Example
By convention elementFormDefault is always set to qualified, but lets look at what it actually does.
First with elementFormDefault set to qualified.
<?xml version="1.0" encoding="utf-8" ?>
<!--Created with Liquid Studio 2017 (https://www.liquid-technologies.com)-->
<xs:schema elementFormDefault="qualified"
targetNamespace="http://base.com"
xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="MyBaseElement">
<xs:complexType>
<xs:sequence>
<xs:element name="ChildA" type="xs:string" />
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
Sample XML Document
<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid Studio 2017 (https://www.liquid-technologies.com) -->
<b:MyBaseElement xmlns:b="http://base.com"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://base.com ElementFormDefault_qualified.xsd">
<b:ChildA>string</b:ChildA>
</b:MyBaseElement>
Notice the element ChildA must also be qualified with the namespace 'b'.
Now lets look at it with elementFormDefault set to unqualified.
<?xml version="1.0" encoding="utf-8" ?>
<!--Created with Liquid Studio 2017 (https://www.liquid-technologies.com)-->
<xs:schema elementFormDefault="unqualified"
targetNamespace="http://base.com"
xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="MyBaseElement">
<xs:complexType>
<xs:sequence>
<xs:element name="ChildA" type="xs:string" />
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
Sample XML Document
<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid Studio 2017 (https://www.liquid-technologies.com) -->
<b:MyBaseElement xmlns:b="http://base.com"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://base.com ElementFormDefault_unqualified.xsd">
<ChildA>string</ChildA>
</b:MyBaseElement>
Notice this time that only the globally defined element MyBaseElement is qualified with the namespace 'b', the inner element ChildA (which is defined in place within the schema) is not qualified.
In the last example we saw that the globally defined elements must be qualified in the XML instance document, but elements defined inplace do not. But this does not just mean the root element, if you have globally defined elements that are referenced, then they need qualifying as well.
<?xml version="1.0" encoding="utf-8" ?>
<!--Created with Liquid Studio 2017 (https://www.liquid-technologies.com)-->
<xs:schema elementFormDefault="unqualified"
targetNamespace="http://base.com"
xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="MyBaseElement">
<xs:complexType>
<xs:sequence>
<xs:element name="ChildA" type="xs:string" />
<xs:element xmlns:q1="http://base.com" ref="q1:MyElement" />
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="MyElement" type="xs:string" />
</xs:schema>
Sample XML Document
<?xml version="1.0" encoding="utf-8"?>
<!-- Created with Liquid Studio 2017 (https://www.liquid-technologies.com) -->
<b:MyBaseElement xmlns:b="http://base.com"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://base.com ElementFormDefault_unqualified.xsd">
<ChildA>string</ChildA>
<b:MyElement>string</b:MyElement>
</b:MyBaseElement>
Notice that MyElement also need qualifiying as it is globally defined.
In conclusion, if you have elementFormDefault set to qualified, then everything needs to be qualified with a namespace (either via a namespace alias or by setting the default namesapce xmlns="..."). However elementFormDefault is set to unqualified, things get complicated and you need to do some quite indepth examination of the schemas to work out if things should be qualified or not.
I assume that is why elementFormDefault is always set to qualified!
