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Check if XAttribute exists inside XElement

Posted on 2008-10-08
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Last Modified: 2013-12-17
Hey experts!

I am working on pulling ASP.NET MVC routes from an external file, and it's going well at the moment. My problem is that I want to be able to return the 'controller' attribute from the 'page' element, or the 'controller' attribute from the 'dir' element (parent of the 'page' element).

The problem is that when this runs, it causes an exception (ArgumentNullException) because I'm trying to retrieve the value of an attribute that doesn't exist.

I realise that, in the snippet, retrieving ControllerName is invalid, but I was trying to highlight what I want to  retrieve.

Any help would be great!

Thanks in advance!
XDocument xDoc = XDocument.Load("urls.xml");
var pages =
  from x in xDoc.Descendants().Elements("page")
  select new
  {
    Name = x.Parent.Attribute("name").Value,
    ControllerName = x.Attribute("controller").Value || x.Parent.Attribute("controller").Value,
    ControllerAction = x.Attribute("action").Value,
    Dir = x.Parent.Attribute("name").Value,
    Url = x.Parent.Attribute("name").Value + "/" + x.Attribute("url").Value,
  };
 
//An example of the urls.xml files is thus:
//
//<urls>
//  <dir name="Cheese" controller="Food"> <!-- Always a 'controller' attr on the 'dir' node -->
//    <page url="mexican.htm" action="mexican" controller="bar" />
//    <page url="cheddar.htm" action="cheddar" /> <!-- Note, no controller attribute here ('controller' attr is optional in 'page' node -->
//    <!--...-->
//  </dir>
//  <dir name="Alcohol" controller="Drink">
//  <!--...-->
//  </dir>
//</urls>

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Question by:DanAtkinson
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Kevin Cross earned 500 total points
ID: 22667753
If I am understanding you correctly and one of the values will at least by blank instead of null, you can use an inline if statement like this:
ControllerName = (x.Attribute("controller")==null ? x.Parent.Attribute("controller").Value : x.Attribute("controller").Value)

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Expert Comment

by:Kevin Cross
ID: 22667768
Guess you could try nesting too.  Don't believe there are any restrictions stopping you from repeating an inline if in the true section of the first if for x.Parent.Attribute("controller") == null then set some default if true of x.Parent.Attribute("controller").Value if not.
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Author Comment

by:DanAtkinson
ID: 22668101
Yep, that's the one! I was checking to see if the attribute value string was null or empty, rather than the attribute itself being null.

How stupid am I! :D

Thanks for your speedy answer!
//INVALID CODE
string.IsNullOrEmpty(x.Attribute("controller").Value) ? x.Parent.Attribute("controller").Value : x.Attribute("controller").Value

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by:Kevin Cross
ID: 22668961
You are welcome!
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