Extend complex types from an existing schema

I need to extend an existing XML document with new elements without losing the ability to val开发者_StackOverflow社区idate it against both the unchanged original schema and an extended schema defined by me.

For example, lets say I want to add Code and Price elements to an existing Book document, as follows:

<aa:Book xmlns:aa="http://www.aa.com"
         xmlns:bb="http://www.bb.com">
  <aa:Title>Complete Works</aa:Title>
  <aa:Author>Shakespeare</aa:Author>
  <bb:Code>98</bb:Code>
  <bb:Price>31.6</bb:Price>
</aa:Book>

The original schema (which I cannot change) would look something like this:

<xs:schema targetNamespace="http://www.aa.com"
           xmlns:xs="http://www.w3.org/2001/XMLSchema"
           xmlns="http://www.aa.com"
           elementFormDefault="qualified">
  <xs:element name="Book" type="Book--type" />
  <xs:complexType name="Book--type">
    <xs:sequence>
      <xs:element name="Title" type="xs:string" />
      <xs:element name="Author" type="xs:string" />
    </xs:sequence>
  </xs:complexType>
</xs:schema>

How would I create a new schema that defines these new elements such that a SchemaSet containing both these schemas can successfully validate the extended XML document above?

I've tried the following usual complex type extension pattern:

<xs:schema targetNamespace="http://www.bb.com"
           xmlns:xs="http://www.w3.org/2001/XMLSchema"
           xmlns="http://www.bb.com"
           xmlns:aa="http://www.aa.com"
           elementFormDefault="qualified">
  <xs:import namespace="http://www.aa.com" />
  <xs:complexType name="Book--type">
    <xs:complexContent>
      <xs:extension base="aa:Book--type">
        <xs:sequence>
          <xs:element name="Code" type="xs:int" />
          <xs:element name="Price" type="xs:double" />
        </xs:sequence>
      </xs:extension>
    </xs:complexContent>
  </xs:complexType>
</xs:schema>

However this causes errors stating that the element aa:Book doesn't define a child element bb:Code, which makes total sense.

Is this even possible?

For reference, here is the code I'm using to validate the document:

var fail = new ValidationEventHandler((sender, e) => Debug.Fail(e.Message));
var schemaSet = new XmlSchemaSet();
schemaSet.Add(XmlSchema.Read(new StringReader(original), fail));
schemaSet.Add(XmlSchema.Read(new StringReader(extension), fail));
var settings = new XmlReaderSettings
    {
        ValidationType = ValidationType.Schema,
        Schemas = schemaSet
    };
settings.ValidationEventHandler += fail;
using (var stream = File.OpenRead(fileName))
using (var reader = XmlReader.Create(stream, settings))
{
    while (reader.Read()) {}
}

It turns out that xs:redefine is what I was looking for. It allows you to redefine an existing element in a new schema without touching the original schema or the document being validated.

Using my original example, a book can be extended using the following two schemas:

<xs:schema targetNamespace="http://www.bb.com"
           xmlns:xs="http://www.w3.org/2001/XMLSchema"
           xmlns="http://www.bb.com"
           elementFormDefault="qualified">
  <xs:element name="Code" type="xs:int" />
  <xs:element name="Price" type="xs:double" />
</xs:schema>

<xs:schema targetNamespace="http://www.aa.com"
           xmlns:xs="http://www.w3.org/2001/XMLSchema"
           xmlns="http://www.aa.com"
           xmlns:bb="http://www.bb.com"
           elementFormDefault="qualified">
  <xs:import namespace="http://www.bb.com" />
  <xs:redefine schemaLocation="http://www.aa.com">
    <xs:complexType name="Book--type">
      <xs:complexContent>
        <xs:extension base="Book--type">
          <xs:sequence>
            <xs:element ref="bb:Code" />
            <xs:element ref="bb:Price" />
          </xs:sequence>
        </xs:extension>
      </xs:complexContent>
    </xs:complexType>
  </xs:redefine>
</xs:schema>

The problem is that even though you have a new type defined in your schema B, the root element in your sample document is of type Book--type from the original schema A.

You need to use a type override in your document, like so:

<aa:Book xmlns:aa="http://www.aa.com"
         xmlns:bb="http://www.bb.com"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:type="bb:Book--type">
  <aa:Title>Complete Works</aa:Title>
  <aa:Author>Shakespeare</aa:Author>
  <bb:Code>98</bb:Code>
  <bb:Price>31.6</bb:Price>
</aa:Book>