• 原config文件

<configuration>
  <appSettings>
    <add key="111" value="222" />
    <add key="111" value="333" />
  </appSettings>
</configuration>

• 添加recorders自定义节点

XmlDocument doc = new XmlDocument();
//加载配置文件
doc.Load("D:\\U8SOFT\\Portal\\App.config");

XmlNode recorders = doc.SelectSingleNode("configuration/recorders");

if (recorders == null)
{
    //找到configuration节点
    XmlNode configuration = doc.SelectSingleNode("configuration");
    //创建recorders节点，添加属性
    XmlElement xml_recorders = doc.CreateElement("recorders");
    xml_recorders.SetAttribute("enabled", "true");
    //创建recorder节点，添加属性
    XmlElement xml_recorder1 = doc.CreateElement("recorder");
    xml_recorder1.SetAttribute("key", "111");
    xml_recorder1.SetAttribute("value", "222");
    xml_recorder1.SetAttribute("type", "0");
    xml_recorders.AppendChild(xml_recorder1);
    //创建recorder节点，添加属性
    XmlElement xml_recorder2 = doc.CreateElement("recorder");
    xml_recorder2.SetAttribute("key", "111");
    xml_recorder2.SetAttribute("value", "222");
    xml_recorder2.SetAttribute("type", "1");
    xml_recorders.AppendChild(xml_recorder2);
    configuration.AppendChild(xml_recorders);
    //保存文件
    doc.Save("D:\\U8SOFT\\Portal\\App.config");

    recorders = doc.SelectSingleNode("configuration/recorders");
}

• 修改后config文件

<?xml version="1.0"?>
<configuration>
  <appSettings>
    <add key="111" value="222" />
    <add key="111" value="333" />
  </appSettings>
  <recorders enabled="true">
    <recorder key="111" value="222" type="0" />
    <recorder key="111" value="222" type="1" />
  </recorders>
</configuration>