在SelectSingleNode中使用XPath：从XML中检索单个元素（如果存在）

Question

My XML looks like :

<?xml version=\"1.0\"?>
<itemSet>
       <Item>one</Item>
       <Item>two</Item>
       <Item>three</Item>
       .....maybe more Items here.
</itemSet>

Some of the individual Item may or may not be present. Say I want to retrieve the element <Item> two </Item> if it's present. I've tried the following XPaths (in C#).

• XMLNode node = myXMLdoc.SelectSingleNode("/itemSet[Item='two']") --- If Item two is present, then it returns me only the first element one . Maybe this query just points to the first element in itemSet, if it has an Item of value two somewhere as a child. Is this interpretation correct?

So I tried:

• XMLNode node = myXMLdoc.SelectSingleNode("/itemSet[Item='two']/Item[1]") --- I read this query as, return me the first <Item> element within itemSet that has value = 'two'. Am I correct?

This still returns only the first element one . What am I doing wrong? In both the cases, using the siblings I can traverse the child nodes and get to two , but that's not what I am looking at. Also if two is absent then SelectSingleNode returns null. Thus the very fact that I am getting a successfull return node does indicate the presence of element two, so had I wanted a boolean test to chk presence of two , any of the above XPaths would suffice, but I actually the need the full element <Item>two</Item> as my return node.

[My first question here, and my first time working with web programming, so I just learned the above XPaths and related xml stuff on the fly right now from past questions in SO. So be gentle, and let me know if I am a doofus or flouting any community rules. Thanks.]

Answer 1

I think you want:

myXMLdoc.SelectSingleNode("/itemSet/Item[text()='two']")

In other words, you want the Item which has text of two, not the itemSet containing it.

You can also use a single dot to indicate the context node, in your case:

myXMLdoc.SelectSingleNode("/itemSet/Item[.='two']")

EDIT: The difference between . and text() is that . means "this node" effectively, and text() means "all the text node children of this node". In both cases the comparison will be against the "string-value" of the LHS. For an element node, the string-value is "the concatenation of the string-values of all text node descendants of the element node in document order" and for a collection of text nodes, the comparison will check whether any text node is equal to the one you're testing against.

So it doesn't matter when the element content only has a single text node, but suppose we had:

<root>
  <item name="first">x<foo/>y</item>
  <item name="second">xy<foo/>ab</item>
</root>

Then an XPath expression of " root/item[.='xy'] " will match the first item, but " root/item[text()='xy'] " will match the second.