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我想排序函数如下所述的可选属性的一些XML节点的LINQ to XML:在可选属性的功能排序XML节点
<ncx xmlns="http://www.daisy.org/z3986/2005/ncx/">
<head>
<meta name="dtb:uid" content="http://www.hxa7241.org/articles/content/epup-guide_hxa7241_2007_1.epub" />
</head>
<docTitle>
<text>Roméo et Juliette</text>
</docTitle>
<navMap>
<navPoint id="toc" playOrder="2">
<navLabel>
<text>Table des Matières</text>
</navLabel>
<content src="9789999997049_toc.html" />
</navPoint>
<navPoint id="tp" playOrder="1">
<navLabel>
<text>Page de Titre</text>
</navLabel>
<content src="9789999997049_tp.html" />
</navPoint>
<navPoint id="p01" playOrder="3">
<navLabel>
<text>ACTE PREMIER</text>
</navLabel>
<content src="9789999997049_p01.html" />
<navPoint id="ch02" playOrder="5">
<navLabel>
<text>Scène 2</text>
</navLabel>
<navPoint id="ch01" playOrder="4">
<navLabel>
<text>Scène première</text>
</navLabel>
<content src="9789999997049_ch01.html" />
</navPoint>
<content src="9789999997049_ch02.html"/>
</navPoint>
</navPoint>
</navMap>
我怎样才能在的navPoint节点进行排序在playOrder属性函数中是否存在,如果不存在则正常检索xml?我应该首先测试playOrder是否存在,然后创建一个不同的Linq查询,或者是否可以用单个语句处理所有事情(可能是一个案例)?