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LINQ: Reconciling objects, relations and XML in the
.NETframework
Erik MeijerMicrosoft CorporationRedmond, USA
[email protected]
Brian BeckmanMicrosoft CorporationRedmond, USA
[email protected]
Gavin BiermanMicrosoft ResearchCambridge, UK
[email protected]
Introduction Many software applications today need tohandle data
from different data models; typically objectsfrom the host
programming language along with the re-lational and XML data. The
ROX impedance mismatchmakes programs awkward to write and hard to
maintain.
The .NET Language-Integrated Query (LINQ) framework,proposed for
the next release of the .NET framework, ap-proaches this problem by
dening a design pattern of general-purpose standard query operators
for traversal, lter, andprojection. Based on this pattern, any .NET
language candene special query comprehension syntax that is
subse-quently compiled into these standard operators (our
codeexamples are in VB).
Besides the general query operators, the LINQ frameworkalso
denes two domain-specic APIs that work over XML(XLinq) and
relational data (DLinq) respectively. The oper-ators over XML use a
lightweight and easy-to-use in-memoryXML representation to provide
XQuery-style expressivenessin the host programming language. The
operators over re-lational data provide a simple OR mapping by
leveragingremotable queries that are executed directly in the
back-endrelational store.
Standard query operators It is well known that col-lections can
be modeled as monoids and queries over themas monoid homomorphisms.
Analogously, LINQ denes anAPI pattern that enables querying of any
collection or .NETarray. This query operator set includes familiar
constructssuch as ltering ( where ), mapping ( select ), monadic
bind(selectMany ), sorting ( orderby ) and partitioning ( groupBy
).
These query operators can be freely composed to formrich
queries; e.g. the following code returns the name andphone numbers
of customers from Seattle ordered by theirage:
Dim cs = From c In CustomersWhere c.Address.City =
"Seattle"Order By c.AgeSelect c.Name, c.Phone
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XLinq The LINQ framework also includes a new, modern,lightweight
XML API that makes it simple to program XMLand integrates smoothly
with the standard query operatorsof LINQ.
In XLinq, nodes are rst-class citizens that can be passedaround
independently of an enclosing document. Nested el-ements are
constructed in an expression-oriented fashion.Elements and
attributes are accessed uniformly using famil-
iar XPath axis-style methods, while namespace handling
issimplied using the notion of universal names.
In addition to the XLinq API, VB 9.0 adds XML liter-als with
full namespace support. A variant of the previousquery that returns
each customer as XML simply uses thefollowing expression in the
select clause:
DLinq The LINQ framework also provides infrastruc-ture for
managing relational data as objects. It leveragesthe ability of
LINQ to provide intensional representations of
delegates as code trees by translating
language-integratedqueries into SQL for execution by the database,
and thentranslating the resulting table into objects. The DLinq
in-frastructure maintains an identity cache and performs
changetracking.
The mapping between database tables and in-memory ob- jects is
done via custom attributes (annotations) such as to indicate
foreignkey associations and to indicate pri-mary keys.
In the following query, the navigation between the cus-tomer and
address objects will automatically be compiledinto the appropriate
joins of the underlying tables.
Dim db = new MyDataBase("...");
Dim cs = From c In db.CustomersWhere c.Address.City ==
"Seattle"Select c.Name, c.Address
As soon as objects are loaded into the data context, eitherby
retrieving them through a query or by constructing newobjects and
inserting them, DLinq tracks all the changesand transmits these
objects back to the database when re-quested, automatically
generating and executing the appro-priate SQL commands.
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