通过 XDocument.Load 加载文档时，您会注意到，您要编写的查询与通过 XElement.Load 加载文档时稍有不同。
XDocument.Load 与 XElement.Load 的比较
通过 XElement.Load 将 XML 文档加载到 XElement 中时，位于 XML 树根部的 XElement 包含所加载文档的根元素。然而，通过 XDocument.Load 将同一个 XML 文档加载到 XDocument 中时，树根部为 XDocument 节点，所加载文档的根元素为 XDocument 所允许的一个子 XElement 节点。LINQ to XML 轴相对于根节点进行操作。
第一个示例使用 Load 加载 XML 树。然后它查询树根部的子元素。
// Create a simple document and write it to a file
File.WriteAllText("Test.xml", @"<Root>
<Child1>1</Child1>
<Child2>2</Child2>
<Child3>3</Child3>
</Root>");
Console.WriteLine("Querying tree loaded with XElement.Load");
Console.WriteLine("----");
XElement doc = XElement.Load("Test.xml");
IEnumerable<XElement> childList =
from el in doc.Elements()
select el;
foreach (XElement e in childList)
Console.WriteLine(e);
' Create a simple document and write it to a file
File.WriteAllText("Test.xml", "<Root>" + Environment.NewLine + _
" <Child1>1</Child1>" + Environment.NewLine + _
" <Child2>2</Child2>" + Environment.NewLine + _
" <Child3>3</Child3>" + Environment.NewLine + _
"</Root>")
Console.WriteLine("Querying tree loaded with XElement.Load")
Console.WriteLine("----")
Dim doc As XElement = XElement.Load("Test.xml")
Dim childList As IEnumerable(Of XElement) = _
From el In doc.Elements() _
Select el
For Each e As XElement In childList
Console.WriteLine(e)
Next
此示例将按预期产生以下输出：
Querying tree loaded with XElement.Load
----
<Child1>1</Child1>
<Child2>2</Child2>
<Child3>3</Child3>
下面的示例与上面的基本相同，不同之处在于 XML 树是加载到 XDocument，而不是加载到 XElement。
// Create a simple document and write it to a file
File.WriteAllText("Test.xml", @"<Root>
<Child1>1</Child1>
<Child2>2</Child2>
<Child3>3</Child3>
</Root>");
Console.WriteLine("Querying tree loaded with XDocument.Load");
Console.WriteLine("----");
XDocument doc = XDocument.Load("Test.xml");
IEnumerable<XElement> childList =
from el in doc.Elements()
select el;
foreach (XElement e in childList)
Console.WriteLine(e);
' Create a simple document and write it to a file
File.WriteAllText("Test.xml", "<Root>" + Environment.NewLine + _
" <Child1>1</Child1>" + Environment.NewLine + _
" <Child2>2</Child2>" + Environment.NewLine + _
" <Child3>3</Child3>" + Environment.NewLine + _
"</Root>")
Console.WriteLine("Querying tree loaded with XDocument.Load")
Console.WriteLine("----")
Dim doc As XDocument = XDocument.Load("Test.xml")
Dim childList As IEnumerable(Of XElement) = _
From el In doc.Elements() _
Select el
For Each e As XElement In childList
Console.WriteLine(e)
Next
此示例产生以下输出：
Querying tree loaded with XDocument.Load
----
<Root>
<Child1>1</Child1>
<Child2>2</Child2>
<Child3>3</Child3>
</Root>
请注意，同样的查询返回一个 Root 节点，而不是返回三个子节点。
处理这一问题的一种方法是在访问轴方法之前使用 Root 属性，如下所示：
// Create a simple document and write it to a file
File.WriteAllText("Test.xml", @"<Root>
<Child1>1</Child1>
<Child2>2</Child2>
<Child3>3</Child3>
</Root>");
Console.WriteLine("Querying tree loaded with XDocument.Load");
Console.WriteLine("----");
XDocument doc = XDocument.Load("Test.xml");
IEnumerable<XElement> childList =
from el in doc.Root.Elements()
select el;
foreach (XElement e in childList)
Console.WriteLine(e);
' Create a simple document and write it to a file
File.WriteAllText("Test.xml", "<Root>" + Environment.NewLine + _
" <Child1>1</Child1>" + Environment.NewLine + _
" <Child2>2</Child2>" + Environment.NewLine + _
" <Child3>3</Child3>" + Environment.NewLine + _
"</Root>")
Console.WriteLine("Querying tree loaded with XDocument.Load")
Console.WriteLine("----")
Dim doc As XDocument = XDocument.Load("Test.xml")
Dim childList As IEnumerable(Of XElement) = _
From el In doc.Root.Elements() _
Select el
For Each e As XElement In childList
Console.WriteLine(e)
Next
此查询现在的执行方式与根部位于 XElement 中的树中的查询相同。此示例产生以下输出：
Querying tree loaded with XDocument.Load
----
<Child1>1</Child1>
<Child2>2</Child2>
<Child3>3</Child3>