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Making cross-domain calls

From: Silverlight 5 Essential Training

Video: Making cross-domain calls

With most browser plug-ins, you are not allowed to communicate with third-party domains. That means that an app loaded from the lynda.com web site does not automatically get to make web requests for content running on the Microsoft.com site. The owner of the foreign server can permit these kinds of cross-domain calls if they set up a cross-domain policy file. This is usually a file called crossdomain.xml. In real world terms, these restrictions mean that your Silverlight application cannot access resources such as videos, images, or other HTML content unless given permission.

Making cross-domain calls

With most browser plug-ins, you are not allowed to communicate with third-party domains. That means that an app loaded from the lynda.com web site does not automatically get to make web requests for content running on the Microsoft.com site. The owner of the foreign server can permit these kinds of cross-domain calls if they set up a cross-domain policy file. This is usually a file called crossdomain.xml. In real world terms, these restrictions mean that your Silverlight application cannot access resources such as videos, images, or other HTML content unless given permission.

But here is the interesting bit. If you are running an elevated trust out-of-browser application, these restrictions no longer apply. I am going to show you how to set up a cross-domain call using Visual Studio. I have opened up our project called MakeCrossDomainCalls. This is my main UI. It consists of a button, a textbox for me to type in an Xbox LIVE alias, and an image control on the bottom of the page that I will load with an avatar from the Xbox LIVE web site.

Let me show you what kind of images I am talking about. I am going to switch over to a browser window and I am going to go to avatar.xboxlive.com. This section of the URL contains the alias. So if I type my alias in here, waltritscher, and hit Enter, you will see my Xbox avatar show up. This is what I want to call from my Silverlight application. So let me switch back to Visual Studio and go to my code behind page. I am going to press F7.

Here on line 37 is the URL from the Xbox LIVE site. Notice that it has a curly brace, 0, curly brace in the URL. I am going to replace that at runtime with the item from the textbox. So that's what's happening on this line of code right here. I get the gamerTag, and then I inject the gamerTag into this location, in the string. Then I set up a webRequest call. I am going to do that here on line 39, and then I am actually gong to invoke the web request in an asynchronous fashion on line 40.

I am going to call a method called BeginGetResponse, which grabs a thread out of the thread pool and sends it off to do the work. When the thread is done, it's going to return on this GetResponseCallback function down here, and inside here, I am going to marshal back to the UI thread, and then I am going to de-serialize the content coming back from Microsoft's servers. That's what's happening on these two lines of code. Then I create a brand-new image called BitmapImage, and then I set the source to that to the de-serialized bits I got from Microsoft's site, by calling GetResponseStream here.

Then I store that image into the image on my page. And then finally, I have a storyboard set up in my XAML that I am going to trigger here on line 60 to animate the image around, make it dance a little bit. Let's see what it looks like. I am going to press F5, and then I am going to click the button and it's going to pass my waltritscher string in and return my avatar. Now, we have an editor here at lynda.com that has graciously granted permission to use their avatar. It's thatgi42, and there you see their avatar.

I am sure you will have fun putting your own avatar in the textbox, or just take a wild guess. Here is one, some random person's avatar. I mentioned earlier that you have to be in full trust to make this happen. Let's see what happens if you're not in a full trust application. I am going to go to Properties, click the Silverlight tab, move down to the Enable running application out of the browser, and unselect that checkbox. Then rerun the application, and when I click on the button, it's going to throw an exception. There it is! Security error. Visual Studio has captured that, taken me over to my code, showing me what's wrong.

I want to fix this so I am going to make a slight modification to my code so that this doesn't happen at runtime. I will shut the application down and then I am going to uncomment this line of code right here, and this one down here. This checks with the application to see if I am currently in an elevated permission mode, and if so, then go ahead and run this code. I apparently put a break point in there without me seeing it, so I am going to press F9 to turn off the break point and then F5 to continue running the application.

What this demo shows is that I am not in an out-of-browser application, but when I click on the button, I don't get an exception happening. So as you see, cross-domain calls are much easier in Silverlight 4 now that we have elevated trust applications.

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This video is part of

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Silverlight 5 Essential Training

106 video lessons · 5449 viewers

Walt Ritscher
Author

 
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  1. 1m 41s
    1. Welcome
      53s
    2. Using the exercise files
      48s
  2. 35m 0s
    1. Overview of Silverlight
      3m 29s
    2. Setting up a developer computer
      2m 46s
    3. Installing the Silverlight Toolkit
      2m 21s
    4. Exploring the toolkit samples
      2m 35s
    5. Using Visual Studio 2010 to create a Silverlight project
      5m 10s
    6. Using Expression Blend to create a Silverlight project
      3m 13s
    7. Getting to know the Visual Studio interface
      8m 15s
    8. Working in the Expression Blend interface
      7m 11s
  3. 29m 46s
    1. Understanding the Visual Studio project structure
      3m 38s
    2. Creating a Silverlight page
      3m 17s
    3. Compiling your first application
      5m 0s
    4. Using other assemblies in an application
      5m 45s
    5. Deploying a Silverlight application
      3m 43s
    6. Understanding the startup process for an application
      3m 13s
    7. Understanding how users get Silverlight on their computers
      5m 10s
  4. 16m 59s
    1. Exploring the relationship between XAML and .NET
      3m 57s
    2. Using C#
      7m 16s
    3. Exploring the code-behind file
      5m 46s
  5. 43m 22s
    1. Working with XAML elements and Property attributes
      4m 56s
    2. Investigating XAML namespaces
      7m 31s
    3. Converting XAML properties with TypeConverters
      5m 1s
    4. Working with Property elements
      4m 24s
    5. Assigning runtime data with XAML markup extensions
      4m 21s
    6. Digging into the dependency property system
      6m 12s
    7. Creating a custom dependency property
      4m 42s
    8. Understanding attached properties
      6m 15s
  6. 9m 35s
    1. Debugging your code
      5m 17s
    2. Special Silverlight debugging techniques
      4m 18s
  7. 36m 0s
    1. Understanding layout
      4m 55s
    2. Using DockPanel and WrapPanel
      4m 2s
    3. Exploring sophisticated layouts with the Grid
      6m 40s
    4. Absolute positioning with the Canvas panel
      5m 20s
    5. Scrolling content with the ScrollViewer
      3m 28s
    6. Adjusting content alignment, margins, and sizing
      5m 6s
    7. Using the TabControl
      2m 17s
    8. Manipulating elements with transforms
      4m 12s
  8. 20m 32s
    1. Painting the user interface (UI) with SolidColorBrush
      6m 7s
    2. Getting colorful with gradient brushes
      3m 15s
    3. Decorating elements with ImageBrush
      4m 56s
    4. Creating effects with VideoBrush
      6m 14s
  9. 25m 42s
    1. Understanding routed events
      3m 12s
    2. Wiring up event handlers in Silverlight
      6m 4s
    3. Understanding event bubbling
      4m 39s
    4. Exploring mouse events
      7m 43s
    5. Exploring keyboard events
      4m 4s
  10. 32m 31s
    1. Displaying text on the screen
      3m 24s
    2. Gathering text input from the user
      5m 30s
    3. Showing complex text with RichTextBox
      6m 7s
    4. Understanding text overflow and text linking
      3m 14s
    5. Searching content with the AutoComplete type-ahead control
      6m 45s
    6. Using and embedding Silverlight fonts
      7m 31s
  11. 26m 47s
    1. Understanding content controls
      5m 7s
    2. Understanding button controls
      6m 16s
    3. Using the BusyIndicator
      4m 40s
    4. Showing tooltips
      4m 34s
    5. Changing content size with ViewBox
      1m 39s
    6. Exploring more controls
      4m 31s
  12. 55m 36s
    1. Connecting elements with binding
      9m 54s
    2. Using business data in a binding
      9m 21s
    3. Listing data with ItemsControls
      6m 39s
    4. Digging into the DataGrid
      8m 28s
    5. Using the DataForm
      3m 49s
    6. Storing data on the client
      7m 10s
    7. Debugging XAML bindings
      5m 41s
    8. Charting data
      4m 34s
  13. 24m 36s
    1. Creating an Out-of-Browser (OOB) application
      6m 16s
    2. Controlling aspects of an OOB application
      2m 49s
    3. Alerting the user with notification windows
      7m 13s
    4. Hosting HTML content in Silverlight applications
      8m 18s
  14. 30m 2s
    1. Exploring Silverlight trust levels
      3m 51s
    2. Creating an elevated trust application
      5m 57s
    3. Reaping the benefits of XAP signing
      6m 5s
    4. Making cross-domain calls
      5m 1s
    5. Getting started with COM Interop using the Speech API
      4m 21s
    6. Using COM Interop to interact with Microsoft Office
      4m 47s
  15. 16m 33s
    1. Showing child windows
      3m 48s
    2. Using a child window as a dialog window
      5m 7s
    3. Creating native windows
      2m 53s
    4. Maximizing a window with full screen mode
      4m 45s
  16. 13m 20s
    1. Dynamically loading UI content with user controls
      4m 46s
    2. Using the Navigation Framework
      8m 34s
  17. 9m 28s
    1. Using resource files
      3m 53s
    2. Using images in an application
      5m 35s
  18. 16m 32s
    1. Playing audio
      4m 27s
    2. Playing sound effects
      5m 4s
    3. Viewing video content with MediaElement
      4m 58s
    4. Changing video playback speed with TrickPlay
      2m 3s
  19. 14m 40s
    1. Capturing video with a web camera
      4m 16s
    2. Working with a microphone
      3m 47s
    3. Creating print output
      6m 37s
  20. 22m 35s
    1. Understanding Silverlight animations
      4m 35s
    2. Creating animations
      7m 52s
    3. Controlling animations
      4m 18s
    4. Simulating physics with animation easings
      5m 50s
  21. 27m 47s
    1. Placing XAML resources within a FrameworkElement
      6m 47s
    2. Centralizing settings in styles
      4m 8s
    3. Creating an alternate control UI with ControlTemplates
      6m 29s
    4. Dressing up your data with DataTemplates
      4m 33s
    5. Explaining the VisualStateManager: a simple way to manage control states
      5m 50s
  22. 10m 55s
    1. Handling global errors in a Silverlight application
      5m 46s
    2. Showing an application loading screen
      5m 9s
  23. 10m 48s
    1. Integrating with XNA
      7m 40s
    2. Exploring data analysis with PivotViewer
      3m 8s
  24. 1m 6s
    1. Goodbye
      1m 6s

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