Standard way to represent null attribute values in XML?

Question

Possible Duplicate:
What is the correct way to represent null XML elements?

Is there a standard way to represent null attribute values in XML?

We have an existing XML format that expects fragments similar to:

<items>
  <item key="key_goes_here" value="value_goes_here" />
  ...
</items>

The original format didn't anticipate the need to distinguish between a null value and an empty string value -- however, that is important (now).

My gut says to create a new format that avoids attributes for nullable values, and use elements for them instead:

<items>
  <item key="key_goes_here_and_is_never_null">
    <value xsi:nil="true" /> 
  </item>
</items>

That said, I'd rather keep attributes if there's a standard way to represent null attribute values in XML.

How is this an exact duplicate? My question pertains to null-valued attributes, not null-valued elements? — jglouie
– jglouie, Commented Dec 1, 2012 at 14:57
It's not duplicated. one is for attribute and the other is for element. — 柯鴻儀
– 柯鴻儀, Commented Apr 13, 2018 at 22:43

Tim Pietzcker · Accepted Answer · 2012-11-30 15:19:52Z

4

I don't know about any standard, but how about using

<item key="key" />

for items that don't have a value,

<item key="key">
    <value /> 
</item>

for items that have an empty string as a value and

<item key="key">
    <value>This is the value</value> 
</item>

for items that actually have a value?

answered Nov 30, 2012 at 15:19

Tim Pietzcker

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7 Comments

jglouie Over a year ago

Thanks -- Is there an advantage or expected difference between <value /> and <value xsi:nil="true" />?

Tim Pietzcker Over a year ago

@LemonBeagle: To be honest, I've never seen the latter before, but I'm not much of an XML expert. It seems to me a simple empty tag conveys the message just fine and is easy to understand.

wds Over a year ago

+1 I'd add you can define this in the schema by making the attribute "value" optional (optional="true" I believe).

tjg184 Over a year ago

We have a similar requirement and we used something like <value /> and <value xsi:nil="true" />, where the former represents empty strings and the latter represents null values. Also, be careful about using no tag as null because there is a difference between something being null to start off with versus being set to null. This is especially true when you're sending changes in as part of a request. stackoverflow.com/questions/774192/… provides some examples, too.

Dan Bechard Over a year ago

@TimPietzcker Although this may have solved the OP's problem, this does not answer the question, as stated. Does anyone know how to represent null attributes? I do not have the power to change the receiving end, which expects attributes not elements.

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