Silverlight unit testing with NUnit: yes you can (without hacks)!

It may be obvious for most of you, but it took a while to my caveman brain to realize this, so I figured I could post it for other cavemen. You (we) have got an excuse though: the short release cycle of Silverlight means that most stackoverflow questions and blog posts on the subject are out-of-date and refer to older versions of Silverlight (<=3) when what I describe was not possible.

Up to Silverlight 3 you had to use a Silverlight-specific unit testing tool, like the Silverlight Toolkit framework. These tools are quite awesome and have been fundamental, but it’s nice to have the full power of NUnit (or xUnit & co.) at disposal. In addition to the community and tooling support, it’s practical to use the same tool you already use for other .NET projects.

The game changer is called binary assembly compatibility, brought by Silverlight 4. In a few words this means that you can add a reference to a Silverlight assembly from a “full” .NET project (provided that you don’t use any Silverlight-only class).

If your application is correctly layered (for ex. with MVVM) in most cases it’s trivial to keep views and viewModels/models in separate assemblies. ViewModels and models usually don’t reference any Silverlight-only class (otherwise you may have a code smell!) and are 100% compatible with .NET, so testing them with NUnit is as easy as

  • create a .NET class library in your solution
  • add a reference to NUnit (NuGet it!)
  • add a reference to your model and/or viewModel assemblies
  • write your unit tests
  • run them with NUnit

no tweaking or hacking required and works fine with F# assemblies as well.

It is typical for views to use Silverlight-only classes, but this is generally not a problem because it doesn’t make much sense to unit-test them anyways as they are mostly XAML with very little amounts of code-behind.

Free tip: if you want to test internal classes and methods you can add the InternalsVisibleTo attribute to the target assembly.

Happy testing!


Dynamic member binding in Silverlight 4

UPDATE: Xavier Decoster already wrote a nice article on this topic some time ago. Please check it out! (Note to self: improve google skills)

First, let me say that I’ll take the long route, so if you are already familiar with dynamic typing in C# you can probably jump straight to the last section. Otherwise read on, you may learn something cool that is not used every day but can save you in some situations.

In the [not so] old days of Silverlight 3 if you wanted to dynamically create a class you had to emit intermediate language instructions, etc. Definitely not so easy. Silverlight 4 (with C# 4.0) introduced support for dynamics and simplified this a lot.

Straight from the DynamicObject documentation, this simple implementation of a dynamic dictionary uses an internal dictionary to store string/object pairs where the key is the member name and value is its associated value.

public class DynamicDictionary : DynamicObject
    Dictionary<string, object> dictionary =
        new Dictionary<string, object>();

    public override bool TryGetMember(
        GetMemberBinder binder, out object result)
        return dictionary.TryGetValue(binder.Name, out result);

    public override bool TrySetMember(
        SetMemberBinder binder, object value)
        dictionary[binder.Name] = value;
        return true;

In a DynamicObject you have two methods (TryGetMember and TrySetMember) that are invoked every time someone tries to access the objects’ members. In this particular implementation, when this code is executed

dynamic myDynamicObject = new DynamicDictionary();
myDynamicObject.FirstName = "John";
myDynamicObject.Age = 18;

two pairs “FirstName”/”John” and “Age”/18 are stored in the internal dictionary. On the other hand when you do

string test = myDynamicObject.FirstName;

instead of calling the getter of FirstName (like any statically typed object would do), TryGetMember is invoked and the value corresponding to key “FirstName” is looked up from the internal dictionary.

The dynamic keyword tells the compiler that the member will be looked up at runtime, so you can set/get any member you want and the compiler won’t complain: he knows the members will be resolved while the program is running.

The binding problem

Now there is only a small problem with this approach (and it’s the whole point of this post): if you create a binding that targets a dynamic member you’ll get an error. It looks like the Silverlight binding engine “cannot discover” dynamic properties.

For example this does not work:

public dynamic MyDynamicDictionary {  get;     set; }

// ...

MyDynamicDictionary = new DynamicDictionary();
MyDynamicDictionary.Label = "Hello, I'm dynamic!";
 <Button Content="{Binding MyDynamicDictionary.Label}"/> 


If you look at the output:

System.Windows.Data Error: BindingExpression path error: 'Label' property not found on 'SilverlightApplication22.DynamicDictionary'

Indexed binding to the rescue

Telerik’s Vladimir Enchev explains on his blog how this approach can be used to implement a dataTable-like structure that can back for ex. a datagrid. The clever bit is that he added to the DynamicDictionary the [] indexer:

public object this[string columnName]
        if (dictionary.ContainsKey(columnName))
            return dictionary[columnName];
        return null;
        if (!dictionary.ContainsKey(columnName))
            dictionary.Add(columnName, value);
            dictionary[columnName] = value;

Now we have two alternatives to access the dynamically-created members:

// like before:
dynamic myDynamicObject2 = new DynamicDictionary();
myDynamicObject2.FirstName = "John";
myDynamicObject2.LastName = "Smith";

// using []:
var myDynamicObject3 = new DynamicDictionary();
myDynamicObject["FirstName"] = "John";
myDynamicObject["LastName"] = "Smith";

The two approaches have exactly the same effect (notice that in the second version the variable is declared with var instead of dynamic).

Using square brackets to access members has the advantage that you can actually create members using strings: let’s say you have a string/object dictionary, it’s easy to loop the dictionary entries and “create” a member for every key while setting the value as the member value. After this you’ll have an object that mirrors the dictionary:

var source = new Dictionary<string, object>();
source.Add("FirstName", "John");
source.Add("LastName", "Smith");
source.Add("Age", 18);

var target = new DynamicDictionary();
foreach (var entry in source)
    target[entry.Key] = entry.Value;

now target is the same as you would have after doing

new something() { FirstName = "John", LastName = "Smith", Age = 18 };

except that it “adapts” to any key/value you have in the dictionary. Cool eh?!

It turns out that the indexer has another side benefit (that solves the binding problem). In fact Silverlight 4 also introduced indexed bindings: you can create bindings that target indexed structures (like a list or a dictionary) simply using square brackets. The nice thing is that our dynamic class happens to have an indexer.

Let’s revisit our code: if we declare the property as DynamicDictyionary instead of dynamic (we now must set the properties using the indexer because the compiler only allows “non-existing” properties on object of dynamic type):

public DynamicDictionary MyDynamicDictionary { get; set; }


MyDynamicDictionary = new DynamicDictionary();
MyDynamicDictionary["Label"] = "Hello, I'm dynamic!";

and change the XAML to look like this (notice the square brackets)

<Button Content="{Binding MyDynamicDictionary2[Label]}"/>

the dynamic binding does work fine:


Happy dynamic binding!

Download the code

Windows Phone 7: how to reset the idle detection countdown

Windows Phone 7 like every other phone OS turns off the screen after a period of inactivity. This is not a problem most of the time because any user activity (namely finger interactions on the screen) resets the countdown, so if you are using an application the screen saver will not get in the way. However there are some particular cases where it is useful to disable the idle detection, for example in games or apps that require long reading (or watching). In that case you can completely disable idle detection:

PhoneApplicationService.Current.UserIdleDetectionMode =

Keep in mind that this disables the “screen saver” at all, so be careful because you could drain the poor user’s battery if you do it without valid reason.

There is another more interesting case, though: suppose your app uses the accelerometer as its main user input. In this case there won’t be any user activity to trigger a countdown reset, but disabling it at all also doesn’t look like the best idea (what if the user puts the phone on the table to go grab a beer?).

The best in this case would be to reset the count-down when a movement is detected, i.e. treating accelerometer events like screen user input. How to do this?

The answer is extremely simple: you can just disable the IdleDetection and re-enable it again. This will reset the count-down. One little caveat: you cannot re-enable it immediately after having disabled it, the OS is smart enough not to be fooled and will ignore your two commands. You’ll have to wait a short while before re-enabling idle detection.

Here is an example: when I get a new reading from the accelerometer (I’m using the AccelerometerHelper) I check if there has been a large enough movement and in that case I disable the idle detection. Otherwise I enable it –this effectively resets the countdown every time the movement goes above a given threshold. Keep in mind that the accelerometer fires 50 times per second, that’s why I used a bool field to avoid unnecessary calls to the system setting. I’m not sure this prevents an actual performance loss, but it would be worth it to experiment and measure a little if you are using this technique in your apps.

double _currentValue;
bool _screenSaverEnabled = true;

private void OnAccelerometerHelperReadingChanged(object sender, AccelerometerHelperReadingEventArgs e)
    Dispatcher.BeginInvoke(() =>
            // you'll have something more useful in your app
            computedValue = e.OptimallyFilteredAcceleration.X;

            var delta = Math.Abs(computedValue - _currentValue);
            if (_screenSaverEnabled)
                if (delta > SOME_ARBITRARY_THRESOLD)
                    _screenSaverEnabled = false;
                    PhoneApplicationService.Current.UserIdleDetectionMode = IdleDetectionMode.Disabled;
                    Debug.WriteLine("Screen saver disabled");
                _screenSaverEnabled = true;
                PhoneApplicationService.Current.UserIdleDetectionMode = IdleDetectionMode.Enabled;
                Debug.WriteLine("Screen saver enabled");
            _currentValue = computedValue;

Happy coding!

Windows Phone 7: correct pinch zoom in Silverlight

Pinch zooming is one of those things that look incredibly simple until you actually try to implement them. At that point you realize it hides quite a number of intricacies that make it hard to get it right. If you tried to implement pinch zooming in Silverlight for Windows Phone 7 you probably know what I’m talking about.

What does it means getting it right?

Adrian Tsai already gave an excellent explanation, so I won’t repeat his words. The test is extremely simple: pick two points in the image (for example two eyes) and zoom with your fingers on them. If at the end of the zoom the two points are still under your fingers you got it right –otherwise you got it wrong.

Multitouch Behavior

Laurent Bugnion, Davide Zordan and David Kelly are the men behind  Multitouch Behavior for SL and WPF. It’s an impressive open source project and you should check it out. In addition to pinch-zooming it gives you rotation, inertia, debug mode and much more. It’s extremely easy to work with as you just need a couple of lines of XAML. The only shortcoming is that at the time of writing it seems that there is no way to read the current zoom state, making it difficult to fully support tombstoning. If you don’t need this, go grab Multitouch Behavior and stop reading: it will probably work better and you’ll save some time.


This is the XAML we are starting with. Notice that our DIY implementation relies on the Silverlight Toolkit’s InputGesture. If you are not yet using it, please install the toolkit and add a reference to Microsoft.Phone.Controls.Toolkit in your project.

<Image x:Name="ImgZoom"
                ScaleX="1" ScaleY="1"
                TranslateX="0" TranslateY="0"/>

The wrong way

I’ve seen this example several times around, I suppose you’ve seen it too somewhere on The Interwebs™:

double initialScale = 1d;

private void OnPinchStarted(object s, PinchStartedGestureEventArgs e)
    initialScale = ((CompositeTransform)ImgZoom.RenderTransform).ScaleX;

private void OnPinchDelta(object s, PinchGestureEventArgs e)
    var transform = (CompositeTransform)ImgZoom.RenderTransform;
    transform.ScaleX = initialScale * e.DistanceRatio;
    transform.ScaleY = transform.ScaleX;

Very simple and good looking. I love simple solutions and I bet you do too, but as someone once said “Things should be as simple as possible, but not simpler.” And unfortunately this is simpler than possible (is this even a sentence?). The problem is that the scaling is always centered in the middle of the image, so this solution won’t pass the poke-two-fingers-in-the-eyes test.

The better but still wrong way

The knee-jerk reaction is to move the scaling center between our fingers as we perform the scaling:

double initialScale = 1d;

private void OnPinchStarted(object s, PinchStartedGestureEventArgs e)
    initialScale = ((CompositeTransform)ImgZoom.RenderTransform).ScaleX;

private void OnPinchDelta(object s, PinchGestureEventArgs e)
    var finger1 = e.GetPosition(ImgZoom, 0);
    var finger2 = e.GetPosition(ImgZoom, 1);

    var center = new Point(
        (finger2.X + finger1.X) / 2 / ImgZoom.ActualWidth,
        (finger2.Y + finger1.Y) / 2 / ImgZoom.ActualHeight);

    ImgZoom.RenderTransformOrigin = center;

    var transform = (CompositeTransform)ImgZoom.RenderTransform;
    transform.ScaleX = initialScale * e.DistanceRatio;
    transform.ScaleY = transform.ScaleX;

This is better. The first time it actually works well too, but as soon as you pinch the image a second time you realize the image moves around. The reason: the zoom state is the sum of all the zoom operations (each one having its center) and by moving the center every time you are effectively removing information from the previous steps. To solve this problem we could replace the CompositeTransform with a TransformGroup and then add a new ScaleTransform (with a new center) at every PinchStart+PinchDelta event group. This will probably work: every scaling will keep its center and all is well. Except your phone will probably catch fire and explode because of the number of transforms you are stacking up. My team has a name for this kind of solutions, and it isn’t a nice one (fortunately there is no English translation for that).

The right way

It is clear by now that simply setting a scale factor and moving the center won’t take us far. As we are real DIYourselfers we will do it with a combination of scaling and translation. In the already mentioned article, Adrian Tsai uses this technique in XNA and we will apply the same concept in Silverlight. If an image is worth a million worth, a line of code is probably worth even more, so I’ll let the c# do the talking.

// these two fully define the zoom state:
private double TotalImageScale = 1d;
private Point ImagePosition = new Point(0, 0);

private Point _oldFinger1;
private Point _oldFinger2;
private double _oldScaleFactor;

private void OnPinchStarted(object s, PinchStartedGestureEventArgs e)
    _oldFinger1 = e.GetPosition(ImgZoom, 0);
    _oldFinger2 = e.GetPosition(ImgZoom, 1);
    _oldScaleFactor = 1;

private void OnPinchDelta(object s, PinchGestureEventArgs e)
    var scaleFactor = e.DistanceRatio / _oldScaleFactor;

    var currentFinger1 = e.GetPosition(ImgZoom, 0);
    var currentFinger2 = e.GetPosition(ImgZoom, 1);

    var translationDelta = GetTranslationDelta(

    _oldFinger1 = currentFinger1;
    _oldFinger2 = currentFinger2;
    _oldScaleFactor = e.DistanceRatio;

    UpdateImage(scaleFactor, translationDelta);

private void UpdateImage(double scaleFactor, Point delta)
    TotalImageScale *= scaleFactor;
    ImagePosition = new Point(ImagePosition.X + delta.X, ImagePosition.Y + delta.Y);

    var transform = (CompositeTransform)ImgZoom.RenderTransform;
    transform.ScaleX = TotalImageScale;
    transform.ScaleY = TotalImageScale;
    transform.TranslateX = ImagePosition.X;
    transform.TranslateY = ImagePosition.Y;

private Point GetTranslationDelta(
    Point currentFinger1, Point currentFinger2,
    Point oldFinger1, Point oldFinger2,
    Point currentPosition, double scaleFactor)
    var newPos1 = new Point(
        currentFinger1.X + (currentPosition.X - oldFinger1.X) * scaleFactor,
        currentFinger1.Y + (currentPosition.Y - oldFinger1.Y) * scaleFactor);

    var newPos2 = new Point(
        currentFinger2.X + (currentPosition.X - oldFinger2.X) * scaleFactor,
        currentFinger2.Y + (currentPosition.Y - oldFinger2.Y) * scaleFactor);

    var newPos = new Point(
        (newPos1.X + newPos2.X) / 2,
        (newPos1.Y + newPos2.Y) / 2);

    return new Point(
        newPos.X - currentPosition.X,
        newPos.Y - currentPosition.Y);

Also note that in the XAML we must set the RenderTransformOrigin to 0,0.
This finally passes the fingers-in-the-eyes test! Now we can add some bells and whistles like handling dragging, blocking the zoom-out when the image is at full screen, and avoiding that the image is dragged outside the visible area. For those extra details please see the sample solution at the end of the article.

What about MVVM?

You are using MVVM-light for your WP7 app, aren’t you? We all agree my code is ugly and not very MVVM friendly, I’ll make no excuses. However it’s all strictly UI code, so it doesn’t feel so bad to have it in the code behind. What you will probably do is wire TotalImageScale and ImagePosition to your ViewModel. Those two values fully define the state of the zoom, so if you save and reload them in your ViewModel you will be good to go.


Here is the full sample project so that you can play with the code within the comfort of your Visual Studio (my daughter is in the picture, please treat her with respect :-) ).
Feel free to use the code in your project. As always, any kind of feedback is deeply appreciated!

Simple Error Reporting on WP7 RE-REDUX

If you haven’t read Rudi Grobler’s Simple Error Reporting on WP7 and Simple Error Reporting on WP7 Redux, please go read them now. Basically it’s about reporting errors at runtime from a Windows Phone 7 application. I always incorporate this kind of functionality in my applications (not only on Windows Phone) and it has proven to be very useful in these years.

While Rudi’s implementation uses the EmailComposeTask to send an error report, I do it in a slightly different and by submitting the report as http POST to a web page that then sends the email. This way the process is entirely silent and nothing is shown to the user (except the prompt). My reasoning is that the less steps she has to perform, the more likely she is to send the report.

In order to use this method you’ll need some server, but nothing fancy is required. I suspect you already have something adequate in place (for example the one hosting your blog).

Client Side

This is the code I put on the phone:

public void Application_UnhandledException(object sender, ApplicationUnhandledExceptionEventArgs e)
    if (MessageBoxResult.OK != MessageBox.Show("We encountered a problem and need to close. Would you like to tell us?",
                                "Oh no!",

        var subj = e.ExceptionObject.Message;
        var body = new StringBuilder("Stacktrace:");

        var wc = new WebClient();
        wc.Headers[HttpRequestHeader.UserAgent] = "Mozilla/5.0";
        wc.Headers[HttpRequestHeader.ContentType] = "application/x-www-form-urlencoded";
        wc.UploadStringCompleted += (s, er) => MessageBox.Show("Thanks for your report!");
            new Uri(""),
    catch (WebException)
        MessageBox.Show("The report could not be sent (no network available?).");

Don’t forget to replace the messages with something better. You can add whatever you like to the report, just keep in mind that if you query DeviceExtendedProperties to get the peak memory usage, your app will need the Identity Capability (which looks a bit scary in the marketplace in my opinion).

Server Side

On the server side you can do pretty much anything: you could insert the report in a database or fill in a ticket in your bug tracker. I keep it simple and send an email (please set up a dedicated address if you do this). My server is rather fast but only supports PHP, so that’s what I use:

	if (isset($_POST["subject"]) &amp;amp;amp;amp;amp;amp;&amp;amp;amp;amp;amp;amp; isset($_POST["body"]))
		mail("", "wp7 app bug report: ".$_POST["subject"], $_POST["body"]);
		echo "ok.";
		echo "missing parameters.";

Please notice that this is not the most secure practice in the world as someone could sniff the traffic generated by your app and exploit this page to flood your mailbox. A simple solution is to limit this page to send a maximum of say 10 emails/hour. Anyways if you are getting a much larger number of legit hits, chances are that the exception is always the same one (or your app is really buggy and you deserve to be flooded :-P).

Is this method better than using the EmailComposeTask? I don’t know, but it’s just another option at your disposal.

An error occurred while accessing IsolatedStorage.

While developing a Silverlight Windows Phone 7 app I ran into this problem: when the application closes, an internal  IsolatedStorageException is raised:

An error occurred while accessing IsolatedStorage.
   at System.IO.IsolatedStorage.IsolatedStorageSecurityState.EnsureState()
   at System.IO.IsolatedStorage.IsolatedStorageFile.get_AvailableFreeSpace()
   at System.IO.IsolatedStorage.IsolatedStorageSettings.Save()
   at System.IO.IsolatedStorage.IsolatedStorageSettings.TrySave()
   at System.IO.IsolatedStorage.IsolatedStorageSettings.SaveAllSettings()
   at MS.Internal.FrameworkCallbacks.ShutdownAllPeers()

IsolatedStorageSettings seemed to be the cause of the problem. Very weird because I wasn’t even using it! Just adding this line in my App class would cause the problem:

private System.IO.IsolatedStorage.IsolatedStorageSettings appSettings = System.IO.IsolatedStorage.IsolatedStorageSettings.ApplicationSettings;

Now I created a new, empty, project (Windows Phone Application) and inserted that line. Weird: no problems.
I thought there was something wrong in my solution, so I made a backup copy and started removing stuff. At some point my solution was identical to the default one (except for the IsolatedStorageSettings line and the project’s GUID). Not “almost identical”, but completely, literally identical, i.e. WinMerge found no difference except the two mentioned. And of course the default solution worked while mine gave the IsolatedStorageException on shutdown.

I honestly haven’t understood the cause of the problem, but at least I’ve found a solution: change the project GUID:

  • close Visual Studio
  • open your solution’s .sln file with notepad
  • replace a couple of numbers in the Project(“{<your GUID here>}”)
  • open the .csproj with notepad
  • apply the same change to the GUID (in the first line after the comment and in all the Build Configurations)
  • Re-open your solution with Visual Studio and the problem is gone.

I suspect the problem has something to do with the emulator but I haven’t had a chance to try a real device yet. I hope I saved you an evening of head scratching.

Best of Swiss Silverlight 2010

During this year’s Shape 2010 conference in Zurich-Oerlikon, Microsoft Switzerland announced the winners of the Best of Swiss Silverlight 2010 Award in collaboration with the Best of Swiss Web Association, simsa and Netzwoche.


Incredibly my application Trails of Switzerland won the Bronze award. I was completely taken by surprise (not to mention super excited) because I didn’t really expect anything when I started the project. In fact it was just a “weekend project” to try a couple of things. When I saw the award application form I thought it could be worth a try so I polished a bit the front-end and added a couple of cool gadgets.

I was familiar with the competition’s application procedure also because I already did it a few times before for my company (that won this year’s .NET Award by the way).


My application leverages Silverlight’s DeepZoom component to show a full topographic map of Switzerland. The base image is a huge 19 Gigapixels (~3 Gigabytes) JPEG, but movement and zooming is wonderfully smooth.

The tricky part was mapping the GPS data to the map and then keeping content synchronized with the DeepZoom zooming and panning.


Currently Trails of Switzerland is in closed-beta at and will probably never go live (except if someone wants to buy it from me) for the simple reason that copyrights on the maps are incredibly expensive and I cannot afford to buy them “just for fun”.
To be honest I must say that the Swisstopo maps are of incredible precision and quality, but still are way too expensive for a no-profit application.

The good part is that Trails of Switzerland could probably be ported to Windows Phone 7 (with a major restyling of course) as DeepZoom seems to work very smoothly there too.

I’d like to congratulate the other contest winners (Coresystems AG/Misapor, Extrafilm AG, VASP Datatecture AG/ETHZ, Portia AG/Immostreet AG): your applications were really mind-blowing!


The conference was very interesting as well. In particular Bob Muglia should have taken notes from Ronnie’s talk on HTML5/Silverlight. If you have watched the PDC2010 keynote (and the twitter/blog-storm that followed) you know what I’m talking about.

As always Laurent’s Bugnion’s talks were interesting, but also many others were worth the trip (in particular from a Windows Phone 7 and design/UX standpoint).

Moral: you never can tell

Moral of the story is you never know where a weekend project is going to bring you. It seems that someone else also agrees on this. This is one of the things I love in this field.


Thanks to Microsoft Switzerland and most of all thanks to my wife for the continuous support and for understanding when I forget stuff/don’t listen because I’m thinking about code (i.e. most of the time) :-)

Silverlight: web service calls fail when OOB

Situation: your Silverlight 3 application is calling a web service and while it works fine in the browser, it miserably fails in out of the browser mode (OOB).
Don’t panic. It seems that you cannot call a webservice (in App.xaml.cs) before having assigned the application’s root visual. As mentioned, this is not a problem when the application runs inside the browser.

So, for example, this will fail (timeout):

private void Application_Startup(object sender, StartupEventArgs e)
    myService.MyMethodCompleted((s, e) => this.RootVisual = new MyPage());

while this will work fine:

private void Application_Startup(object sender, StartupEventArgs e)
    this.RootVisual = new MyPage();

On a side note, remember that RootVisual can only be set once. I haven’t checked what happens with Silverlight 4, but if you are seeing weird behaviours it may be worth it to check this before setting everything on fire.

I hope I saved you long hours sniffing http traffic, checking for crossdomain issues, etc… :-)

Happy coding!

Silverlight/WPF RGB color in c#

Sometimes you have a color in XAML and want to use it in c#. In other words you want to translate something like:

<Border Background=“#AA0FCC1B”>;


Border.Background = …something…

So you start looking for an IValueConverter that create a color from a RGB string, or translate the hex values to decimal, etc… STOP it!
All you need is:

Border.Background = new SolidColorBrush(
                            Color.FromArgb(0xaa, 0x0f, 0xcc, 0x1b));


Ok, it may sound stupid, but you never can tell…

Silverlight 4 is out…

…just a word of caution for developers: if you are still developing with Silverlight 3 and VS2008 don’t install the Silverlight 4 runtime. If you do, you won’t be able to build your SL3 application anymore and you’ll spend the next hour

a) looking for a way to make your app build again

b) looking for the SL3 runtime (that you won’t find anywhere –and won’t correct the situation anyway).

This problem should not arise if you are already on VS2010 because it allows you to choose your target between SL3 and SL4, but if you are stuck with VS2008 you are out of luck.
It seems that the folks at MS think that everybody can just go ahead and migrate all their solutions to VS2010 and SL4 the next day things are released.

Epic fail! Rant over.

P.S. the correct way to make things work again is:

– uninstall the Silverlight 4 runtime (listed as “Silverlight” in the Programs & Features panel)
– restart your machine (no, you cannot skip this step!!!)
– download the Silverlight3 Developer Runtime and install it (hurry up because as soon as they’ll notice they will remove it from the download server!)

And no, you won’t be able to view SL4 websites –but at least your app will build.

UPDATE: forget the rubbish above. If you install the Silverlight 4 developer runtime you will be able to run SL4 apps and build SL3 apps (of course if you had the SL3 SDK). Just don’t install the “normal” SL4 runtime.