ADO.NET Data Services is a platform to build services based
on REST-approach. It is a complex tool which allows construction of a complicated
REST-service thanks to usage of an "out of box" in several minutes.
As it has already been said, REST-services work via
HTTP/HTTPS. ADO.NET Data Services work is also based on this protocol. Besides
that, a universal syntax URI is supported there. It allows us to address any
part of the information from a data model. This is the reason of unavailability
to modify format of URI working with web service.
To create a new web service based on ADO.NET Data Services
it is necessary to have a ready-made data model. It can be LINQ to SQL, ADO.NET
Entity Framework or another data model. As a model of data is ready, you should
use tools of Visual Studio to add a new web service in a web application.
Figure 1: Creation ADO.NET Data Services
web-service
When you create a service it represents as an inheritor of
DataService generic-class. In a generic-parameter it is necessary to specify a
type which defines a data context. As a result of a service creation, a static
method InitializeService, which is required to configure a web service,
appears. An object of IDataServiceConfiguration type, which allows us to change
service options, is sent as parameters of this method. Thanks to first
acquaintance with ADO.NET Data Services it is necessary to grant full access
rights to all collections in data models. In real projects you should specify
containers explicitly and they will be accessible via a web service. Thus, the
code of a service which gives access to a data model will look as follows.
Listing 1 - Definition of simple web-service
public class WebDataService1 : DataService<NorthwindEntities>
{
public static void InitializeService(IDataServiceConfiguration config)
{
config.SetEntitySetAccessRule("*", EntitySetRights.All);
}
} As a result of access to a service it will generate an
answer in AtomPub format. This response will contain a list of accessible
tables.
Figure 2: Response from web-service
You can access to any tables of this data model now thanks
to changing address. For example, access to the address
"http://localhost:9580/WebDataService1.svc/Customers" allows us to
receive the list of objects from the table "Customers." When you
specify values of a primary key in parentheses, you can get access to a single
entity. For example, getting access to the address
"http://localhost:9580/WebDataService1.svc/Customers('ALFKI ')" web
service will return data only for one entity. More over, it is possible to
refer to a field of this entity. To do this you need to specify a name of a
field after the slash. For example, you can apply to field "CompanyName"
using the address "http://localhost:9580/WebDataService1.svc/Customers('ALFKI')
/ CompanyName."
Operations of data sets manipulation - sorting, filtration
and paging - are accessible on a server side. For these purposes use operators
"$orderby," "$filter," "$top," and
"$skip." Here we have some examples.
Listing 2 - Sample queries to web-service
/Bookmarks?$orderby=Name
/Bookmarks?$filter=Created gt '2007-05-07'
/Bookmarks?$top=10&$skip=30
Building a service side is only half of a work. An important
part is client building. To create a client within .NET applications you can
use "Add Service Reference" wizard.
Figure 3: "Add service reference" window
A proxy-class will be generated on a client side as a result
of adding a link to a service. This class will translate accesses to the web
service and work with them like with a simple collection.
Listing 3 - Sample of client code
var client = new NorthwindEntities(new
Uri(@"http://localhost:9580/WebDataService1.svc/"));
foreach (Customers customer in client.Customers)
{
}In this service all collections implement IQueryable interface.
For this reason you need to create a LINQ-queries to these collections; they
will be transformed into URI which corresponds to ADO.NET Data Services syntax.
For example, you can get a sample page using syntax of LINQ.
Listing 4 - Sample of paging
var client = new NorthwindEntities(new Uri(@"http://localhost:9580/WebDataService1.svc/"));
var query = client.Customers.Skip(40).Take(20);
foreach (Customers customer in query)
{
}At execution of a similar inquiry, it will be transformed in
corresponding URI. It is well visible at the debugging of this code.
Figure 4: Transformation of LINQ-query
After execution of a query, the service returns data in
AtomPub format that will be parsed by a client and presented as objects.
