[xsde-users] XSD/e and xsi:nil support?

[Boris Kolpackov]

Hi Witek,

Witek Piet <travelwitek at gmail.com> writes:

> I have read in the XSD support list, that XSD does not support nillable and
> xsi:nil. Based on my tests I assume that this (nillable and xsi:nil) is not
> supported by XSD/e either.

That's correct.


> If not are there any plans on implementing this feature in future releases
> of XSD and XSD/e?

No, not at the moment. This is one of those XML Schema "mis-features" that
doesn't make much sense, doesn't map cleanly to object-oriented models, and,
as a result, hardly used by anybody.

The main problem with the nillable elements in XML Schema is the fact that
an element declared nil can still have a full set of attributes. Consider,
for example:

  <complexType name="person">
    <sequence>
      <element name="first-name" type="string"/>
      <element name="last-name" type="string"/>
    </sequence>
    <element name="id" type="unsignedLong"/>
    <element name="email" type="string"/>
  </complexType>

  <element name="person" type="person" nillable="true"/>

Then in your XML document you can have:

  <person xsi:nil="true"/>

Which makes sense. But then you can also have this:

  <person xsi:nil="true" id="123" email="null at null.com"/>

Which doesn't make much sense. In a sense, this means that only half of the
object is NULL. There is no way in programming languages such as C++ to have
pointer or object that is "half NULL".


> Does XSD and XSD/e support any other mechanism for assigning a null 
> value to an element? If this is the case could you please give me a 
> hint?

The cleanest and most straightforward way is simply to declare an element
optional by setting its minOccurs="0". For example:

  <complexType name="bank_account">
    <sequence>
      <element name="primary_holder" type="person"/>
      <element name="secondary_holder" type="person" minOccurs="0"/>
    </sequence>
  </complexType>

Here we have a bank account type which must have a primary account holder.
It also may or may not have the secondary holder. For example:

  <bank_account>
    <primary_holder/>
      ...
    </primary_holder>
  </bank_account>

  <bank_account>
    <primary_holder/>
      ...
    </primary_holder>

    <secondary_holder/>
      ...
    </secondary_holder>
  </bank_account>

In the C++/Hybrid object mode you will check whether secondary holder
is set like this:

bank_account& acc = ...

if (acc.secondary_holder_present ())
{
  person& sh (acc.secondary_holder ());
  ...
}

Which, in a way, is similar to checking whether a pointer is NULL.

Boris