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Creating your First Silverlight Client Application: Twitter and COM, of course

Pete Brown - 28 May 2010

I'm creating some "create your first" videos for MSDN, and thought it would be useful to offer up a text version of the Out-of-Browser one in addition to the video.

In this example, we'll build a simple Twitter Search client, using Silverlight out-of-browser mode. I'll also show how to use the Automation (IDispatch) API to load the search results into Microsoft Excel.


The XAML for this demo includes just a simple page with one button and a listbox. The button is there to retrieve the tweets, the listbox to display them. No data templates yet.

<UserControl x:Class="SilverlightApplication50.MainPage"
    d:DesignHeight="300" d:DesignWidth="400"

    <Grid x:Name="LayoutRoot" Background="White">
        <Button Content="Get Tweets"
                Click="GetTweets_Click" />
        <ListBox x:Name="TweetList"

The resulting page, at design time in the Visual Studio 2010 editor, looks like this:


Next, we need to make a service call to Twitter to retrieve the latest tweets.

Calling Twitter

We're going to call the Twitter Search API, using the URL http://search.twitter.com/search.atom?q=silverlight . That searches for anything with the word "silverlight", and returns the results back in the atompub XML format. You can also return JSON if you prefer (I do not)

You can, of course, use a third-party library to make your twitter integration easier. As I'm only doing a simple search, I decided to parse the API return results myself.

Result Format

When you hit that URL, you'll get back XML that includes header information like this:

<?xml version="1.0" encoding="UTF-8"?>
<feed xmlns:google="http://base.google.com/ns/1.0" xml:lang="en-US" xmlns:openSearch="http://a9.com/-/spec/opensearch/1.1/" xmlns="http://www.w3.org/2005/Atom" xmlns:twitter="http://api.twitter.com/">
  <link type="text/html" href="http://search.twitter.com/search?q=silverlight" rel="alternate"/>
  <link type="application/atom+xml" href="http://search.twitter.com/search.atom?q=silverlight" rel="self"/>
  <title>silverlight - Twitter Search</title>
  <link type="application/opensearchdescription+xml" href="http://search.twitter.com/opensearch.xml" rel="search"/>
  <link type="application/atom+xml" href="http://search.twitter.com/search.atom?q=silverlight&amp;since_id=14847443030" rel="refresh"/>
  <twitter:warning>adjusted since_id to 14795621913 due to temporary errorsince_id removed for pagination.</twitter:warning>
  <link type="application/atom+xml" href="http://search.twitter.com/search.atom?lang=en&amp;max_id=14847443030&amp;page=2&amp;q=silverlight" rel="next"/>

Plus an entry for each search result:

  <link type="text/html" href="http://twitter.com/onDev/statuses/14847443030" rel="alternate"/>
  <title>Don Syme: F# on Silverlight 4 - http://cut.ms/UDe</title>
  <content type="html">Don Syme: F# on &lt;b&gt;Silverlight&lt;/b&gt; 4 - &lt;a href=&quot;http://cut.ms/UDe&quot;&gt;http://cut.ms/UDe&lt;/a&gt;</content>
  <link type="image/png" href="http://s.twimg.com/a/1274739546/images/default_profile_2_normal.png" rel="image"/>
  <twitter:source>&lt;a href=&quot;http://twittsync.com/default.aspx&quot; rel=&quot;nofollow&quot;&gt;TwittSync&lt;/a&gt;</twitter:source>
    <name>onDev (Aggregated blogs)</name>

Tweet Entity

The first thing we'll want to do is add a new class called "Tweet". Add that to your project, and populate it so it looks like this:

public class Tweet
    public string Title { get; set; }
    public Uri Image { get; set; }
    public Uri Link { get; set; }

Calling the Service

Now, in the code-behind, you can add the code to call the service and load the results into a collection. First, make sure you have System.Xml and System.Xml.Linq referenced in your Silverlight project. Then, double-click the "Get Tweets" button and add the following handler code:

private ObservableCollection<Tweet> _tweets = new ObservableCollection<Tweet>();

private void GetTweets_Click(object sender, RoutedEventArgs e)
    WebClient client = new WebClient();

    client.DownloadStringCompleted += (s,ea) =>
            XDocument doc = XDocument.Parse(ea.Result);

            var items = from item in doc.Descendants("entry")
                        select new Tweet()
                            Title = item.Element("title").Value,

                            Image = new Uri((from XElement xe in item.Descendants("link")
                                             where xe.Attribute("type").Value == "image/png"
                                             select xe.Attribute("href").Value).First<string>()),

                            Link = new Uri((from XElement xe in item.Descendants("link")
                                            where xe.Attribute("type").Value == "text/html"
                                            select xe.Attribute("href").Value).First<string>()),

            foreach (Tweet t in items)

    client.DownloadStringAsync(new Uri("http://search.twitter.com/search.atom?q=silverlight"));

The Linq query above goes through the document and pulls out all the entries. For each entry it finds, it looks for the title, a Link of type image/png (even jpegs are listed as image/png) and the tweet link (text/html)

If you'd rather, you could use the built-in SyndicationFeed class rather than hand-parsing using Linq. For most types of basic feed parsing, you'll find it much simpler.

Binding the ListBox

We'll bind the listbox from code. You can do it from XAML as well; both methods are equally valid. We want the ListBox to show all the tweets returned from the network call, so we'll bind it to the _tweets ObservableCollection.

public MainPage()

    Loaded += new RoutedEventHandler(MainPage_Loaded);

void MainPage_Loaded(object sender, RoutedEventArgs e)
    TweetList.ItemsSource = _tweets;

All good there. The next step is to make the application actually work in out-of-browser mode.

Making it Out-of-Browser

Making an application run out-of-browser is pretty simple. If the app is new, all you need to do is set the properties on the property page. If the app is something you've worked on for a while, you'll also want to make sure you aren't using any HTML interop, or anything else which relies on the browser.

Right-click the Silverlight application project, and select properties. On the Silverlight tab in properties, check the "Enable running application out of the browser." This will enable the right-click menu option to take the app out of the browser, as well as provide access to the OOB APIs from within code.


Starting up in Out-of-Browser Mode

Finally, we want the application to start up in out-of-browser mode, so we need to tell that to Visual Studio. In Project->Properties for the Silverlight app, click the "Debug" tab and change the start action to "Out-of-Browser application". Note that it will list the web project where the xap resides, not the Silverlight project; that's correct.


Next, right-click your Silverlight project, and set it as startup:


Now run the project and click the button. Hmm. As is often the case with a first run of a new application (especially one without any unit tests) nothing happened. Let's try a little debugging.

Debugging Out-of-Browser Applications

With the out-of-browser application set to start in that mode, it's easy to insert some breakpoints. I set a breakpoint inside the DownloadStringCompleted handler and saw that there are no items in the XDocument. Hmm. Let's fire up Fiddler2 and see what's going on. If you haven't used Fiddler2 yet, stop now, and go get it. It is one of the single most useful applications for debugging Silverlight and Ajax apps. Seriously, stop reading and head over there now.

Well, I see the failed clientaccesspolicy check followed by a crossdomain.xml check. Since crossdomain.xml is there and is permissive, Silverlight makes the networking request. Note that in elevated permissions mode, the checks are not required.


Looking in the response inspector in Fiddler2, I can see that the search returned correct results, so the issue is not a networking one. Next step is to see if it is perhaps a namespace issue with XML. I don't know about you, but those always get me. In general, if the XML document is well-formed and you aren't seeing any results, but your query is correct, it's a namespace issue. Luckily, with XDocument, it's really easy to add the correct namespace code. Here's the updated method with the namespaces added.

private void GetTweets_Click(object sender, RoutedEventArgs e)
    WebClient client = new WebClient();

    client.DownloadStringCompleted += (s,ea) =>
            XDocument doc = XDocument.Parse(ea.Result);

            XNamespace ns = "http://www.w3.org/2005/Atom";

            var items = from item in doc.Descendants(ns + "entry")
                        select new Tweet()
                            Title = item.Element(ns + "title").Value,

                            Image = new Uri((from XElement xe in item.Descendants(ns + "link")
                                                where xe.Attribute("type").Value == "image/png"
                                                select xe.Attribute("href").Value).First<string>()),

                            Link = new Uri((from XElement xe in item.Descendants(ns + "link")
                                            where xe.Attribute("type").Value == "text/html"
                                            select xe.Attribute("href").Value).First<string>()),

            foreach (Tweet t in items)

    client.DownloadStringAsync(new Uri("http://search.twitter.com/search.atom?q=silverlight"));

Note how I added the "ns" to all the elements in the linq query as well. Namespaces are an unfortunate reality when working with XML. 99% of the time, you'll wish they were never there, but that other 1% when you need the differentiation, they're super important.

Run it again, and you should see something show up in the ListBox.


But wait, that information isn't very useful. Instead, you are seeing the default "ToString" representation of the Tweet class. To get a better display, we need to create a data template to format the items in the listbox.

Creating the List Box Data Template

A DataTemplate is a bit of XAML that is used to format information. While not exclusive to ItemsControl-derived controls like the ListBox, it is particularly useful with them.

<ListBox x:Name="TweetList" 
            <Grid Margin="10">
                    <ColumnDefinition Width="Auto" />
                    <ColumnDefinition Width="*" />

                <Image Source="{Binding Image}"
                <TextBlock Text="{Binding Title}"
                            TextWrapping="Wrap" />

The DataTemplate results in a grid for each row, with a 56x56 (50 + 3 + 3 for margins) first column with an image in it, and a TextBlock in the second column. The second column is star-sized, that is, it takes up the remaining space. Image and Title are both properties of the Tweet class. I didn't use the Link property, but you can add that in using the HyperlinkButton control if you so desire.

When you run it now, you'll get a much nicer experience:


Now that that is working, let's add a little bit of elevated trust functionality.

Making it Elevated Trust

So far, everything we've done works 100% cross-browser, cross-platform. Elevated Trust also works cross-platform, but we're going to use a feature that lights up only on windows: COM Automation.

First, we need to tell Silverlight that this is an elevated trust application. Back to the property pages, open up the Out-of-Browser settings dialog.


The next thing you should do is sign your XAP. We won't do that here, but there is a great walkthrough on XAP signing for elevated trust apps here. The walkthrough also shows what the various install dialogs look like.

In production, you always want to sign your elevated trust applications using a trusted root certificate authority. If you fail to sign your app, the install dialog the user gets is a pretty scary one. Signed applications have a much nicer dialog.

Now that we have an elevated trust app, let's do a little local light-up.

Automating Microsoft Excel

Elevated trust applications have a number of ways they can integrate with the local system: they have increased access to files, custom chrome, and can (on Windows) use the IDispatch COM Automation API to call automation servers such as Microsoft Office, and even the Location API.

To support automating excel, add a second button to the page:


Add the button using the designer, or via XAML. Ideally, place it after the "GetTweets" button and before the "TweetList" ListBox. The XAML for the button looks like this:

<Button Content="Export"
        Width="75" />

Because the Automation API is available only on Windows, you'll want to first check to see if it is present and available.

Checking for the Presence of the Automation API

The first check we need to do is to ensure the automation API is available. If it is not available, we need to disable the functionality. First, in the code-behind, add the following using statement to gain access to the automation classes.

using System.Runtime.InteropServices.Automation;

Next, modify the MainPage_Loaded event to include a check for the COM Automation API:

void MainPage_Loaded(object sender, RoutedEventArgs e)
    TweetList.ItemsSource = _tweets;

    ExportToExcel.IsEnabled = AutomationFactory.IsAvailable;

That will enable or disable the button based upon the availability of the automation API. You can test this by unchecking the "requries elevated permissions" option and running the application.

Now we'll get down to the actual Excel export code.

Exporting Tweets to Excel

The event handler code for the "Export" button is going to contain the export functionality. Since the COM Automation API is based on IDispatch, and is therefore bound at runtime, there's no intellisense or other help for you when coding. One trick you can use is to write your code in a full .NET framework application using the early-bound version of the API (assuming it is compatible) and then paste the code into Silverlight, making adjustments as required. When looking to see what APIs you can use, one good source is scripting documentation. Anything which is available on Windows to scripting via Automation is also available to Silverlight via Automation.

An aside on patterns

This is your first Silverlight OOB application, so I don't want to muddy the waters with patterns just yet. However, keep in mind that the code-behind approach we're using here is not the only way to provide functionality in your application. One very popular pattern is MVVM or ViewModel. Once you progress beyond the basics, learn about that pattern so you can write applications that have better structure and are more easily tested.

Using Automation in Silverlight means you'll be using the dynamic keyword. Support for that keyword is included in the Microsoft.CSharp.dll, so you'll need to add a reference to that from your Silverlight project.


Next, double-click the export button on the design surface to create an event handler. We'll put our export code inside the handler.

private void ExportToExcel_Click(object sender, RoutedEventArgs e)
    if (_tweets.Count == 0)
        MessageBox.Show("No tweets to export. Please get latest tweets first.");

        dynamic excel = AutomationFactory.CreateObject("Excel.Application");


        dynamic sheet = excel.ActiveSheet;

        int row = 1;

        // headers
        dynamic linkHeaderCell = sheet.Cells[row, 1];
        dynamic textHeaderCell = sheet.Cells[row, 2];

        linkHeaderCell.Value = "Url";
        textHeaderCell.Value = "Message Text";

        // rows
        foreach (Tweet t in _tweets)

            dynamic linkCell = sheet.Cells[row, 1];
            dynamic textCell = sheet.Cells[row, 2];

            linkCell.Value = t.Link.ToString();
            textCell.Value = t.Title;

        excel.Visible = true;

    catch (Exception ex)
        MessageBox.Show("Error automating Excel: " + ex.Message);

That's all there is to it. Example code to automate Excel, Word, Outlook and other applications can be found all over the internet. You could easily expand upon this example, or create your own automation code to produce a Word document, or even email the search results using Outlook.

This particular machine still has Office 2007 installed. The Automation API works with 2007, 2010 and likely versions below that. The resulting spreadsheet, once you open up the columns a little, looks like this:



There you have it: your first Silverlight out-of-browser application. In this sample, we went over:

  • Creating a Silverlight application
  • Making a RESTful network service call
  • Making an application work in out-of-browser mode
  • Debugging out-of-browser applications
  • Debugging Silverlight applications using Fiddler2
  • Basic data binding
  • Creating Data Templates
  • Making applications require elevated trust
  • Using COM Automation from Silverlight to automate Excel

I hope you found this example helpful.


Source Code and Related Media

Download /media/56247/firstsilverlightclientapptwittercomsource.zip
posted by Pete Brown on Friday, May 28, 2010
filed under:        

13 comments for “Creating your First Silverlight Client Application: Twitter and COM, of course”

  1. Petesays:

    Just so I can understand how you use examples like this: do you run the app and that's it, or do you trace through the existing source code using the walkthrough here?

    All the source code required is in the post, but it requires you to build it step-by-step, so I'm trying to understand if this is a useful type of tutorial, or if there's a different approach some/many/most folks prefer.

  2. Randysays:
    Thanks and appreciate the tutorial, Pete. I followed the tutorial by building it step-by-step and was very happy about the results. Along the walkthrough, I also learned other useful stuffs e.g. Debugging using Fiddler etc. I will surely buy your upcoming book. All the best.
  3. Damansays:
    I am getting this error
    An exception occurred during the operation, making the result invalid. Check InnerException for exception details.
    error is coming at this line of code XDocument doc = XDocument.Parse(ea.Result);
    ea.Result is throwing this up.
    Please advice how to resolve this.

  4. lugerovacsays:
    He Pete, I have a problem :(

    Twitter no longer supports atom searches so the tutorial here, and in the book, can no longer be used. What could be used as a substitute?

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