为什么linq to xml无法识别我的XML文件中的属性？

Question

I'm brand new to LINQ to XML (comfortable with LINQ to sql) and learning my way around with this tutorial http://www.joe-stevens.com/2010/01/08/linq-to-xml-tutorial/

When I write this linq query

Dim data As XDocument = XDocument.Load(xmlFileLocation)
Dim outputFileLoc = (From c In data.Descendants("Program") Where c.Attribute("ProgramName").Equals("EnviroEpi") Select c)

on this XML file:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Programs>
   <Program ProgramName="EnviroEpi">
      <Type>csv</Type>
      <Location>test</Location>
   </Program>
   <Program ProgramName="Epi">
       <Type>HL7</Type>
       <Location>test</Location>
   </Program>
   <Program ProgramName="Lead">
      <Type>csv</Type>
      <Location>test</Location>
   </Program>
</Programs>

I get an error:

Sequence contains no elements

Does this mean that LINQ can't see the ProgramName attribute of my program nodes? Or am I missing something else? As far as I can tell, I'm doing everything exactly like in the tutorial.

Thanks again for the help. I'm really just getting a feel for LINQ to XML.

Edit

... where c.attribute("ProgramName").Equals("EnviroEpi") // returns no elements. 

This works:

 c.attribute("ProgramName").value.equals("EnviroEpi") //added in .value to get it to work

Answer 1

You don't want Descendants("Programs").Descendants , that would return any descendant of the Programs elements - most of which are unrelated and don't have a ProgramName attribute, hence you get a NullReferenceException .

You also need to use Attribute("ProgramName").value.Equals("EnviroEpi") to get to the value of the program name attribute.

You want Descendants("Program") instead:

 Dim outputFileLoc = (From c In data.Descendants("Program") Where c.Attribute("ProgramName").Equals("EnviroEpi") Select c).First

Answer 2

The issue is most likely that data.Descendants("Programs") or data.Descendants("Programs").Descendants is null. You need to do null checks along the way, you can't really just chain a bunch of objects together like you've done.