The next step is to build a concrete image provider class by deriving from ImageProvider. Listing 2 contains the skeleton for a SQL Server image provider that fetches images from a SQL Server database. SqlImageProvider supports image retrieval, but does not support image storage. Note the false return from CanSaveImages, and the SaveImage implementation that throws a NotSupportedException.
Listing 2. SQL Server image provider

SqlImageProvider's Initialize method expects to find a configuration attribute named connectionStringName identifying a connection string in the <connectionStrings> configuration section. This connection string is used by the RetrieveImage method to connect to the database in preparation for retrieving an image. SqlImageProvider also accepts an applicationName attribute if provided but assigns ApplicationName a sensible default if no such attribute is present.

Configuring Custom Provider-Based Services

In order to use a provider-based service, consumers must be able to configure the service, register providers for it, and designate which provider is the default. The Web.config file in Listing 3 registers SqlImageProvider as a provider for the image service and makes it the default provider. The next challenge is to provide the infrastructure that allows such configuration directives to work.

Listing 3. Web.config file configuring the image service

Simce <imageService> is not a stock configuration section, you must write a custom configuration section that derives from System.Configuration.ConfigurationSection. Listing 4 shows how. ImageServiceSection derives from ConfigurationSection and adds two properties: Providers and DefaultProvider. The [ConfigurationProperty] attributes decorating the property definitions map ImageServiceSection properties to <imageService> attributes. For example, the [ConfigurationProperty] attribute decorating the DefaultProvider property tells ASP.NET to initialize DefaultProvider with the value of the <imageService> element's defaultProvider attribute, if present.
Listing 4. Class representing the <imageService> configuration section

Next, you must register the <imageService> configuration section and designate ImageServiceSection as the handler for it. The Web.config file in Listing 5, which is identical to the one in Listing 3 except for the changes highlighted in bold, makes <imageService> a valid configuration section and maps it to ImageServiceSection. It assumes that ImageServiceSection lives in an assembly named CustomSections. If you use a different assembly name, you'll need to modify the <section> element's type attribute accordingly.

Listing 5. Making <imageService> a valid configuration section and registering ImageServiceSections as the configuration section handler

<configuration >
  <configSections>
    <sectionGroup name="system.web">
      <section name="imageService"
        type="ImageServiceSection, CustomSections"
        allowDefinition="MachineToApplication"
        restartOnExternalChanges="true" />
    </sectionGroup>
  </configSections>
  <connectionStrings>
    <add name="ImageServiceConnectionString" connectionString="..." />
  </connectionStrings>
  <system.web>
    <imageService defaultProvider="SqlImageProvider">
      <providers>
        <add name="SqlImageProvider" type="SqlImageProvider"
          connectionStringName="ImageServiceConnectionString"/>
      </providers>
    </imageService>
  </system.web>
</configuration>

Loading and Initializing Custom Providers

The final piece of the puzzle is implementing the image service and writing code that loads and initializes the providers registered in <imageService>'s <providers> element.

Listing 6 contains the source code for a class named ImageService that provides a programmatic interface to the image service. Like ASP.NET's Membership class, which represents the membership service and contains static methods for performing membership-related tasks, ImageService represents the image service and contains static methods for loading and storing images. Those methods, RetrieveImage and SaveImage, call through to the provider methods of the same name.

Listing 6. ImageService class representing the image service

Before delegating calls to a provider, ImageService.RetrieveImage and ImageService.SaveImage call a private helper method named LoadProviders to ensure that the providers registered in <imageService>'s <providers> element have been loaded and initialized. LoadProviders uses the .NET Framework's System.Web.Configuration.ProvidersHelper class to do the loading and initializing. One call to ProvidersHelper.InstantiateProviders loads and initializes the registered image providers-that is, the providers exposed through the Providers property of the ImageServiceSection object that LoadProviders passes to ProvidersHelper.InstantiateProviders.

Observe that the ImageService class implements public properties named Provider and Providers that provide run-time access to the default image provider and to all registered image providers. Similar properties may be found in the Membership class and in other ASP.NET classes representing provider-based services.

Click here to continue on to part 9, Hands-on Custom Providers: The Contoso Times