List<string> list = new List<string>();
            list.Add("王小");
            list.Add("张三");
            list.Add("汪一");
            XElement file = XElement.Load(@"../../Info2.xml");
            var query = from item in list
                        let elems = from elem in file.Elements()
                                    select (string)elem.Attribute("Name")
                        where elems.Contains(item) == false
                        select item;

            foreach (var e in query)
            {
                XElement temp = new XElement("USer",
                    new XAttribute("Name", e),
                    new XElement("SendTime", "新时间")
                    );
                file.AddFirst(temp);
            }
            Console.WriteLine(file);