how to check an attribute value when that attribute may or may not exist

Question

My LINQ statement looks like

List<XElement> filteredset 
= (from file in matchingsets.Elements<XElement>(Global.xnFile) 
where (((string)file.Attribute(Global.xnToKeep).Value != "*")
&& ((string)file.Attribute(Global.xnProtected).Value != "*") &
& File.Exists(file.Attribute(Global.xnPath).Value)) 
select file).ToList<XElement>();

My problem is that neither file.Attribute(Global.xnToKeep). nor file.Attribute(Global.xnProtected) are guaranteed to exist in file.  So, I need to test their existence before testing what their value is.  How do I do that in a LINQ statement?

Answer 1

Try this,

List<XElement> filteredset 

= (from file in matchingsets.Elements<XElement>(Global.xnFile) 
where (
(file.Attribute(Global.xnToKeep) != null &&(string)file.Attribute(Global.xnToKeep).Value != "*")
&& (file.Attribute(Global.xnProtected) != null &&(string)file.Attribute(Global.xnProtected).Value != "*") 
&& File.Exists(file.Attribute(Global.xnPath).Value)
)select file).ToList<XElement>();

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Best Regards Sanjay Pant [Metadesign Solutions]

Answer 2

The LINQ to XML API supports your use case easily, simply do

List<XElement> filteredset

= (from file in matchingsets.Elements<XElement>(Global.xnFile)

where (string)file.Attribute(Global.xnToKeep) != "*" && (string)file.Attribute(Global.xnProtected) != "*" && File.Exists(file.Attribute(Global.xnPath).Value))

select file).ToList<XElement>();

instead of what you have. If the attribute of a certain name (say "foo") does not exist then

  file.Attribute("foo")

is null and the conversion operators ensure that

  (string)file.Attribute("foo")

is also null. And the comparison

  null != "*"

is possible whereas in your case with

 file.Attribute("foo").Value

you get a NullReferenceException if the attribute does not exist.

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MVP Data Platform Development My blog