Question

I’m trying to learn LINQ and have a question on querying XML using VB.

My XML:

 <Procedure-Text>     <A ID='marker1'></A>Do This Procedure  </Procedure-Text>  <Procedure-Text>     <A ID='marker2'></A>Do That Procedure  </Procedure-Text> 

How can I specify my query to get only the procedure text that has the ID attribute marker2? In other words, I would like a resulting string that says Do That Procedure.

Thanks

Answer 1

Use VB XML literals:

Dim marker2 = From x In data...<Procedure-Text> _               Where x.<A>.@ID = 'marker2' _               Select x 

The triple dot syntax produce ‘all descendants’ of an xml element, i.e. data...<Procedure-Test> will produce a list of <Procedure-Test> tags inside data

The dot syntax on XML literals means ‘first descendant’ so x.<A> will produce the first occurence of <A> inside x. Of which x is now instance of <Procedure-Test>

And now that you have the desired <A> element, comparing its id to a string is trivial with the @attr attribute selector. <A>.@ID = 'marker2' will evaluate to True if the ID attribute of the <A> tag is equal to ‘marker2’

So x.<A>.@ID means ‘The ID attribute of the first <A> tag inside x’

And you want the <Procedure-Text> element, so you specify Select x

Full example:

Sub Main()      Dim data = <doc>                    <Procedure-Text>                        <A ID='marker1'></A>Do This Procedure                    </Procedure-Text>                    <Procedure-Text>                        <A ID='marker2'></A>Do That Procedure                    </Procedure-Text>                </doc>      Dim marker2 = From x In data...<Procedure-Text> _                   Where x.<A>.@ID = 'marker2' _                   Select x      ' prints the second procedure-text element     Console.WriteLine(marker2.FirstOrDefault().ToString())      Console.ReadKey()  End Sub