What is the correct way to represent null XML elements

Question

I have seen null elements represented in several ways   The element is present with xsi nil  true      lt book gt        lt title gt Beowulf lt  title gt        lt author xsi nil  true   gt    lt  book gt    The element is present  but represented as an empty element  which I believe is wrong since  empty  and null are semantically different      lt book gt        lt title gt Beowulf lt  title gt        lt author  gt    lt  book gt     lt  -- or  -- gt    lt book gt        lt title gt Beowulf lt  title gt        lt author gt  lt  author gt    lt  book gt    The element is not present at all in the returned markup     lt book gt        lt title gt Beowulf lt  title gt    lt  book gt    The element has a  lt null  gt  child element  from TStamper below      lt book gt        lt title gt Beowulf lt  title gt        lt author gt  lt null  gt  lt  author gt    lt  book gt    Is there a correct  or canonical way to represent such a null value  Are there additional ways than the above examples   The XML for the examples above is contrived  so don t read too far into it

User · Answer

It depends on how you validate your XML  If you use XML Schema validation  the correct way of representing null values is with the xsi nil attribute    Source

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The documentation in the w3 link   http   www w3 org TR REC-xml  sec-starttags  says that this are the recomended forms    lt test gt  lt  test gt   lt test  gt    The attribute mentioned in the other answer is validation mechanism and not a representation of state  Please refer to the http   www w3 org TR xmlschema-1  xsi nil     XML Schema  Structures introduces a mechanism for signaling that an   element should be accepted as   valid   when it has no content despite a   content type which does not require or even necessarily allow empty   content  An element may be   valid   without content if it has the   attribute xsi nil with the value true  An element so labeled must be   empty  but can carry attributes if permitted by the corresponding   complex type    To clarify this answer                                                                 Content                                       lt Book gt       lt  --Invalid construct since the element attribute xsi nil  true  signal that the element must be empty-- gt       lt BuildAttributes HardCover  true  Glued  true  xsi nil  true  gt         lt anotherAttribute name  Color  gt Blue lt  anotherAttribute gt       lt  BuildAttributes gt       lt Index gt  lt  Index gt       lt pages gt         lt page pageNumber  1  gt Content lt  page gt                   lt  pages gt       lt  --Missing ISBN number could be confusing and misguiding since its not present-- gt     lt  Book gt   lt  Books gt

User · Answer

In many cases the purpose of a Null value is to serve for a data value that was not present in a previous version of your application   So say you have an xml file from your application  ReportMaster  version 1   Now in ReportMaster version 2 a some more attributes have been added that may or not be defined   If you use the  no tag means null  representation you get automatic backward compatibility for reading your ReportMaster 1 xml file

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Simply omitting the attribute or element works well in less formal data   If you need more sophisticated information  the GML schemas add the attribute nilReason  eg  in GeoSciML    xsi nil with a value of  true  is used to indicate that no value is available nilReason may be used to record additional information for missing values  this may be one of the standard GML reasons  missing  inapplicable  withheld  unknown   or text prepended by other   or may be a URI link to a more detailed explanation    When you are exchanging data  the role for which XML is commonly used  data sent to one recipient or for a given purpose may have content obscured that would be available to someone else who paid or had different authentication  Knowing the reason why content was missing can be very important    Scientists also are concerned with why information is missing  For example  if it was dropped for quality reasons  they may want to see the original bad data

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There is no canonical answer  since XML fundamentally has no null concept  But I assume you want Xml Object mapping  since object graphs have nulls   so the answer for you is  whatever your tool uses   If you write handling  that means whatever you prefer  For tools that use XML Schema  xsi nil is the way to go  For most mappers  omitting matching element attribute is the way to do it

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You use xsi nil when your schema semantics indicate that an element has a default value  and that the default value should be used if the element isn t present   I have to assume that there are smart people to whom the preceding sentence is not a self-evidently terrible idea  but it sounds like nine kinds of bad to me   Every XML format I ve ever worked with represents null values by omitting the element    Or attribute  and good luck marking an attribute with xsi nil

User · Answer

xsi nil is the correct way to represent a value such that  When the DOM Level 2 call getElementValue   is issued  the NULL value is returned  xsi nil is also used to indicate a valid element with no content even if that elements content type normally doesn t allow empty elements   If an empty tag is used  getElementValue   returns the empty string      If the tag is omitted  then no author tag is even present  This may be semantically different than setting it to  nil   Ex  Setting  Series  to nil may be that the book belongs to no series  while omitting series could mean that series is an inapplicable element to the current element    From  The W3C     XML Schema  Structures introduces a   mechanism for signaling that an   element should be accepted as   valid     when it has no content despite a   content type which does not require or   even necessarily allow empty content    An element may be   valid   without   content if it has the attribute   xsi nil with the value true  An   element so labeled must be empty  but   can carry attributes if permitted by   the corresponding complex type    A clarification   If you have a book xml element and one of the child elements is book series you have several options when filling it out    Removing the element entirely - This can be done when you wish to indicate that series does not apply to this book or that book is not part of a series  In this case xsl transforms  or other event based processors  that have a template that matches book series will never be called  For example  if your xsl turns the book element into table row  xhtml tr  you may get the incorrect number of table cells  xhtml td  using this method  Leaving the element empty - This could indicate that the series is     or is unknown  or that the book is not part of a series  Any xsl transform  or other evernt based parser  that matches book series will be called  The value of current   will be     You will get the same number of xhtml td tags using this method as with the next described one  Using xsi nil  true  - This signifies that the book series element is NULL  not just empty  Your xsl transform  or other event based parser  that have a template matching book series will be called  The value of current   will be empty  not empty string   The main  difference between this method and  2  is that the schema type of book series does not need to allow the empty string      as a valid value  This makes no real sense for a series element  but for a language element that is defined as an enumerated type in the schema  xsi nil  true  allows the element to have no data  Another example would be elements of type decimal  If you want them to be empty you can union an enumerated string that only allows    and a decimal  or use a decimal that is nillable

[xml] What is the correct way to represent null XML elements?

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