Before delving into the ADO.NET Data Service, let's sum up
the typical data access solutions used in Silverlight applications.
Overview of Silverlight Data Access Technology
Experienced ASP.NET developers know, with the use of data
access, they can write corresponding logics so that application can connect to
various external data sources and further query and process data. With
sophisticated data access provided by ASP.NET, developers can quickly accomplish
the realization of the above process, almost without writing any code. The
reason is that, ASP.NET operating environment is a complete installation of .NET
framework, so ASP.NET can invoke ADO.NET to access the database, and then the
results are transformed into HTML to be sent to the client.
But, for Silverlight applications, the same problem is not
so simple. Because the Silverlight plug-in runs on the client browser, the
corresponding micro .NET Framework is not possible to contain huge ADO.NET
components. Aiming at this particular structure of Silverlight applications,
the realization of Microsoft's technology is first to submit HTTP requests to
the server to let the server carry out the task of data access, and then the
results are sent back to the client in accordance with the format of the two
sides agreed to. Thus, developers can use any SOAP-based technologies or WCF
service to obtain data. Once the data are returned to the client, you can process
them by calling the corresponding data components.
In summary, Silverlight applications can use the following
three typical techniques for data access:
·
Traditional Web Service to access server-side database.
·
Using WCF service shipped with.NET 3.0 framework to access the
server database.
·
Through the up-to-date and the most closely Silverlight bound
technology--ADO.NET data service, to access the server-side database.
For Web service and WCF solution, readers may refer to the
relevant information in MSDN. The sample in this article only shows interest in
ADO.NET data service technology. Let's next start the more detailed discussion.
ADO.NET Data Services Technology
ADO.NET Entity Framework is Microsoft's next generation of
ADO.NET. This framework, in fact, is not a brand-new and independent
infrastructure. It is only another new choice based upon the traditional
ADO.NET framework.
On the whole, ADO.NET Entity Framework has the following
characteristics:
·
Entity data model that allows of high abstraction for data
modeling.
·
Powerful mapping engine that allows developers to easily set up the
mapping between the data model and data storage definition.
·
Supporting the use of entity SQL syntax and LINQ to query the
definition of EDM data.
·
Object services layer that allows you to choose whether or not to
present the results of a query as the row/column records or .NET objects.
·
Opening data provider model, allowing other storage mechanisms to
be combined with ADO.NET Entity Framework.
For a simple schematic diagram of the ADO.NET Entity
Framework, you can refer to here.
Use ADO.NET Data Services to Access the Server-side Image
Data
Next, we are going to give details in the image management
system, on how to use ADO.NET data services to access the image data on the
server side.
(A) Implement Data Model
1. Right-click the ASP.NET Web Site
project in Solution Explorer, point to Add, and then select New Item.
2. In the Add New Item dialog box,
select ADO.NET Entity Data Model. Name the data model PhotoAlbumModel.edmx, and then click Add.
3. In the Entity Data Model Wizard,
select Generate from Database, and then click Next.
4. In the Generate from Database
dialog box, create a connection to the AjaxAlbumDB database.
Notice that the entity connection settings that are shown will be added to the Web.config file of the application. The connection will be
named AjaxAlbumDBEntities. The data service requires
this connection but the Silverlight client will not use it. Click Next.
5. In the Choose Database Objects
dialog box, select the two tables Category and Photo in the AjaxAlbumDB database. Then
name the model namespace PhotoAlbumModel and click Finish.
This finishes the AjaxAlbumDB data
model implementation and opens the Entity Data Model Designer.
In the designer, we touch nothing. So you can see an association relationship
diagram between two tables Category and Photo, as shown in Figure 7 given previously.
(B) Implement Data Service
1. Right-click the ASP.NET Web application project in
Solution Explorer, point to Add, and then select New Item.
2. In the Add New Item dialog box,
select ADO.NET Data Service. For this sample app, name
the service PhotoAlbumDataService.svc, and then click Add. This adds the data service template that contains the
required reference to the System.Data.Services library.
3. Visual Studio will open a file named PhotoAlbumDataService.cs
that contains the source code that initializes the ADO.NET data service. The
type of the template class identifies the data model. Set the type in the
DataService declaration to AjaxAlbumDBEntities, as
shown in the following example. As you see, the data service class is derived
from System.Data.Services.DataService<T>.
4. Modify the config.SetEntitySetAccessRule
method as shown in the following code. Authorization to read and write all data
is defined by the call to config.SetEntitySetAccessRule("*",
EntitySetRights.All). The asterisk applies the rule to all data sets in
the data model.
5. Also, we modify the config. SetServiceOperationAccessRule method to grant all access rights
in service operations.
Listing 1-- Initialize ADO.NET Data Service
public class PhotoAlbumDataService : DataService<AjaxAlbumDBModel.AjaxAlbumDBEntities>
{
//Initialize Service
public static void InitializeService(IDataServiceConfiguration config)
{
//Authorization to read and write all data
config.SetEntitySetAccessRule("*", EntitySetRights.All);
//Service operations access configuration
config.SetServiceOperationAccessRule("*", ServiceOperationRights.All);
}
}(C) Generating Data Service Client Library
This step will generate data service classes from the
running data service. Note these classes represent each of the entities that
are defined in the data service.
To do this, please first make sure the data service is
running. Then, right-click the Silverlight project and select Add
Service Reference. In the Add Service Reference dialog
box, we just let Visual Studio discover the base URL of the data service, since
the service and Silverlight project are in the same solution. If you click OK, Visual Studio goes and generates all the client side
classes for you, adds the related references, and you can start using the
service right away.
In our case, such as in the MainPage.xaml.cs, add the
following using directive: using System.Data.Services.Client.
Since everything gets ready, let's come to the final phase--implementing
the data access on the client side.
(D) Data Client Implementation
1. Open the *.xaml.cs source code files that will access
ADO.NET Data Service in the Silverlight project.
2. Add a using directive to System.Data.Services.Client.
3. Add using directives to the PhotoServiceReference
Service classes that are generated by Add Service Reference
or DataSvcUtil.exe. The generated classes make it
possible for the client application to materialize the data that is received
from the data service.
Listing 2-- Add necessary using directives
using System.Collections.ObjectModel;
using System.Data.Services.Client;
using ADONET = S2PhotoAlbum.PhotoServiceReference;
4. Create an instance of the data service context of the PhotoServiceReference data service. Then by invoking the
relevant methods (such as BeginExecute<(Of
<(TElement>)>)(Uri, AsyncCallback, Object)), you can implement your
client-side data access.
Listing 3-- Instantiate the proxy
DataServiceContext
namespace S2PhotoAlbum
{
public partial class MainPage : UserControl
{
//Define a variable of DataServiceContext proxy
ADONET.AjaxAlbumDBEntities proxy;
public MainPage()
{
InitializeComponent();
//others omitted…
//Define the instantiation of proxy DataServiceContext
proxy = new ADONET.AjaxAlbumDBEntities(new
Uri("PhotoAlbumDataService.svc", UriKind.Relative));
}For details in using the instantiation of proxy
DataServiceContext to implement the typical CRUD operations, please refer to
the later on modules.
In the next section, we are to preview the main finished
line of the RIA image management system, and then set out to develop the first
module of the sample system.
