获取xml指定xpath的文本内容

public static string ReadInnerXml(string xmlfile,string parentnode){
    string returnstr = null;
    System.Xml.XmlTextReader xreader = null;
    try{
        xreader = new System.Xml.XmlTextReader(xmlfile);
        while(xreader.Read()){
            if((xreader.NodeType== System.Xml.XmlNodeType.Element) && (xreader.Name==parentnode)){    
                returnstr = xreader.ReadInnerXml();
                break;
            }
        }
    }catch(Exception e){
        System.Web.HttpContext.Current.Response.Write("App.ReadInnerXml:"+e.Message);
    }finally{
        if(xreader!=null)xreader.Close();
    }
    return returnstr;
}