Remove element from XML based on attribute value?

Question

I was trying to remove a descendant element from an XElement (using .Remove()) and I seem to get a null object reference, and I'm not sure why.

Having looked at the previous question with this title (see here), I found a way to remove it, but I still don't see why the way I tried 1st didn't work.

Can someone enlighten me ?

  String xml = "<things>"
             + "<type t='a'>"
             + "<thing id='100'/>"
             + "<thing id='200'/>"
             + "<thing id='300'/>"
             + "</type>"
             + "</things>";

  XElement bob = XElement.Parse(xml);

  // this doesn't work...
  var qry = from element in bob.Descendants()
            where element.Attribute("id").Value == "200"
            select element;
  if (qry.Count() > 0)
     qry.First().Remove();

  // ...but this does
  bob.XPathSelectElement("//thing[@id = '200']").Remove();

Thanks, Ross

svick · Accepted Answer · 2012-01-16 10:00:36Z

3

The problem is that the collection you are iterating contains some element that don't have the id attribute. For them, element.Attribute("id") is null, and so trying to access the Value property throws a NullReferenceException.

One way to solve this is to use a cast instead of Value:

var qry = from element in bob.Descendants()
          where (string)element.Attribute("id") == "200"
          select element;

If an element doesn't have the id attribute, the cast will returns null, which works fine here.

And if you're doing a cast, you can just as well cast to an int?, if you want.

answered Jan 16, 2012 at 10:00

svick

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1 Comment

Black Light Over a year ago

Thanks svick, I see what the issue is now.

ColinE · Accepted Answer · 2012-01-16 10:04:16Z

1

Try the following:

  var qry = bob.Descendants()
               .Where(el => el .Attribute("id") != null)
               .Where(el => el .Attribute("id").Value = "200")

  if (qry.Count() > 0)
     qry.First().Remove();

You need to test for the presence of the id attribute before getting its value.

answered Jan 16, 2012 at 10:04

ColinE

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3 Comments

Black Light Over a year ago

Thanks for the useful advice re. checking attributes - I see why its relevant in the light of svick's answer.

ColinE Over a year ago

@BlackLight oh well, it was worth a try!

Basit Nizami Over a year ago

Well, no offense but it's been over three years and I'm really surprised how no body saw any errors in this code. There are 3 very noticeable mistakes. el .Attribute has white space both times and Attribute("id").Value = "200" should have been Attribute("id").Value == "200" instead. Notice the double = sign.

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