XML namespaces

XML namespaces provide a way to create globally unique names
to distinguish between elements and attributes with the same name
but of different terminologies. For example, in an XML invoice document
for a bookstore, the name “date” could be used by accounting for
the date of the order and by order fulfillment for the date of publication.An
XML namespace is identified by a Uniform Resource Identifier (URI),
a short string that uniquely identifies resources on the Web. The
elements and attributes in each namespace can be uniquely identified
by prefixing the element or attribute name (the local name) with
the URI of the namespace.

Associating a prefix with a namespace

You declare an XML namespace using xmlns as
part of a namespace declaration attribute. With the namespace declaration
attribute, you can associate a prefix with the namespace.

For example, the following namespace declaration attribute
declares the http://www.pre.com namespace
and associates the prefix pre with
this namespace:

Default XML namespace

If an XML namespace declaration does not specify a prefix,
the namespace becomes a default XML namespace. For example, the
following element, digicom, declares the namespace http://www.digital_software.com:

The
namespace http://www.digital_software.com is
the in-scope default namespace for the element digicom and
any child elements that digicom might contain.
The child elements of digicom will automatically
be in this namespace.

The NONAMESPACE declaration

The following namespace declaration is known as the NONAMESPACE declaration:

The
containing element and its child elements are declared to be in
no namespace. An element that is in the NONAMESPACE namespace has
its namespace prefix and URI set to empty strings.

Initial state

When a PBDOM_ELEMENT or a PBDOM_ATTRIBUTE
is first created, it has no name, and the namespace information
is by default set to the NONAMESPACE namespace (that is, its namespace
prefix and URI are both empty strings). The SetName method
is used to set the local name and the SetNamespace method
is used to set the namespace prefix and URI.

The name is required The name is a required property of a PBDOM_ELEMENT
and PBDOM_ATTRIBUTE, but the namespace information is not.

Retrieving from a parsed document

If a PBDOM_ELEMENT or PBDOM_ATTRIBUTE is
retrieved programmatically from a parsed document, then its name
and namespace information are inherited from the Element or Attribute
contained in the parsed document. However, even after parsing, the
name and namespace information of the PBDOM_ELEMENT and
PBDOM_ATTRIBUTE can be further modified with the SetName and SetNamespace methods.The
name and namespace information are stored separately internally. Changing
the name of a PBDOM_ELEMENT or PBDOM_ATTRIBUTE
does not affect its namespace information, and changing its namespace
information has no effect on its name.

Setting the name and namespace of a PBDOM_ATTRIBUTE

The W3C “Namespaces in XML” specification (in section 5.3)
places restrictions on setting the name and namespace of a PBDOM_ATTRIBUTE. No
tag can contain two attributes with identical names, or with qualified
names that have the same local name and have prefixes that are bound
to identical namespace names.

The specification provides the following examples of illegal
and legal attributes:

In the first example, <bad a=”1″ a=”2″ /> violates
the rule that no tag can contain two attributes with identical names.
In the second tag, the attributes have the same local name but different
prefixes, so that their names are different. However, their prefixes
point to the same namespace URI, http://www.w3.org,
so it is illegal to place them inside the same owner element.

PBDOM scenarios

The following scenarios illustrate how PBDOM conforms to these requirements.

• When the PBDOM_ATTRIBUTE SetName method
  is invoked:
  If the PBDOM_ATTRIBUTE pbdom_attr1 has
  an owner PBDOM_ELEMENT that contains an existing PBDOM_ATTRIBUTE with
  the same name that is to be set for pbdom_attr1 and
  has the same namespace URI as pbdom_attr1,
  the EXCEPTION_INVALID_NAME exception is thrown.
• When the PBDOM_ATTRIBUTE SetNamespace method
  is invoked:
  If the PBDOM_ATTRIBUTE pbdom_attr1 has
  an owner PBDOM_ELEMENT that contains an existing PBDOM_ATTRIBUTE with
  the same name as pbdom_attr1 and the
  same namespace URI that is to be set for pbdom_attr1,
  the EXCEPTION_INVALID_NAME exception is thrown.
• When the PBDOM_ELEMENT SetAttribute(pbdom_attribute pbdom_attribute_ref)
  method is invoked:
  If the PBDOM_ELEMENT already contains an attribute
  that has the same name and namespace URI as the input PBDOM_ATTRIBUTE,
  the existing attribute is replaced by the input PBDOM_ATTRIBUTE.
  The existing attribute is thus removed (detached) from the owner
  element.
• When the PBDOM_ELEMENT SetAttributes(pbdom_attribute pbdom_attribute_array[])
  method is invoked:
  If any two PBDOM_ATTRIBUTE objects in the array have
  the same name and namespace URI, the EXCEPTION_INVALID_NAME exception
  is thrown. If there is no name or namespace conflict within the array,
  all the existing attributes of the PBDOM_ELEMENT are replaced by
  the PBDOM_ATTRIBUTE objects in the array.

  Note All the above scenarios apply to PBDOM_ATTRIBUTE
  objects that are contained in the NONAMESPACE namespace.
  

• When the PBDOM_ELEMENT SetAttribute(string strName, string strValue)
  method is invoked:
  A new PBDOM_ATTRIBUTE with the specified name and
  value is created and set into the PBDOM_ELEMENT. If the
  PBDOM_ELEMENT already contains an attribute that has the
  same name and that is contained within the NONAMESPACE namespace,
  it is removed (detached) from the PBDOM_ELEMENT.
• When the PBDOM_ELEMENT SetAttribute(string strName, string strValue, string strNamespacePrefix, string strNamespaceUri, boolean bVerifyNamespace)
  method is invoked:
  A new PBDOM_ATTRIBUTE with the specified name, value,
  and namespace information is created and set into the PBDOM_ELEMENT.
  If the PBDOM_ELEMENT already contains a PBDOM_ATTRIBUTE
  that has the same name and namespace URI as the input namespace
  URI, it is removed (detached) from the PBDOM_ELEMENT.

Example

The following example demonstrates the impact of
setting a PBDOM_ATTRIBUTE for a PBDOM_ELEMENT
where the PBDOM_ELEMENT already contains an attribute of
the same name and namespace URI as the input PBDOM_ATTRIBUTE.

The example creates a PBDOM_DOCUMENT based on the
following document:

Then it creates a PBDOM_ATTRIBUTE object and set
its name to a and its prefix
and URI to pre2 and http://www.pre.com.
The bVerifyNamespace argument is set to FALSE because
this PBDOM_ATTRIBUTE has not been assigned an owner PBDOM_ELEMENT
yet, so that the verification for a predeclared namespace would
fail. The text value is set to 456.The child1 element
already contains an attribute named a that
belongs to the namespace http://www.pre.com,
as indicated by the prefix pre1. The new PBDOM_ATTRIBUTE
uses the prefix pre2, but it represents the
same namespace URI, so setting the new PBDOM_ATTRIBUTE
to child1 successfully replaces the existing pre1:a with
the new PBDOM_ATTRIBUTE pre2:a.

The XML output from SaveDocument looks
like the following :

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