What's the difference between <a_element /> and <a_element xsi:nil=“true” />?

问题

do you know if there's a difference between these tags on XML/XSD?

<a_element /> and <a_element xsi:nil="true"/>

e.g:

<SpreadCurve>
      <Index>3M</Index>
      <IndexNumber>4587</IndexNumber>
      <BusinessArea xsi:nil="true" />
</SpreadCurve>

and

<SpreadCurve>
      <Index>3M</Index>
      <IndexNumber>4587</IndexNumber>
      <BusinessArea />
</SpreadCurve>

Are these equivalent ?

If I have a XSD element:

<xsd:element name="BusinessArea" type="xsd:string"/>

this means that it is by default xsi:nil="false". And this means it will not accept a null value for this element.

My doubt is, will it accept this one?

<BusinessArea />

What does this really mean to the XSD?

Best regards

回答1:

You get this as your XSD BusinessArea should be defined as nillable="true". Something like:

<xsd:element name="BusinessArea" nillable="true">
.....
</xsd:element> 

What this mean is that BusinessArea element can have null value i.e. empty.

And if element in XML doesn't contain any value then it must have attribute xsi:nil="true":

<BusinessArea xsi:nil="true" />

This should be invalid :

<BusinessArea />

Two examples you showed should not be equivalent.

Check this out for understanding xsi:nil and nillable:

http://www.zvon.org/xxl/XMLSchemaTutorial/Output/ser_over_st0.html

http://www.w3.org/TR/xmlschema-0/#Nils

回答2:

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.

Source: http://www.w3.org/TR/xmlschema-1/#xsi_nil

回答3:

<a_element />  

is the equivalent of an empty string and will render valid against xsd:string, but not against types like xsd:date, xsd:datetime, xsd:decimal etc.

<a_element xsi:nil="true"/>

is the equilalent of null and will render valid for all elements which have nillable="true" in the schema-definition

回答4:

My understanding is that they are not the same. At least if you want to validate the xml against a schema. If in your schema you define the element as nillable, let's say:

<xsd:element name="SomeName" type="xsd:double" nillable="true"/>

You need to explicitly in your xml set that element to null, like this:

<SomeName xsi:nill="true" />

If in your xml the element is like <SomeName /> it will not be valid according to the schema.

来源：https://stackoverflow.com/questions/3760629/whats-the-difference-between-a-element-and-a-element-xsinil-true