Why does this XDocument fail validation?

Question

Given the schema (anonymised, the key points of interest are renamed and the rest omitted):

<?xml version="1.0" encoding="utf-8"?>
<xs:schema id="inspec"
    targetNamespace="the_right_namespace"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
>
  <xs:element name="inspec">
    <xs:complexType>
      <xs:all>
        <xs:element name="a_scalar_property" type="xs:int"/>
        <xs:element name="a_collection_property">
          <xs:complexType>
            <snip>
          </xs:complexType>
        </xs:element>
        <xs:element name="another_collection_property">
          <xs:complexType>                
            <snip>
          </xs:complexType>
        </xs:element>                       
      </xs:all>
    </xs:complexType>
  </xs:element>
</xs:schema>

and the instance (declared using VB xml literals):

Dim xDocument = 
<x:inspec xmlns:x='the_right_namespace'>
<a_collection_property/>
<another_collection_property/>
</x:inspec>

validation fails with the message The element 'inspec' in namespace 'the_right_namespace' has incomplete content. List of possible elements expected: 'a_scalar_property'.

Why? The all element, according to W3Schools:

"The all element specifies that the child elements can appear in any order and that each child element can occur zero or one time."

Omitting a_scalar_property is the same as including it zero times. Why does this document fail to validate?

And don't say things like 'post the full code' - this is not my IP and I have anonymised it for a good reason. There is very little else to it, and I have tested with this minimal example, it gives the same result.

Answer 1

You need to specify the minOccurs="0" for every optional element in xs:all:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema id="inspec"
    targetNamespace="the_right_namespace"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
>
    <xs:element name="inspec">
        <xs:complexType>
            <xs:all>
                <xs:element name="a_scalar_property" type="xs:int" minOccurs="0" />
                <xs:element name="a_collection_property" minOccurs="0">
                    <xs:complexType>
                        <!-- snip -->
                    </xs:complexType>
                </xs:element>
                <xs:element name="another_collection_property" minOccurs="0">
                    <xs:complexType>
                        <!-- snip -->
                    </xs:complexType>
                </xs:element>
            </xs:all>
        </xs:complexType>
    </xs:element>
</xs:schema>

Answer 2

To make an element optional, the minOccurrs attribute should be 0, even in an <all> group. Getting that from reading the XML schema specification is really cumbersome, but relying on w3schools is not a good alternative.