XmlDocument doc = new XmlDocument();
//這里為創建節點等代碼，省略....
//保存
doc.Save(filename);
結果生成的節點有換行：
<UserName>
</UserName>

using (XmlTextWriter xtw = new XmlTextWriter(filename, null))
{
//None表示不應用特殊格式，另一個相反枚舉值Indented表示縮進
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;
}