Xml-SelectNodes with default-namespace via XmlNamespaceManager not working as expected

Question

I have some xml with default namespace

<a xmlns='urn:test.Schema'><b/><b/></a>

and want to count the number of <b/>

How do I have to define

XmlNamespaceManager nsmgr = ????
Assert.AreEqual(2, doc.SelectNodes("//b", nsmgr).Count);

so that the assert becomes true?

I have tried so far (using nunit):

[Test]
[Ignore("Why does this not work?")]
public void __DoesNotWork_TestSelectWithDefaultNamespace()
{
    // xml to parse with defaultnamespace
    string xml = @"<a xmlns='urn:test.Schema'><b/><b/></a>";

    XmlDocument doc = new XmlDocument();
    doc.LoadXml(xml);

    // fails because xpath does not have the namespace
    //!!!!
    Assert.AreEqual(2, doc.SelectNodes("//b").Count);

    // using XPath defaultnamespace 
    XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
    nsmgr.AddNamespace("", "urn:test.Schema");

    // This will fail with dotnet 3.5sp1. Why?
    //!!!!
    Assert.AreEqual(2, doc.SelectNodes("//b", nsmgr).Count);
}

[Test]
public void TestSelectWithoutNamespaces_Ok()
{
    // xml to parse without namespace
    string xml = @"<a><b/><b/></a>";

    XmlDocument doc = new XmlDocument();
    doc.LoadXml(xml);

    // works ok
    Assert.AreEqual(2, doc.SelectNodes("//b").Count);

    // works ok
    XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
    Assert.AreEqual(2, doc.SelectNodes("//b", nsmgr).Count);
}

[Test]
public void TestSelectWithNamespacesPrefixed_Ok()
{
    // xml to parse with defaultnamespace
    string xml = @"<a xmlns='urn:test.Schema'><b/><b/></a>";

    XmlDocument doc = new XmlDocument();
    doc.LoadXml(xml);

    // using XPath namespace via alias "t". works ok but xpath is to complicated
    XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
    nsmgr.AddNamespace("t", doc.DocumentElement.NamespaceURI);
    Assert.AreEqual(2, doc.SelectNodes("//t:b", nsmgr).Count);
}

Answer 1

// This will fail with dotnet 3.5sp1. Why? 
//!!!! 
Assert.AreEqual(2, doc.SelectNodes("//b", nsmgr).Count);

This is a FAQ. In XPath any unprefixed name is assumed to be in "no namespace". In order to select elements that belong to a namespace, in any XPath expression their names must be prefixed with a prefix that is associated with this namespace. The AddNamespace() method serves exactly this purpose. It creates a binding between a specific namespace and a specific prefix. Then, if this prefix is used in an XPath expression, the element prefixed by it can be selected.

It is written in the XPath W3C spec: "A QName in the node test is expanded into an expanded-name using the namespace declarations from the expression context. This is the same way expansion is done for element type names in start and end-tags except that the default namespace declared with xmlns is not used: if the QName does not have a prefix, then the namespace URI is null".

See this at: w3.org/TR/xpath/#node-tests .

So, any unprefixed name is considered to be in "no namespace". In the provided XML document there are no b elements in "no namespace" and this is why the XPath expression //b selects no nodes at all.

Use:

XmlNamespaceManager nsmanager = new XmlNamespaceManager(doc.NameTable);
nsmanager.AddNamespace("x", "urn:test.Schema");

and later:

Assert.AreEqual(2, doc.SelectNodes("//x:b", nsmanager).Count);

Remember: The whole purpose of registering the namespace is to be able to use the prefix (in this case x) in any XPath expression.