XML Serialization of the default values of optional attributes

Question

I have a set of classes build using xsd.exe, and I am trying to serialise them. However, an attribute is not being included in the resulting XML. Here is part of the schema where the problem lies.

<xsd:element name="Widget">
    <xsd:complexType>
        /* sequence removed for brevity */
        <xsd:attribute name="Version" type="Version" use="optional" default="1.1"/>
    </xsd:complexType>
</xsd:element>
<xsd:simpleType name="Version">
    <xsd:restriction base="xsd:string">
        <xsd:enumeration value="1.0"/>
        <xsd:enumeration value="1.1"/>
    </xsd:restriction>
</xsd:simpleType>

xsd.exe generated a property called "Version" on a "Widget" class and another property called "VersionSpecified", but this not appear to generate the attribute when I serialize even when set to true:

        /// <remarks/>
    [System.Xml.Serialization.XmlAttributeAttribute()]
    [System.ComponentModel.DefaultValueAttribute(Version.Version_1_1)]
    public Version Version
    {
        get
        {
            return this.version;
        }
        set
        {
            this.version = value;
        }
    }

    /// <remarks/>
    [System.Xml.Serialization.XmlIgnoreAttribute()]
    public bool VersionSpecified
    {
        get
        {
            return this.versionSpecified;
        }
        set
        {
            this.versionSpecified = value;
        }
    }

And this is the enumeration on which it is based:

    /// <remarks/>
[System.CodeDom.Compiler.GeneratedCodeAttribute("xsd", "2.0.50727.3038")]
[System.SerializableAttribute()]
public enum Version
{

    /// <remarks/>
    [System.Xml.Serialization.XmlEnumAttribute("1.0")]
    Version_1_0,

    /// <remarks/>
    [System.Xml.Serialization.XmlEnumAttribute("1.1")]
    Version_1_1,
}

Code snippet as per request

        Widget widget = new Widget();
        widget.Version = Version.Version_1_1;
        widget.VersionSpecified = true;    

        XmlSerializer serializer = new XmlSerializer(widget.GetType());
        serializer.Serialize(/*Memory Stream object*/, widget);

Does anyone have any thoughts on why the serialization refuses to introduce the attribute?

Answer 1

You have to set the VersionSpecified flag to true before serializing. That's how it knows whether or not this optional attribute is to be serialized.

Answer 2

Its because you specified the default value as "1.1". The serialiser will not create the element/attribute when the property is equal to its default value.