Strange behaviour with identical XmlNode.SelectSingleNode(xpathQuery) queries

[Rune]

I have two queries that appear to be exactly the same, but one of them
returns null while the other one returns a valid result!
Can anyone provide an explanation to why this is so? Below is an nunit
test that exposes the problem. I have run the test under both the 1.0
and 1.1 framework with the same result.

[Test]
public void XPathBooks()
{
XmlDocument smallDoc = new XmlDocument();
smallDoc.LoadXml("<book><title>Bugs</title></book>");
XmlNode book1 = smallDoc.SelectSingleNode(@"book");
XmlNode title1 = book1.SelectSingleNode(@"/book/title");
Assert.IsNotNull(title1, "title1");

XmlDocument largeDoc = new XmlDocument();
largeDoc.LoadXml("<books><book><title>Bugs</title></book></books>");
XmlNode book2 = largeDoc.SelectSingleNode(@"books/book");
Assert.AreEqual(book1.OuterXml, book2.OuterXml);
XmlNode title2 = book2.SelectSingleNode(@"/book/title");
Assert.IsNotNull(title2, "title2"); // This test fails!!
}

Thanks,
Rune

[Derek Harmon]

"Rune" <ru*****@yahoo.no> wrote in message news:f6**************************@posting.google.c om...

I have two queries that appear to be exactly the same, but one of them
returns null while the other one returns a valid result!
Can anyone provide an explanation to why this is so?

Rune,

The queries are the same, but the XML node tree you're querying
and the context in which you're querying it has changed.

The first query is "/book/title" against the following XML document,

<book>
<title> Bugs </title>
</book>

Where the context node for the XPath expression is <book>
(observe that <book>'s ParentNode is "#document"). The
XPath expression is absolute, so it starts at #document,
finds #document has child named <book>, and that <book>
has child named <title>.

The second query is "/book/title" against the following XML
document,

<books>
<book>
<title> Bugs </title>
</book>
</books>

The context node for the XPath expression is <book> (observe
that <book>'s ParentNode is "books" and not "#document"). The
XPath expression is absolute, so it begins at #document. It finds
that #document has no immediate child named "book" (it has one
named "books," but that doesn't match), therefore the query
returns null.

The XPath expression equivalent to the first expression when
the context node is <book> in the second document would be:

XmlNode title2 = book2.SelectSingleNode(@"title");

As a general practice when writing XPath expressions, try not
to spell out the context node explicitly. For instance, instead
of using the relative expression "../book/title" (which may not
produce a single node if <book> has siblings), just say "title".
Derek Harmon

[Derek Harmon]

"Rune" <ru*****@yahoo.no> wrote in message news:f6**************************@posting.google.c om...

I have two queries that appear to be exactly the same, but one of them
returns null while the other one returns a valid result!
Can anyone provide an explanation to why this is so?

Rune,

The queries are the same, but the XML node tree you're querying
and the context in which you're querying it has changed.

The first query is "/book/title" against the following XML document,

<book>
<title> Bugs </title>
</book>

Where the context node for the XPath expression is <book>
(observe that <book>'s ParentNode is "#document"). The
XPath expression is absolute, so it starts at #document,
finds #document has child named <book>, and that <book>
has child named <title>.

The second query is "/book/title" against the following XML
document,

<books>
<book>
<title> Bugs </title>
</book>
</books>

The context node for the XPath expression is <book> (observe
that <book>'s ParentNode is "books" and not "#document"). The
XPath expression is absolute, so it begins at #document. It finds
that #document has no immediate child named "book" (it has one
named "books," but that doesn't match), therefore the query
returns null.

The XPath expression equivalent to the first expression when
the context node is <book> in the second document would be:

XmlNode title2 = book2.SelectSingleNode(@"title");

As a general practice when writing XPath expressions, try not
to spell out the context node explicitly. For instance, instead
of using the relative expression "../book/title" (which may not
produce a single node if <book> has siblings), just say "title".
Derek Harmon

[Rune]

Of course :) Thanks a lot

[Rune]

Of course :) Thanks a lot