How to write a query that finds elements based on context (LINQ to XML)
Sometimes you might have to write a query that selects elements based on their context. For example, you might want to filter based on preceding or following sibling elements, or on child or ancestor elements.
You can do this by writing a query and using the results of the query in the where
clause. If you have to first test against null, and then test the value, it's more convenient to do the query in a let
clause, and then use the results in the where
clause.
Example: Select p
elements that are immediately followed by a ul
element
The following example selects all p
elements that are immediately followed by a ul
element.
XElement doc = XElement.Parse(@"<Root>
<p id=""1""/>
<ul>abc</ul>
<Child>
<p id=""2""/>
<notul/>
<p id=""3""/>
<ul>def</ul>
<p id=""4""/>
</Child>
<Child>
<p id=""5""/>
<notul/>
<p id=""6""/>
<ul>abc</ul>
<p id=""7""/>
</Child>
</Root>");
IEnumerable<XElement> items =
from e in doc.Descendants("p")
let z = e.ElementsAfterSelf().FirstOrDefault()
where z != null && z.Name.LocalName == "ul"
select e;
foreach (XElement e in items)
Console.WriteLine("id = {0}", (string)e.Attribute("id"));
Dim doc As XElement = _
<Root>
<p id='1'/>
<ul>abc</ul>
<Child>
<p id='2'/>
<notul/>
<p id='3'/>
<ul>def</ul>
<p id='4'/>
</Child>
<Child>
<p id='5'/>
<notul/>
<p id='6'/>
<ul>abc</ul>
<p id='7'/>
</Child>
</Root>
Dim items As IEnumerable(Of XElement) = _
From e In doc...<p> _
Let z = e.ElementsAfterSelf().FirstOrDefault() _
Where z IsNot Nothing AndAlso z.Name.LocalName = "ul" _
Select e
For Each e As XElement In items
Console.WriteLine("id = {0}", e.@<id>)
Next
This example produces the following output:
id = 1
id = 3
id = 6
Example: In a namespace select p
elements that are immediately followed by a ul
element
The following example shows the same query as above, but for XML that's in a namespace. For more information, see Namespaces overview.
XElement doc = XElement.Parse(@"<Root xmlns='http://www.adatum.com'>
<p id=""1""/>
<ul>abc</ul>
<Child>
<p id=""2""/>
<notul/>
<p id=""3""/>
<ul>def</ul>
<p id=""4""/>
</Child>
<Child>
<p id=""5""/>
<notul/>
<p id=""6""/>
<ul>abc</ul>
<p id=""7""/>
</Child>
</Root>");
XNamespace ad = "http://www.adatum.com";
IEnumerable<XElement> items =
from e in doc.Descendants(ad + "p")
let z = e.ElementsAfterSelf().FirstOrDefault()
where z != null && z.Name == ad.GetName("ul")
select e;
foreach (XElement e in items)
Console.WriteLine("id = {0}", (string)e.Attribute("id"));
Imports <xmlns='http://www.adatum.com'>
Module Module1
Sub Main()
Dim doc As XElement = _
<Root>
<p id='1'/>
<ul>abc</ul>
<Child>
<p id='2'/>
<notul/>
<p id='3'/>
<ul>def</ul>
<p id='4'/>
</Child>
<Child>
<p id='5'/>
<notul/>
<p id='6'/>
<ul>abc</ul>
<p id='7'/>
</Child>
</Root>
Dim items As IEnumerable(Of XElement) = _
From e In doc...<p> _
Let z = e.ElementsAfterSelf().FirstOrDefault() _
Where z IsNot Nothing AndAlso z.Name = GetXmlNamespace().GetName("ul") _
Select e
For Each e As XElement In items
Console.WriteLine("id = {0}", e.@<id>)
Next
End Sub
End Module
This example produces the following output:
id = 1
id = 3
id = 6
See also
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