domainExist = ConfigurationManager.AppSettings["domainExist"];

UpdAppSettings("domainExist", "true");

ConfigurationManager.RefreshSection("appSettings");

/// <summary>

    /// 修改web.config中appSettings键的值

    /// </summary>

    /// <param name="keyName">键的名称</param>

    /// <param name="keyValue">键的值</param>

    public static void UpdAppSettings( string keyName, string keyValue)

    {

        System.Xml.XmlDocument doc = new System.Xml.XmlDocument();

        try

        {

            //获取 Web.config 的路径

            string configPath = HttpRuntime.AppDomainAppPath.ToString()+"Web.config";

            doc.Load(configPath);

            System.Xml.XmlNode node;

            System.Xml.XmlElement element;

            node = doc.SelectSingleNode("//appSettings");

            element = (System.Xml.XmlElement)node.SelectSingleNode("//add[@key='" + keyName + "']");

            if (element != null)

            {

                element.SetAttribute("value", keyValue);

                doc.Save(configPath);

            }

        }

        catch (Exception)

        {

            throw;

        }

    }