Why this LINQ to XML doesn't work?

Question

I receive this XML from an external source:

<epp xmlns="urn:ietf:params:xml:ns:epp-1.0">
  <response>
    <result code="1300">
      <msg>Command completed successfully; no messages</msg>
    </result>
    <trID>
      <clTRID>TEST-12345</clTRID>
      <svTRID>IRNIC_2011-09-21T13:44:25+04:30_f1u</svTRID>
    </trID>
  </response>
</epp>

I want to extract the value of the code attribute of the result element. I used this code:

XDocument doc = XDocument.Parse(response);
XNamespace ns = "urn:ietf:params:xml:ns:epp-1.0";
string value = doc.Descendants(ns + "result").First().Attribute("code").Value;

However, it throws null value exception, because doc.Descendants(ns + "result") is null.

What's wrong here?

Descendants("urn:ietf:params:xml:ns:epp-1.0result")? Is that right? — bzlm
– bzlm, Commented Sep 21, 2011 at 9:21
Looks fine to me - have you inspected'doc' with the debugger to verify that it is the XML document you have included above? — ColinE
– ColinE, Commented Sep 21, 2011 at 9:21
@ColinE, yeah, it's valid and everything is as I've posted here. — Saeed Neamati
– Saeed Neamati, Commented Sep 21, 2011 at 9:22
I have just put this in a unit test and it works fine for me! — Tim Rogers
– Tim Rogers, Commented Sep 21, 2011 at 9:26
@bzlm - the first part is a namespace, not a string - XNamespace. The result element is in the namespace, so this is correct. — ColinE
– ColinE, Commented Sep 21, 2011 at 9:30

Marc Gravell · Accepted Answer · 2011-09-21 09:32:59Z

3

However, it throws null value exception, because doc.Descendants(ns + "result") is null

I would not expect doc.Descendants to ever return a null - it might return an empty sequence, but that wouldn't be a "null value exception". Likewise, First() - it either returns something, or it throws an exception (and while that "something" can be a null, I do not expect Descendants would be yielding nulls). So that leaves .Attribute("code").Value. Now yes; .Attribute("code") can return a null, if an attribute doesn't exist. If you are happy to obtain a null in those cases, I recommend:

string value = (string)doc.Descendants(ns + "result").First().Attribute("code");

the conversion operator handles null (non-existent) attributes.

answered Sep 21, 2011 at 9:32

Marc Gravell

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Comments

Henk Holterman · Accepted Answer · 2011-09-21 09:28:18Z

1

No repro. Your code is basically OK.

So: Debug. Break the statement up and check things.

is your doc loaded OK ?
what does just doc.Descendants(ns + "result").First() deliver?
etc

answered Sep 21, 2011 at 9:28

Henk Holterman

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Rased Dot Net · Accepted Answer · 2011-09-21 10:08:24Z

1

Check response variable because i loaded the xml from an xml file and your code worked great.

In case of testing, you can save the response in an xml file and load it from there.

use XDocument.Load("a.xml") to load an xml file.

answered Sep 21, 2011 at 10:08

Rased Dot Net

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Trygve · Accepted Answer · 2011-09-23 09:41:45Z

Tested this too. Easiest way to test is to download LinqPad, create new query, then for Language select "C# statement(s)" from the dropdown. Click Run, should work with the code provided. I would also check the response variable as suggested by "Rased Dot Net"

string xml =
@"<epp xmlns=""urn:ietf:params:xml:ns:epp-1.0"">
  <response>
    <result code=""1300"">
      <msg>Command completed successfully; no messages</msg>
    </result>
    <trID>
      <clTRID>TEST-12345</clTRID>
      <svTRID>IRNIC_2011-09-21T13:44:25+04:30_f1u</svTRID>
    </trID>
  </response>
</epp>";

XDocument doc = XDocument.Parse(xml);
Console.WriteLine(doc.ToString());
XNamespace ns = doc.Root.Name.Namespace;
Console.WriteLine("Namespace: " + ns.ToString());
string value = doc.Descendants(ns + "result").First().Attribute("code").Value;
Console.WriteLine(value);

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