Null reference exception in my LINQ to XML code

Question

I have been playing around with linking XML files to dropdown lists and gridviews.

I have managed to populate one dropdown list from the XML document and then a gridview to another but when try to add a where clause, I get a null reference exception and not sure why. How can I resolve this?

XDocument xmlDoc = XDocument.Load(Server.MapPath("XMLFile.xml"));
var q = from c in xmlDoc.Descendants("Images")
        where c.Attribute("PropertyId").Value == DropDownList1.SelectedValue.ToString()
        select new
        {
            PropertyID = c.Element("ThumbUrl").Value,
        };
GridView1.DataSource = q;
GridView1.DataBind();

Is nullreference thrown at where part? Are you sure "PropertyId" attribute exists on "Images" node...? — k.m
– k.m, Commented Apr 22, 2011 at 8:05
ok thanks for that managed to get it working to a point so much appreciated - one last qestion if you can help....hte gridview now shows the correct ThumbUrl when dropdown box changes - but how do i change the gridview to link to the image as opposed to just showing the url? — Kev
– Kev, Commented Apr 22, 2011 at 9:15
Kev - to get the best out of StackOverflow you need to vote-up / accept answers that help you, and you stand a much better chance of getting an answer to your second question if you post it in its own right rather than in a comment. — Will Dean
– Will Dean, Commented Apr 26, 2011 at 8:06

Marc Gravell · Accepted Answer · 2011-04-22 08:18:32Z

6

Avoid using .Value; a range of null-safe implicit conversion operators are available:

var q = from c in xmlDoc.Descendants("Images")
        where (string)c.Attribute("PropertyId")
               == DropDownList1.SelectedValue.ToString()
        select new
        {
            PropertyID = (string)c.Element("ThumbUrl"),
        };

answered Apr 22, 2011 at 8:18

Marc Gravell

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1 Comment

Marcus Over a year ago

This deserves more recognition

Will Dean · Accepted Answer · 2011-04-22 08:08:45Z

1

Any of these:

c.Attribute("PropertyId")
c.Element("ThumbUrl")
DropDownList1.SelectedValue

could be null, and then calling .ToString() or .Value on them would give you the exception you're seeing.

If you're not happy to catch XML problems via NullReferenceExceptions, then you need to take the value of the Attribute() call into a local variable and then test against that (or call it twice and test the first call for null).

answered Apr 22, 2011 at 8:08

Will Dean

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Comments

Arnab Das · Accepted Answer · 2011-04-22 08:31:23Z

0

Try: where c.Attribute("PropertyId") != null && c.Attribute("PropertyId").Value == DropDownList1.SelectedValue.ToString() for the condition part and c.Element("ThumbUrl") != null . Your code should look like this:

XDocument xmlDoc = XDocument.Load(Server.MapPath("XMLFile.xml"));
var q = from c in xmlDoc.Descendants("Images")
        where c.Attribute("PropertyId") != null 
        && c.Attribute("PropertyId").Value == DropDownList1.SelectedValue.ToString() 
        && c.Element("ThumbUrl") != null
        select new
        {
            PropertyID = c.Element("ThumbUrl").Value,
        };
GridView1.DataSource = q;
GridView1.DataBind();

edited Apr 22, 2011 at 8:31

answered Apr 22, 2011 at 8:03

Arnab Das

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Stuntman · Accepted Answer · 2014-03-11 10:41:01Z

0

from x in document.Descendants("Images")
let xElement = x.Element("PropertyId")
where xElement != null && xElement.Value == DropDownList1.SelectedValue.ToString()
select new
{
   PropertyID = c.Element("ThumbUrl").Value,
};

answered Mar 11, 2014 at 10:41

Stuntman

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