XmlDocument doc = new XMLDocument();
//这里为创建节点等代码，省略....
//保存
doc.Save(filename);
结果生成的节点有换行：
<UserName>
</UserName>

using (XmlTextWrITer xtw = new XmlTextWriter(filename, null))
{
//None表示不应用特殊格式，另一个相反枚举值Indented表示缩进
xtw.Formatting = Formatting.None;
doc.Save(xtw);
}

public static XmlElement CreateNodeWithNullAttr(XmlDocument doc, string nodeName)
{
XmlElement element = doc.CreateElement(nodeName);
XmlAttribute attr = doc.CreateAttribute("xsi", "nil", "http://www.w3.org/2001/XMLSchema-instance");
attr.Value = "true";
element.setattributeNode(attr);
//element.Attributes.ApPEnd(attr);
return element;
}