want to change xml attribute name.

Question

Hi, I am having an xml file. I want to change the attribute name of one node. 

 

<?xml version="1.0" encoding="utf-8"?>

<!DOCTYPE book PUBLIC "-//NLM//DTD Book DTD v2.3 20070202//EN" "book.dtd">

<book xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" dtd-version="2.3">

<book-meta>

<book-id pub-id-type="doi">10.1163/9789004318168</book-id>

<book-title-group>

<book-title>Lay Prophets in Lutheran Europe (<italic>c</italic>. 1550–1700)</book-title>

</book-title-group>

<volume>74</volume>

<series>Brill’s Series in Church History and Religious Culture</series>

<contrib-group>

<contrib contrib-type="author">

<name>

<surname>Beyer</surname>

<given-names>Jürgen</given-names>

</name>

</contrib>

</contrib-group>

<publisher>

<publisher-name>Brill</publisher-name>

<publisher-loc>Leiden</publisher-loc>

</publisher>

<isbn>9789004156289</isbn>

<isbn content-type="e-ISBN">9789004318168</isbn>

<pub-date pub-type="epub"><year>2017</year></pub-date>

<permissions>

<copyright-statement>Copyright 2017 by Koninklijke Brill NV, Leiden, The Netherlands</copyright-statement>

<copyright-year>2017</copyright-year>

<copyright-holder>Koninklijke Brill NV incorporates the imprints Brill, Brill Hes &amp; De Graaf, Brill Nijhoff, Brill Rodopi and Hotei Publishing.</copyright-holder>

</permissions>

</book-meta>

 </book>

 

 

in this i want to change the  pub-id-type into book-id-type. i tried with following code. but i always got null error. what is the problem in that.

 

doc.XPathSelectElement("//book-meta/book-id").Attribute("pub-id-type").Remove();

doc.XPathSelectElement("//book-meta/book-id").SetAttributeValue("book-id-type", "doi");

 

 

Thanks in advance.