用IXMLDOMElement怎么获得项目的个数啊

用IXMLDOMElement建立xml文件,得到了以下一个文件
<MyDatas>
<key>1</key>
<valuecount>1</valuecount>
<value>aaaaaa</value>
</MyDatas>
<MyDatas>
<key>2</key>
<valuecount>1</valuecount>
<value>bbbbbb</value>
</MyDatas>
<key>3</key>
<valuecount>1</valuecount>
<value>ddddd</value>
</MyDatas>怎么才能获得有多少个mydatas项啊