说明： 读写XML文件，根据不同的选择显示不同的XML节点信息，超级棒的例子。
(read and write XML documents, according to the different options show different XML node information, the super rods example.)
文件列表：
MSXML_Sample
............\AssemblyInfo.
cs
............
\bin
............\...
\Debug
............\books.
xml
............\Form1.
cs
............\Form1.
resx
............\MSXMLTest.
csproj
............\MSXMLTest.csproj.
user
............\MSXMLTest.
sln
............
\obj
............\...
\Debug
............\...\Interop.MSXML2.
dll
