I'm currently on it to write a SDK around a pretty old and convoluted xml standard. There is a xml schema given and I'm confused by this construct:

<xs:complexType>
    <xs:choice maxOccurs="unbounded">
        <xs:choice>
            <xs:element ref="A" minOccurs="0" />
            <xs:element ref="B" />
        </xs:choice>
        <xs:element ref="C" minOccurs="0" maxOccurs="unbounded" />
        <xs:element ref="D" minOccurs="0" maxOccurs="unbounded" />
    </xs:choice>
</xs:complexType>

I have no clue how to understand this... especially the inner choice. So choice means take exactly one element of the listed ones, right? But B has no min or max occurs so it defaults to 1. So exactly 1 element of B is required. How to choose A then?

I'm currently on it to write a SDK around a pretty old and convoluted xml standard. There is a xml schema given and I'm confused by this construct:

<xs:complexType>
    <xs:choice maxOccurs="unbounded">
        <xs:choice>
            <xs:element ref="A" minOccurs="0" />
            <xs:element ref="B" />
        </xs:choice>
        <xs:element ref="C" minOccurs="0" maxOccurs="unbounded" />
        <xs:element ref="D" minOccurs="0" maxOccurs="unbounded" />
    </xs:choice>
</xs:complexType>

I have no clue how to understand this... especially the inner choice. So choice means take exactly one element of the listed ones, right? But B has no min or max occurs so it defaults to 1. So exactly 1 element of B is required. How to choose A then?

• xml
• xsd
• schema

• The inner <xs:choice> forces B to always be present, while A is optional. – Hermann12 Commented 2 days ago
• 1 The choice also makes B optional as you can choose A and not have B. – Dijkgraaf Commented 2 days ago

2 Answers 2

As Michael said, you could simply it to a structure like below and it will validate the same structure.
If you want at enforce at least one element the set the minOccurs on the choice to 1.

Note: I've defined the elements inline as string rather than a ref, as you didn't supply that part of the schema.

<?xml version="1.0" encoding="utf-16"?>
<xs:schema xmlns="http://Scratch2.Schema.SO79550752" xmlns:b="http://schemas.microsoft/BizTalk/2003" targetNamespace="http://Scratch2.Schema.SO79550752" xmlns:xs="http://www.w3./2001/XMLSchema">
  <xs:element name="Root">
    <xs:complexType>
      <xs:sequence>
        <xs:choice minOccurs="0" maxOccurs="unbounded">
          <xs:element name="A" type="xs:string" />
          <xs:element name="B" type="xs:string" />
          <xs:element name="C" type="xs:string" />
          <xs:element name="D" type="xs:string" />
        </xs:choice>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

Alternatively to using a choice you could use a sequence, and then you actually have to make the elements minOccurs 0, which you don't need with the choice

<?xml version="1.0" encoding="utf-16"?>
<xs:schema xmlns="http://Scratch2.Schema.SO79550752" xmlns:b="http://schemas.microsoft/BizTalk/2003" targetNamespace="http://Scratch2.Schema.SO79550752" xmlns:xs="http://www.w3./2001/XMLSchema">
  <xs:element name="Root">
    <xs:complexType>
      <xs:sequence>
        <xs:sequence minOccurs="0" maxOccurs="unbounded">
          <xs:element minOccurs="0" name="A" type="xs:string" />
          <xs:element minOccurs="0" name="B" type="xs:string" />
          <xs:element minOccurs="0" name="C" type="xs:string" />
          <xs:element minOccurs="0" name="D" type="xs:string" />
        </xs:sequence>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

I think the inner choice is basically nonsensical. The content model allows any number of elements, each of which can be either an A, a B, a C, or a D, and they can be in any order. There are simpler ways of expressing this.