Question

Considering the following code

IEnumerable<String> query = null;    
query = from x in xml.Descendants(xmlMasterContainerName).Descendants(xmlElementName)
                    let guid = x.Attribute("guid") ?? new XAttribute("guid", "-1")
                    where x.Attribute(xmlAttributeIdName).Value == xmlAttributeIdValue
                    select guid.Value;

I get the ‘object reference not set’ when trying query.ToList()

This is very probably caused by ‘select guid.Value’ when ‘x.Attribute(xmlAttributeIdName).Value == xmlAttributeIdValue’ does not exist.

How can I check the where statement for existing value before selecting?
Thanks

Answer 1

In XLinq, you usually don’t use Attribute().Value directly, because of the exact error you are getting. Instead, you cast it. The cast will result in null if Attribute() returned null, so there will be no exception.

So, you would change your where clause to this:

where ((string)x.Attribute(xmlAttributeIdName)) == xmlAttributeIdValue

and your select to this:

select (string)guid

BTW: I would write that code like this:

var query = xml.Descendants(xmlMasterContainerName)
               .Descendants(xmlElementName)
               .Where(x => ((string)x.Attribute(xmlAttributeIdName)) ==
                               xmlAttributeIdValue)
               .Select(x => (string)x.Attribute("guid") ?? "-1");