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Hosting Silverlight applications in BLOB storage (part 2) - Communicating with third-party sites

3/14/2011 10:36:41 PM
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2. Communicating with third-party sites

Suppose your Silverlight application requires communication with an external website or web service (WCF, ASMX, REST, POX, or HTTP). For example, what if you want to host a Silverlight exchange rate calculator?

Your Silverlight application needs to poll a web service for live data, so you might be thinking that you should host your entire solution as an ASP.NET-hosted website. Although this is a perfectly valid solution, it’s still quite expensive for what you require. In this scenario, it might be more cost effective to host your web service in ASP.NET (or use a third-party service if one is available), but host your Silverlight application (and website) in BLOB storage. Figure 2 shows a Silverlight application communicating with an external web service.

Figure 2. Silverlight application communicating with an external web service


For the Silverlight application to communicate with a third-party domain, the external site must host a suitable cross-domain policy.

Hosting a Cross-Domain Policy

The following listing shows a typical cross-domain policy file (ClientAccessPolicy.xml) used to give permissions to a Silverlight application.

Listing 2. ClientAccessPolicy.xml file for Silverlight permissions

Cross-domain policy

For security purposes, Silverlight applications (and Flash applications) can, by default, communicate only with the domain that the container web page is hosted on. If your Silverlight application needs to communicate with a third-party domain, the external website needs to host a cross-domain policy file (ClientAccessPolicy.xml or CrossDomain.xml) to give your application permission to communicate with it.


The ClientAccessPolicy.xml file displayed in listing 2 states at that any Silverlight application hosted at http://silverlightukstorage.blob.core.windows.net/ (the BLOB storage account of your Silverlight application) can access the third-party web service. Your application would also be able to access the service if the domain URI at was set to *, which would indicate that any Silverlight application hosted at any website could access it.

ClientAccessPolicy.xml is a cross-domain policy file that’s used solely by Silverlight applications. Silverlight applications can also access web services that host a CrossDomain.xml file (a format that’s supported by both Flash and Silverlight). The following listing shows a CrossDomain.xml file that your BLOB storage-hosted application could communicate with.

Listing 3. CrossDomain.xml file

The CrossDomain.xml file displayed in listing 3 states at that any website hosting a browser-based application (Silverlight or Flash) can communicate with this service. If the owner of the web service wanted to restrict access to your application only, then it would replace the * at with silverlightukstorage.blob.core.windows.net (your BLOB storage account).

$root container

If you need to allow third-party Silverlight or Flash applications to access assets held in your BLOB storage account, store your CrossDomain.xml or ClientAccessPolicy.xml file in a public container named $root.

This special container allows you to store any file in the root of your URI, for example: http://chrishayuk.blob.core.windows.net/crossdomain.xml.


Now let’s build a Silverlight web search application that uses Yahoo’s Search API to search the internet.

Building a Silverlight Web Search Application

Figure 3 shows the Silverlight application that we’re going to show you how to build.

Figure 3. A Silverlight search application hosted in BLOB storage that communicates with the Yahoo Search API.


The HTML page for the application that you’re going to build (shown in figure 3) is no different from the one in listing 1 (well, there is one small difference: the source parameter value references the new Silverlight application (.XAP file).

To create the application, create a new Silverlight application as you normally would; then rename the default XAML (Extensible Application Markup Language) to YahooSearch.xaml. Replace the default Grid provided in the template with the XAML shown in the following listing.

Listing 4. XAML for Silverlight web search application

After you’ve pasted the XAML in listing 4 into your Silverlight page, you can hook up the code-behind for the page, as shown in listing 5.

Listing 5. Code-behind for Silverlight web search application XAML page

In the code-behind for the Silverlight page shown in listing 5, you can see that when the Search button is clicked, the execute method of your Search class is invoked at . The search term is sent to the Yahoo Search API, and then the results are returned asynchronously. When the search results are received, the results are bound to the data grid at .

Yahoo Search

Yahoo was a popular search engine many years ago, at a time before we Googled for everything. Here at Azure in Action, we like to support the little guy; come on Yahoo, you can be big again.

Actually, the reason we use Yahoo is that it has a nice, simple, REST-based API that we can easily use from Silverlight.


Listing 6 shows the code-behind for the Yahoo Search class.

Listing 6. Code-behind for the Yahoo Search class

We’re not going to spend any time explaining the code in listing 6, except to say that it makes an HTTP request to the Yahoo Search API and returns a set of results that you can bind to your Silverlight data grid.

Now that you have an idea of how to store static HTML websites and Silverlight applications in BLOB storage, let’s look at how you can use BLOB storage as a media server.

Other -----------------
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- Hosting static HTML websites (part 1) - Creating a static HTML website
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- Doing CRUDy stuff with the Table service (part 3) - Deleting entities & Updating entities
- Doing CRUDy stuff with the Table service (part 2) - Adding entities & Listing entities
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- Developing with the Table service
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