XML encoding of "null" Variant is incorrect

[kevinherron]

Type of issue

• Bug
• Enhancement
• Compliance
• Question
• Help wanted

Current Behavior

The way XmlEncoder encodes a "null" Variant value is incorrect.

A Variant value is encoded as an xs:complexType with the following XML schema:

<xs:complexType name="Variant">
  <xs:sequence>
    <xs:element name="Value" minOccurs="0" nillable="true">
      <xs:complexType>
        <xs:sequence>
          <xs:any minOccurs="0" processContents="lax"/>
        </xs:sequence>
      </xs:complexType>
    </xs:element>
  </xs:sequence>
</xs:complexType>

Which means the expected output for something like:

var context = new ServiceMessageContext();
string actualXmlValue;

using (IEncoder xmlEncoder = new XmlEncoder(context))
{
    Variant value = Variant.Null;

    xmlEncoder.PushNamespace(Namespaces.OpcUaXsd);
    xmlEncoder.WriteVariant("Test", value);
    xmlEncoder.PopNamespace();
    actualXmlValue = xmlEncoder.CloseAndReturnText();
}

would be some variation of:

<Test xmlns:uax="http://opcfoundation.org/UA/2008/02/Types.xsd"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <uax:Value xsi:nil="true" />
</Test>

yet the implementation of XmlEncoder yields this:

<Test xmlns="http://opcfoundation.org/UA/2008/02/Types.xsd">
  <Value>
    <Null />
  </Value>
</Test>

Ignore the namespaces and prefixes - the problem is the element named "Null" for which a reference does not exist in either Part 6 nor the the schema defined by http://opcfoundation.org/UA/2008/02/Types.xsd.

I believe another valid encoding based on the schema includes:

<Test>
  <Value></Value>
</Test>