一小时了, 写了三行代码! 把上面三行换成下面三行.

//XmlNodeList xmlNodeList = inXmlDoc.SelectNodes("//" +field);
    //    foreach(XmlNode node in xmlNodeList)
    //    {
    //     node.ParentNode.RemoveChild(node);
    //    }

    XmlNodeList xmlNodeList = inXmlDoc.GetElementsByTagName(field);

    while ( xmlNodeList[0] != null )
    {
     xmlNodeList[0].ParentNode.RemoveChild(xmlNodeList[0]); 
    }