Getting a specific node from xml using linq

Question

Im having some trouble with a linq query into an xml tree. Here is what the tree structure looks like:

<Student>
  <ID> Hello </Hello>
  <Classroom>
     <Name> 1B </Name>
     <Year> 1 </Year>
  </Classroom>
  <Classroom>
     <Name> 2B </Name>
     <Year> 2 </Year>
  </Classroom>
  <Classroom>
     <Name> 3B </Name>
     <Year> 3 </Year>
  </Classroom>
</Student>

Now this is one student entry among 5. say i am passed an XElement Student node from another method and i want to search for a classroom given the XElement Student node and the classroom name. So i have to write a method like this:

getClassRoomNode(XElement StudentNode, string classroomName)

This is what ive tried. Please let me know where i am wrong

XElement classroom = StudentNode.Descendants("Classroom")
                                .Where(arg => arg.Element("Name").Value == classroomName)
                                .Select(arg => arg.Parent)
                                .First();

This returns the StudentNode back again instead of a classroom node. Can anyone please help me out with this?

There is an error in your XML, not well formed "<ID>"

Jodrell
– Jodrell

2011-08-10 15:54:58 +00:00
Commented Aug 10, 2011 at 15:54 — Jodrell
– Jodrell, Commented Aug 10, 2011 at 15:54
sorry, its just an example... fixed now

Joe
– Joe

2011-08-10 15:55:54 +00:00
Commented Aug 10, 2011 at 15:55 — Joe
– Joe, Commented Aug 10, 2011 at 15:55

Jodrell · Accepted Answer · 2014-11-14 09:43:28Z

2

You don't need to select parent, just the get the First(), or even better Single() class room.

XElement classroom = StudentNode.Descendants("Classroom")
    .Single(arg => arg.Element("Name").Value == classroomName);

I suggest it is better to error rather than ignore subsequent matches. If the intention is to match the first of many matches then obviously,

XElement classroom = StudentNode.Descendants("Classroom")
    .First(arg => arg.Element("Name").Value == classroomName);

edited Nov 14, 2014 at 9:43

answered Aug 10, 2011 at 15:57

Jodrell

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Comments

Bob Vale · Accepted Answer · 2011-08-10 15:55:29Z

1

Get rid of the Select, you are reselecting the parent!

XElement classroom = StudentNode.Descendants("Classroom").Where(arg => arg.Element("Name").Value == classroomName).First();

answered Aug 10, 2011 at 15:55

Bob Vale

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1 Comment

Jodrell Over a year ago

First() could replace Where()

dtb · Accepted Answer · 2011-08-10 15:56:30Z

1

Your code does exactly what it's told to: select the <classrom> node where the content of the name element equals classroomName, and then select the parent of that <classrom> node, i.e. the <student> node.

Fix:

XElement classroom =
    StudentNode.Elements("Classroom")
               .First(arg => (string)arg.Element("Name") == classroomName);

This returns the first <classrom> node where the content of the name element equals classroomName.

answered Aug 10, 2011 at 15:56

dtb

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3 Comments

Jodrell Over a year ago

Wouldn't it complain because classroomName is not an XElement

dtb Over a year ago

@Jodrell: No, because (string)arg.Element("Name") returns the content of the <name> element as string.

Jodrell Over a year ago

you are right, I missed the explicit cast msdn.microsoft.com/en-us/library/bb155263.aspx note the CLS compliance.

jdavies · Accepted Answer · 2011-08-10 16:08:26Z

0

You are selecting the parent node of the Classroom node. Change the query like so:

XElement classroom = StudentNode.Descendants("Classroom").First(arg => arg.Element("Name").Value == classroomName);

Instead of using First you should use FirstOrDefault and check for a null value to stop any exceptions which may occur if no Classroom nodes are found based on your query. (Ignore this if you are handling this exception elsewhere in your code)

Enumerable.FirstOrDefault

edited Aug 10, 2011 at 16:08

answered Aug 10, 2011 at 15:55

jdavies

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2 Comments

Dan Abramov Over a year ago

.Select(arg => arg) is still redundant, why including it at all?

Jodrell Over a year ago

Additionaly, First() could replace Where()

Bob Vale · Accepted Answer · 2011-08-10 16:19:22Z

0

This is because you are selecting the parent of the classroom nodes. Just get rid of the last selector:

XElement classroom = StudentNode.Descendants("Classroom").Where(arg => arg.Element("Name").Value == classroomName).First();

Or even simpler, as was pointed out combine First and Where together.

XElement classroom = StudentNode.Descendants("Classroom").First(arg => arg.Element("Name").Value == classroomName)

This will however throw an exception if there is no match, FirstOrDefault on the otherhand would return null on no match;

XElement classroom = StudentNode.Descendants("Classroom").FirstOrDefault(arg => arg.Element("Name").Value == classroomName)

edited Aug 10, 2011 at 16:19

Bob Vale

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answered Aug 10, 2011 at 15:55

Oved D

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1 Comment

Jodrell Over a year ago

First() could replace Where()

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