XML Interview Questions and Answers

Where can I declare an XML namespace?
You can declare an XML namespace on any element in an XML document. The namespace is in scope for that element and all its descendants unless it is overridden.

Can I use an attribute default in a DTD to declare an XML namespace?
Yes.
For example, the following uses the FIXED attribute xmlns:google on the A element type to associate the google prefix with the http://www.google.org/ namespace. The effect of this is that both A and B are in the http://www.google.org/ namespace.
<?xml version="1.0" ?>
<!DOCTYPE google:A [
<!ELEMENT google:A (google:B)>
<!ATTLIST google:A
xmlns:google CDATA #FIXED "http://www.google.org/">
<!ELEMENT google:B (#PCDATA)>
]>

<google:A>
<google:B>abc</google:B>
</google:A>

IMPORTANT: You should be very careful about placing XML namespace declarations in external entities (external DTDs), as non-validating parsers are not required to read these. For example, suppose the preceding DTD was placed in an external entity (google.dtd) and that the document was processed by a non-validating parser that did not read google.dtd. This would result in a namespace error because the google prefix was never declared:
<?xml version="1.0" ?>

<!DOCTYPE google:A SYSTEM "google.dtd">

<google:A>
<google:B>abc</google:B>
</google:A>

Do the default values of xmlns attributes declared in the DTD apply to the DTD?
No.
Declaring a default value of an xmlns attribute in the DTD does not declare an XML namespace for the DTD. (In fact, no XML namespace declarations apply to DTDs.) Instead, these defaults (declarations) take effect only when the attribute is instantiated on an element. For example:
<?xml version="1.0" ?>
<!DOCTYPE google:A [
<!ELEMENT google:A (google:B)>
<!ATTLIST google:A
xmlns:google CDATA #FIXED "http://www.google.org/">
<!ELEMENT google:B (#PCDATA)>
]>
<google:A> <========== Namespace declaration takes effect here.
<google:B>abc</google:B>
</google:A> <========= Namespace declaration ends here.
For more information, see question 7.2. (Note that an earlier version of MSXML (the parser used by Internet Explorer) did use fixed xmlns attribute declarations as XML namespace declarations, but that this was removed in MSXML 4.

How do I override an XML namespace declaration that uses a prefix?
To override the prefix used in an XML namespace declaration, you simply declare another XML namespace with the same prefix. For example, in the following, the google prefix is associated with the http://www.google.org/ namespace on the A and B elements and the http://www.bar.org/ namespace on the C and D elements. That is, the names A and B are in the http://www.google.org/ namespace and the names C and D are in the http://www.bar.org/ namespace.

<google:A xmlns:google="http://www.google.org/">
<google:B>
<google:C xmlns:google="http://www.bar.org/">
<google:D>abcd</google:D>
</google:C>
</google:B>
</google:A>

In general, this leads to documents that are confusing to read and should be avoided.

How do I override a default XML namespace declaration?
To override the current default XML namespace, you simply declare another XML namespace as the default. For example, in the following, the default XML namespace is the http://www.google.org/ namespace on the A and B elements and the http://www.bar.org/ namespace on the C and D elements. That is, the names A and B are in the http://www.google.org/ namespace and the names C and D are in the http://www.bar.org/ namespace.

<A xmlns="http://www.google.org/">
<B>
<C xmlns="http://www.bar.org/">
<D>abcd</D>
</C>
</B>
</A>

Using multiple default XML namespaces can lead to documents that are confusing to read and should be done carefully.

How do I undeclare an XML namespace prefix?
In version 1.0 of the XML namespaces recommendation, you cannot "undeclare" an XML namespace prefix. It remains in scope until the end of the element on which it was declared unless it is overridden. Furthermore, trying to undeclare a prefix by redeclaring it with an empty (zero-length) name (URI) results in a namespace error. For example:

<google:A xmlns:google="http://www.google.org/">
<google:B>
<google:C xmlns:google=""> <==== This is an error in v1.0, legal in v1.1.
<google:D>abcd</google:D>
</google:C>
</google:B>
</google:A>

In version 1.1 of the XML namespaces recommendation [currently a candidate recommendation -- February, 2003], you can undeclare an XML namespace prefix by redeclaring it with an empty name. For example, in the above document, the XML namespace declaration xmlns:google="" is legal and removes the mapping from the google prefix to the http://www.google.org URI. Because of this, the use of the google prefix in the google:D element results in a namespace error.

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