Oleg Tkachenko writes

Just found new beast in the Longhorn SDK documentation - OPath language:

The OPath language is the query language used to query for objects using an ObjectSpace. The syntax of OPath also allows you to query for objects using standard object oriented syntax. OPath enables you to traverse object relationships in a query as you would with standard object oriented application code and includes several operators for complex value comparisons.

Orders[Freight > 5].Details.Quantity > 50 OPath expression should remind you something familiar. Object-oriented XPath cross-breeded with SQL? Hmm, xml-dev flamers would love it.

The approach seems to be exactly opposite to ObjectXPathNavigator's one - instead of representing object graphs in XPathNavigable form, brand new query language is invented to fit the data model. Actually that makes some sense, XPath as XML-oriented query language can't fit all. I wonder what Dare think about it. More studying is needed, but as for me (note I'm not DBMS-oriented guy though) it's too crude yet

Oleg is right that an XML oriented query language like XPath doesn't fit for querying objects. There is a definitely an impedance mismatch between XML and objects, a good number of which were pointed out by Erik Meijer in his paper Programming with Circles, Triangles and Rectangles. A significant number of constructs and semantics of XPath simply don't make sense in a language designed to query objects. The primary construct in XPath is the location step which consists of an axis, a node test and zero or more predicates, of which both the axis and the node test are out of place in an object query language.

From the XPath Grammar, there are 13 axes of which almost none make sense for objects besides self. They are listed below

[6]    AxisName    ::=    'ancestor'
| 'ancestor-or-self'
| 'attribute'
| 'child'
| 'descendant'
| 'descendant-or-self'
| 'following'
| 'following-sibling'
| 'namespace'
| 'parent'
| 'preceding'
| 'preceding-sibling'
| 'self'

The ones related to document order such as preceding, following, preceding-sibling and following-siblings don't really apply to objects since there is no concept of order amongst the properties and fields of a class. The attribute axis is similarly unrelated since there is no equivalent of the distinction between elements and attributes among the fields and properties of a class. 

The axes related to document hierarchy such as parent, child, ancestor, descendent, etc look like they may make sense to map to object oriented concepts until one asks what exactly is meant to be the parent of an object? Is it the base class or the object to which the current object belongs as a field or property? Most would respond that it is the latter. However what happens when multiple objects have the same object as a field which is often the case since objects structures are graph-like not tree-like as XML structures? It also gets tricky when an object that is a field in one class is a member of a collection in another class. Is the object a child of the collection? If so what is the parent of the object, if not what is the relationship of the object to the collection then? The questions can go on...

On the surface the namespace axes sounds like it could map to concepts from object oriented programming since languages like C#, C++ and Java all have a concept of a "namespace". However namespace nodes in the XPath data model have distinct characteristics (such as the fact that each element node in document has a distinct set of namespace nodes regardless of whether each of these namespace nodes represent the same mapping of a prefix to a namespace URI). 

A similar argument can also be made around node tests which are the second primary constructs in XPath location steps. A node test either specifies a name or a type of node to match. A number of XPath node types don't have equivelants in the object oriented world such as comment and processing instruction nodes. Other nodes such as text and element nodes are problematic when one begins to try to tie them in to the various axes such as the parent axis.

Basically, a significant amount of XPath is not really applicable to querying objects without changing the semantics of certain aspects of the language in a way that conflicts with how XPath is used when querying XML documents.

As for how this compares to my advocacy of XML to object mapping techniques such as the ObjectXPathNavigator, the answer is simple; XML is the universal data interchange format and the software world is moving to a situation where all the major sources of important data can be accessed or viewed as XML from office documents to network messages to information locked within databases. It makes sense then that in creating this universal data access layer that one create a way for all interesting sources of data to be viewed as XML so they to can participate as input for data aggregation technologies such as XSLT or XQuery and enable the reuse of XML technologies for processing and manipulating them.