[SOLVED] Does the sequence schema element guarantee order of child elements?

Issue

Given this xml schema (fragment):

<xs:element name="GetIEnumerableResponse">
  <xs:complexType>
    <xs:sequence>
      <xs:element minOccurs="0" name="GetIEnumerableResult" nillable="true" xmlns:q4="http://schemas.microsoft.com/2003/10/Serialization/Arrays" type="q4:ArrayOfstring" />
    </xs:sequence>
  </xs:complexType>
</xs:element>

In this xml fragment:

<ArrayOfstring xmlns="http://schemas.microsoft.com/2003/10/Serialization/Arrays">
  <string>string1</string>
  <string>string2</string>
  <string>string3</string>
</ArrayOfstring>

can the <string></string> elements occur in any order? Thus, are these two fragments of XML semantically equvalent:

<ArrayOfstring xmlns="http://schemas.microsoft.com/2003/10/Serialization/Arrays">
  <string>string1</string>
  <string>string2</string>
  <string>string3</string>
</ArrayOfstring>

<ArrayOfstring xmlns="http://schemas.microsoft.com/2003/10/Serialization/Arrays">
  <string>string3</string>
  <string>string2</string>
  <string>string1</string>
</ArrayOfstring>

Or does the sequence element in the schema mean the <string></string> elements have to occur in the same order to be semantically equivalent?

Does the presence of the in the schema require the parser/deserializer to keep the elements in the order they exist in the xml text? If I understand correctly, normally (i.e. without a schema) there is no requirement to do so (even if most parsers usually do).

Solution

The Sequence element means that the individual elements (not the elements in the array) are supposed to preserve order.

For instance

<xs:element name="GetIEnumerableResponse">
  <xs:complexType>
    <xs:sequence>
      <xs:element minOccurs="0" name="GetIEnumerableResult" nillable="true" xmlns:q4="http://schemas.microsoft.com/2003/10/Serialization/Arrays" type="q4:ArrayOfstring" />
      <xs:element name="Dummy" type="xs:string" />
    </xs:sequence>
  </xs:complexType>
</xs:element>    

In this example, Dummy appears after GetIEnumerableResult in the sequence, and to validate it should always appear in this order in this complex type.

The order of “repeating” items in the “ArrayOfString” complex type is not enforcable in the schema, and because arrays do not imply or enforce any explicit order the semantics of order are not guaranteed in the CLR either.

One way to guarantee order would be to impose order on the collection by serializing an index.

Answered By – Blue Toque

Answer Checked By – Dawn Plyler (BugsFixing Volunteer)

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