HTML5 CSS3 Javascript vs XAML .net and Silverlight

Introduction >> The clash of the Titans is on

Developers right this second are at some bar / restaurant / lounge / god knows where else being “social” i.e. having geeky discussions about technology, what language/platform is best to use or even better “what’s coolest right now” Smile

I am one of those developers and I must say, when it comes to such discussions I am always biased towards XAML technologies. Yet recently I started diving in what I used to call the “darker side” i.e. HTML5, JS and CSS3 (the “dark side” is reserved for things like Java Smile ). It’s been years (6 to be exact) since I was doing some HTML and Co and I must say; I am really impressed. HTML5 just takes everything to a whole new level BUT what impresses me most is JS and CSS3.

JS because the browsers are natively exposing APIs such as GeoLocation, Canvas drawing, local storage and also indexed DB (and more APIs). BESIDES THAT, what sells JS for me, is the new libraries developers are building such as JQuery and Knockout.js. Working with javascript today is just loads and loads of fun !!

CSS3 because WOW its frekin cool!! The new box modelling with the Flexible box model, the gardients, the transforms, the transitions and I can go on and on…

So yea I must admit, web development (presentation layer) of today is becoming loads of fun! Backend for Web with MVC its also frekin cool! so I guess the questions that readers of my blog / people who know me would ask “Is Marlon turning his back on XAML?” The answer is simple and its NO, my car number plate still says WPF so I guess I cannot change everything now Smile Joking aside, I see both technologies as super awesome and both of them have a different realms (to a certain extend). “to a certain extend” for the reason that WinRT will also support HTML5 JS and CSS3 as a first class citizen just like it will for XAML and C# / C++.

Does this mean we should say bye bye to XAML and just focus on HTML5 and Co?? If it can do web + desktop why bother looking at XAML??

Ok so lets be a bit metaphoric about this and let’s compare technologies with cars. There are many different brands, there is Audi, there is Honda and you can go on and on… They all take you from place A – B. How they take you is a bit different, some are super confortable, some are fast. Some are perfect for one thing some are perfect for another! You buy a car that fits your needs and the same can be said to technology you decide to use. There are those scenarios, for example when you are moving house, where you do not have the luxury to use your “preferred car” because hey; if its a sports car, it will be no good when you are moving, so in such a case you are coerced to use a specific type. Such an example for developers is the web / mobile phones … Yet again just like in real life there are many different trucks for moving Smile there is HTML5, and then there are the plugins (Flash, Silverlight) …

Which one should you go for? HTML5 or Plugins?

Looking at web tech today I would say go for HTML5. It will work on devices such as phones, if done properly it will be super fast. On the other hand I would strongly suggest you consider SL / Flash if your users cannot install/already have a browser that supports HTML5. In that case then yea I would not go for the HTML < 5 because you’re in for nightmares! Maybe mix and match! Some things are better done with plugins some other with HTML. Do not be afraid to try both worlds, it will not hurt.

What about WPF??

WPF will still have a place but I guess we will slowly see it being used lesser and lesser, reason being with WinRT you are not using WPF you are using WinRT, yes its XAML but NO it is not WPF. There are cases such as VS and similar heavy weight products that would require something like WPF. Trading applications and internal banking applications is another example for WPF apps. But yea if you are building a simple app, going forward one would be better off with something Metro style i.e. WinRT based. So should WPF developers say “cr*p I wasted a lot of time learning something I will not use anymore”?? Whoever says that is really talking bull***t! WPF thought us a lot, WPF gave us so many concepts and ideas… WPF will always continue living in other technologies… Its concepts that really are hard to learn not how to use a framework, example DataTemplates, ControlTemplates, Binding and many other concepts … For me WPF showed me what an awesome API should look like! Besides that you learnt XAML that is something that is not a waste of time since XAML is here to stay !

Final verdict – Who will be the last technology standing?

None probably Smile technology evolves … sorry my fellow geeks, I like you used to treat a technology stack (and must admit still do sometimes) as my one and true love and I could not use another technology without feeling that I am being MARLONCHANGE… well its not the case, when it comes to technology you can mix and match as much as you want and no you will not be called a “Cheater” Smile

Take it from me, if you did not have a look at HTML5 and co, maybe its time for you to do so. Maybe you will not use it but I am sure you’ll get something out of it. I am learning so much, getting so many new ideas (that yea I can even apply for WPF) from doing some HTML5 and Co.

Hope I did not bore you to death with this post but I felt I needed to share this with you guys! I’ll be posting more on HTML5 and Co this year but yea of course I will also do the same for XAML and Co.

Long live XAML

Long live HTML5 (and co)

>> No source code to download just thoughts …

MEFedMVVM NavigationExtension


Most of the MEFedMVVM features so far were all around discoverability of ViewModels, yet when building MVVM application sometimes you want to also have a mechanism to discover and launch views. If we look at web development its all about resources sitting on a server and you can launch/load a specific resource via a URI (Unique Resource Identifier). This mechanism proved to be a very easy and scalable way of locating views and within a Web Page you can link to other pages very easily. When building WPF applications that are purely content based such a mechanism would really come in handy, and if you think about it MEFedMVVM is all about discoverability so why not support this scenario.

Since this is not really part of the core MEFedMVVM I created an extension that you can use to accomplish this, MEFedMVVM NavigationExtension.

MEFedMVVM.NavigationExtension support both WPF and Silverlight 4.


Enter MEFedMVVM Navigation Extensions

The idea is to be able to specify to a View that it can be located by a unique identifier (a string) and then you can have someway of launching that view and render it in some container/host. Something like this


And you make the view discoverable by decorating it with this attribute


As you can see in the figure above, there are 3 magic attached properties that are attached to the “Invoker”

  • NavigationExtensions.NavigateTo
    • Specify the unique identifier (string) to locate the view. I use a URI format but you can use whatever you like as long as its unique
  • NavigationExtensions.NavigationHost
    • Specify where you want the View to be rendered
  • NavigationExtensions.NavigationParameter
    • Specify a Parameter to be passed to the ViewModel of the View. The reason why the parameter is passed to its ViewModel is because if you are doing MVVM then your View has no need for parameters, its the ViewModel that needs the parameter(after all the ViewModel controls the logic). We will see how you can still cheat and do whatever you like at the end of the day, the parameter can be passed to the View.

So one might wonder how will my ViewModel receive the parameter. This is done by your ViewModel being set as DataContext of the View (if you are using MEFedMVVM to link the View to the ViewModel this happens automatically) and also your ViewModel has to implement the INavigationInfoSubscriber interface. This interface defines 1 method OnNavigationChanged which will pass along the parameter and also give you an instance of the INavigationManager responsible for starting the Navigation.




So till now we can

  1. Make a View discoverable by specifying a Unique Identifier
  2. Specify an Invoker and give it enough information on what to render and where to render it
  3. And also specify a parameter to be passed

This pretty much covers the bare basics, let’s get a better understanding of what is a Host and what is an Invoker before we deep dive in more complex scenarios.


Host and Invoker Deep Dive

When building the NavigationExtensions I wanted to make sure that you can create your own handlers both for Hosts and Invokers, and what is the best way to do so if not with MEF Smile

There are 2 base classes you need to write in order to create your own handlers.

  • ControlNavigationHost
    • This is to create your own hosting control. Out of the box you get one which is ContentControlNavigationHost (it handles any ContentControl)
  • ControlNavigationHandler
    • This is to create your own invoker for a control. Out of the box you get one which is the ButtonNavigationHandler (it handles any ButtonBase)

The ControlNavigationHost has 4 methods that you need to implement (all method implementation would be usually one liners)


In order to make your own ControlNavigationHost discoverable by the NavigationExtensions simple Export it like this


The ControlNavigationHandler has 3 methods you need to implement


In the implementation you simple have to register to the Event you want and then call the OnEventFired protected method of the base class. here is an example


And again to make the handler discoverable you Export it like so


Please note: that its up to you how you want the creation policy to be (i.e. If MEF should create a new instance of the NavigationHandler or not but in this case you should always make it NonShared so that for each invoker in your application you have a different ControlNavigationHandler instance)

Apps are usually more complicated, so let’s dive into more complicated scenarios

Before we start going through these scenarios let’s have a look at some interfaces and classes that MEFedMVVM exposes for you to consume/implement




Implement this interface on a class that will be passed as NavigationParameter and you will get injected with a INavigationManager responsible for that Navigation



Implement this interface in your ViewModel to get passed the NavigationParameter.



A NavigationCommand is just a DelegateCommand<T> BUT it implements the INavigationManagerProvider interface. When used as a NavigationParameter it will hold the instance of the INavigationManager so that you can do things such as Closing a navigation. We will see the NavigationCommand<T> being used in the first scenario below.


Scenario 1

Let’s say you have a dialog that shows some settings and when you are done you want to get those settings back to the original ViewModel that “started” the navigation to the Settings screen. Here are a couple of screen shots for such a scenario.


In order to do this we need the MainViewModel to expose a NavigationCommand<T>


and the Execute handler for this would be something like this


We will revisit the code inside the Execute Handler in a bit**…

Now we can specify that the NavigationParameter is this command so that the SettingsViewModel can execute this command when it is done and give us the ApplicationSettings object instance.


The Settings ViewModel implements the INavigationInfoSubscriber thus it will get injected with the NavigationCommand that we are passing to it via the NavigationParameter attached property


Once the Settings ViewModel calls the Execute on the _onSettingChangedCommand it will invoke the method inside the MainViewModel (OnSettingChangedExecuted) passing the new ApplicationSettings.

**One thing to note is that the MainViewModel is also calling CloseNavigation on the NavigationManager of the NavigationCommand. This is so that as soon as its done applying the new settings the Settings screen disappears.

Download the sample and play around with it to get a better feel of how this all works together (its under Samples/TestNavigation)

Scenario 2

Let’s say you have a sort of Wizard Step by Step UI.


In this case we want to chain the Navigation so that the CreateUserProfileViewModel send the UserProfile not to the MainViewModel (the ViewModel that started the Navigation) but to the ViewModel next in the chain i.e. the RenderUserProfileViewModel.

In order to do so both “Invokers” (i.e. the button for the CreateUserProfile and the button for the RenderUserProfile) must have the same navigation “invoker”. You do so by explicitly setting the NavigationHander attached property (this is an attached property that exposes the Navigation handler for an “invoker”).


Ok so now we have both “invokers” using the same NavigationHandler; because of this we can register to the NavigatingAway event of the INavigationManager inside the CreateProfileViewModel and pass the data we want to the RenderUserProfileViewModel (which is the NewNavigationInfoSubsciber in the NavigationEventArgs passed by the event)


So basically the CreateUserProfileViewModel (Step 1) could pass along data to RenderUserProfileViewModel (Step 2) and you can continue chaining like this one step after another.

NOTE: For Silverlight you instead of using the NavigationExtensions.NavigationHandler use the NavigationExtensions.ChainToElement and specify the other button (this is because there are issues around binding to custom attached properties in SL). This approach can also be used in WPF.



Download the sample and play around with it to get a better feel of how this all works together (its under Samples/TestNavigation)


One thing I love about this Extension is that it enables you to use View-First approach to MVVM in nearly any scenario. Yes granted sometimes its better to have ViewModel-First approach but in my experience if you can always work using View-First life becomes much more easy because your code is more loosely coupled. In fact this is one of the things I love about MVC and Web in general… Controllers never reference each other, A View has a controller and thats it. In MVVM we tend to complicate things by having Parent ViewModels that have Child ViewModels yada yada yada… just my 2 cents…

This is all still work in progress, it needs more testing from my end to make sure there are no side effects such as memory leaks etc yet feel free to poke around and play around with it. As always feedback/bug reports are very welcome.

Download the code from

Making a generic UpdateSourceTrigger for PropertyChanged in Silverlight

In my previous post I explained how Silverlight 4 lacks the UpdateSourceTrigger for PropertyChanged. I focused on how you can overcome this issue for one of the biggest use cases, which is the TextBox.

In this post I will show how one can do this for any Dependency Property of any Framework element. Please note that this post is quite an experimental one. Probably the use case you are looking for is for the textbox scenario, if that is the case I would suggest that you use the more explicit approach i.e the one I show in my previous post. The take away from this post should be more the idea of how things work rather than the actual code I am using here, this code was never tested in production thus it might contain memory leaks and other issues.

The key for updating the binding when a property changes is to actually know when the property has changed and then force the binding to update the source. In WPF there are multiple ways of doing this, one of which is to use the DependencyPropertyDescriptor class. This class allows you to hook an event handler for when the specified property has changed. Unfortunately in Silverlight there is no DependencyPropertyDescriptor class thus one has to resort to some ninja trick Smile After doing some internet crawling I found an interesting approach using attached property creating and hooking to the property changed. The idea is that you create an attached property and you bind the newly created attached property to the property that is consuming the binding.

Let’s dig a bit deeper

Let’s say you have the following binding

<TextBox Text=”{Binding SomeText, Mode=TwoWay}”

The Text dependency property is consuming a binding and you want to update the source of the binding as soon as the property changes.

You create an attached property and you bind it to the Text property of that TextBox. This will get you notified when the text changes because when creating the attached property you can have a property changed handler. In the property changed handler you update the source of the binding.

You would create the attached property and the binding for notifications like so

// Create attached property

var listeningProperty = System.Windows.DependencyProperty.RegisterAttached(

    "ListenAttached" + propertyName,



    new System.Windows.PropertyMetadata(OnPropertyChanged));

//Create a binding that will be updated when the property changes

var listeningBinding = new Binding(propertyName) { Source = element };



Ok so I showed how you can workaround the lack of DependencyPropertyDescriptor in Silverlight for property changed notification, now how do we update the binding?

In order to do this we will need to get the instance of the Text Dependency Property. Unfortunately we will need to resort to reflection in order to get the instance of the Dependency property. We can do this by name

var propertyFieldInfo = frameworkElement.GetType().GetField(newPropertyName + "Property");

if (propertyFieldInfo == null)

    throw new InvalidOperationException(String.Format(

        "The property {0} does not exist in the element {1}. Make sure you specified the correct property.",

        newPropertyName, frameworkElement.GetType().FullName));

//Get the dependency property the binding is applied to

var property = (DependencyProperty)


As you can see I am attaching a string “Property” to the actual property name (in this case Text) when I try to get the dependency property by reflection. Even though in XAML you specify “Text” as the property name, the actual dependency property name is TextProperty (which is a coding standard for dependency properties), “Text” is how the property is registered to the framework.

So why did we need to get the actual instance of the Dependency property? We had to do this in order to get the Binding Expression so that we can force the binding expression to update the source of the binding with the new property value. This would look like this

_bindingExpression = element.GetBindingExpression(BoundProperty);

And to update the source (i.e in the property changed handler of the attached property) we would do this


How is this approach generic?

Well let’s see what we did in the “Dig Deeper” section.

– We are getting a dependency property by name

– We get the Binding Expression from that dependency property

– We create an attached property which will be our way of hooking to property changes of a specific dependency property.

mmm… So as such we can have 1 attached property of type string that will generate this hook and update the source for us. If we do this we would have something like this in our XAML

<TextBox Text=”{Binding SomeText, Mode=TwoWay}” local:UpdateSourceTrigger.PropertyName=”Text” />

Not great but not too bad, it will do the trick for now… So here is the code for the attached property (you have to download the sample project)… Its exactly what I explained in the section above.

Please note that I am creating the hook inside a separate class UpdateSourceTriggerProxy. I am doing that so that the operation is atomic and everytime you use this approach we have a different instance of UpdateSourceTriggerProxy doing the hook for updates and the actual update. The object instance we be kept alive because of the property changed delegate.

Download sample project here.

UpdateSourceTrigger PropertyChanged for Silverlight 4 Binding ?

Problem Definition

In Silverlight 4 there is no way out of the box to specify that a certain binding should update the source on PropertyChanged, the options for the UpdateSourceTrigger are LostFocus and Explicit.

These options are more or less what you really need for most scenarios, yet sometimes you want to have the ability to update the binding while a property is changing. A use case would be, you have a textbox and you want to apply some validations while the user is typing, maybe you enable a button if the text entered is correct or disable it if it is not correct.

Many times developers end up writing code behind in order to achieve this. Problem with this is that if you need it in multiple places you end up copying and pasting code for code behind. In this article I will show how one can workaround this problem by resorting to the mighty Attached Properties.

Using an Attached Property to workaround this problem

By using an attached property we can have the code to update the source on property changed. For this demo I will focus on how to make this work for a TextBox in Silverlight. In future posts I will show how you can have a generic way of dealing with this issue.

The attached property will give us the instance of the object the attached property is being applied to, with the reference of the object (in our case the TextBox) at hand we can hook to the TextChanged and update the binding. In order to update the binding you can use the GetBindingExpression method of the UI element. Then we can simple call the UpdateSource on the BindingExpression and viola the trick is done…

The Attached Property would look like this

using System.Windows;

using System.Windows.Controls;


namespace SilverlightUpdateSourceTrigger


    public class UpdateSourceTrigger


        #region TextChangeUpdateSourceTrigger


        /// <summary>

        /// TextChangeUpdateSourceTrigger Attached Dependency Property

        /// </summary>

        public static readonly DependencyProperty TextChangeUpdateSourceTriggerProperty =

            DependencyProperty.RegisterAttached("TextChangeUpdateSourceTrigger", typeof(bool), typeof(UpdateSourceTrigger),

                new PropertyMetadata((bool)false,

                    new PropertyChangedCallback(OnTextChangeUpdateSourceTriggerChanged)));


        /// <summary>

        /// Gets the TextChangeUpdateSourceTrigger property. This dependency property 

        /// indicates ....

        /// </summary>

        public static bool GetTextChangeUpdateSourceTrigger(DependencyObject d)


            return (bool)d.GetValue(TextChangeUpdateSourceTriggerProperty);



        /// <summary>

        /// Sets the TextChangeUpdateSourceTrigger property. This dependency property 

        /// indicates ....

        /// </summary>

        public static void SetTextChangeUpdateSourceTrigger(DependencyObject d, bool value)


            d.SetValue(TextChangeUpdateSourceTriggerProperty, value);



        /// <summary>

        /// Handles changes to the TextChangeUpdateSourceTrigger property.

        /// </summary>

        private static void OnTextChangeUpdateSourceTriggerChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)


            TextBox textBox = d as TextBox;

            if (textBox == null)



            bool newTextChangeUpdateSourceTrigger = (bool)d.GetValue(TextChangeUpdateSourceTriggerProperty);


            if (newTextChangeUpdateSourceTrigger)

                textBox.TextChanged += OnTextChanged;


                textBox.TextChanged -= OnTextChanged;



        static void OnTextChanged(object sender, TextChangedEventArgs e)


            TextBox textBox = sender as TextBox;

            var binding = textBox.GetBindingExpression(TextBox.TextProperty);







Enjoy and Happy XMAS Smile

Adding more goodies in MEFedMVVM

Thanks to some good feedback from the community and also some really awesome help from Glenn Block and Sasha Barber, I added some new stuff to MEFedMVVM.

The core 3 additions are

- Reloading of design time data in Blend as you compile in Visual Studio (Patch by Chris Szabo)

      Before this patch you had to close and re open Blend everytime you did a change in your ViewModel for Blend to pick up this change. Chris Szabo sent me a code snippet showing me how I could overcome this issue. Very cool Chris!!!!!

- Added a new method in IComposer so that you can specify a list of custom ExportProviders.

      A user of Cinch suggested this feature since he had some ExportProvider he wanted to use in MEFedMVVM for versioning of objects. Now the IComposer has a new method that looks like this


You can return a list of ExportProviders or if you do not want to just return null. MEFedMVVM will attach its own ExportProvider even if you return null. (MEFedMVVM uses the ExportProvider for IContextAware services such as IVisualStateManager so that it can inject the View that requested the ViewModel)


- Added 2 new attached properties to make things more clear.

By default Exports in MEF are treated as Shared. This implies that if before you did an [ExportViewModel(“MyVM”] without specifying a [PartCreationPolicy(CreationPolicy.NonShared)] the ViewModel would be exported as a shared item so all imports get the same instance of the ViewModel. Also this implies that the ViewModel would never be garbage collected since MEF will keep its instance alive. The workaround for this would be to specify [PartCreationPolicy(CreationPolicy.NonShared)] where you put the ExportViewModel attribute. Once you know this its all good but if you are new to MEF maybe it is not that obvious that exports are by default Shared. So in order to make this crystal clear I added 2 new attached properties SharedViewModel and NonSharedViewModel, which will impose the CreationPolicy on the ViewModel you want to export (this is like doing the CreationPolicy on the Import if you were doing standard MEF). Glenn Block came up with this idea, kudos to Glenn!!!!


Besides these changes I also did some bug fixing here and there and some minor changes as per request …

- Export for ViewModel had an incorrect ImportCardinality. now the ImportCardinality is set to ExactlyOne.
- DataContextAware ViewModels used to call the DesignTimeInitialization 2 times, this is now fixed.
- Exposed the MEFedMVVM CompositionContainer, you can now access the CompositionContainer directly by doing this ViewModelRepoitory.Instance.Resolver.Container. This is useful if you want to get some Exported object from the CompositionContainer that MEFedMVVM is using.
- Fixed issue with ImportMany. This was a problem in the ExportProvider of MEFedMVVM.
- Added propertyObserver. you can see more about this here
- Fixed issue with DelegateCommand (was not hocking automatically to the CanExecute of the Command Manager in WPF)

Hope you enjoy MEFedMVVM and as always keep feedback coming !

INotifyPropertyChanged… I am fed up of handling events just to know when a property changed

The INotifyPropertyChanged is what tells the WPF/SL binding that a property changed and that the binding should be updated. For this purpose its quite good. But what many times we end up doing is; handling the PropertyChanged in code so that we get notified that a property changed. An example is a ViewModel that need to know when a property in the underlying model changed and if it did it will do something to facilitate the View.

Doing this in code is a bit ugly because you base everything on “magic strings”.. something like this


Another problem is that if you use an anonymous delegate you need to do loads of tricks in order to unregister to the event.

In order to over come this I created a PropertyObserver that handles these 2 issues. (Please note that there are already others, Philip Sumi has a good one here and  Josh Smith has a good one here. I created my own since I wanted to have extra + different way of doing it). Also this problem can be solved easily using Rx, I am doing this sample so that you do not need to depend on Rx.

The implementation I did is very simple, I created a PropertyObserver class that uses lambdas so that you tell it what property you want to subscribe to and pass an action to do your “Property Changed handling”. What I also did is an attached method so that all objects that implement INotifyPropertyChanged can simple handle the property changed… so the code looks like this


One would ask and how do I unregister? is this using weak event pattern? The answer is NO. I wanted to keep it simple and cross platform (for WPF and SL). what you can do is a more Rx approach… basically in RX if you subscibe to something you get an IDisposable in return so that you can Dispose (i.e in our case unsubscribe) when you want like this


as you can see here I am wrapping the call in a using statement… but if you need to unsubscribe in any specific state you can store the IDisposable that you get from the Do call and call Dispose to unsubscribe.

Many times you need this for only one time i.e a property changes you do something and you do not want to get called any more times. For this I created a DoOnce. This will unsubscribe for you once the property changes once. The API is very similar and looks like this


That’s more or less it.

Big thanks go to Diego Colombo and John Rayner (two champs that work with me) for the help and input they gave me on this :) You rock guys!

You can download the code here.

OR if you have MEFedMVVM download the latest version since its in MEFedMVVM.

MEF DeploymentCatalog and Reference assemblies

In the Silverlight version of MEF one of the most powerful catalogs is the Deployment catalog. The Deployment catalog enables you to do some really cool stuff because you can download parts of the application when you need them.

I am using the Deployment Catalog on one of my projects and today I hit an issue that made me bang my head for over 30 minutes! The error was clear but it was a mystery why it was happening… 

The Error

I could see the error I was getting inside the event arguments of the DownloadCompleted event (e.Error) of the Deployment catalog. It was saying I had an import that could not be resolved since I had more then 1 export matching that requested import >> “More than one export was found that matches the constraint ”

The Import I had looked like this

public ILogService Logger { get; set; }

But I had a single export like this

public class Loger : ILogService

The Logger class was not in the assembly that I was trying to download it was in an assembly called CommonServices. I had a reference to the CommonServices assembly.

The main application that was using the Deployment Catalog to download the other assembly also had a reference to the CommonServices. Besides that it was also creating an Assembly Catalog for the CommonServices and put it in the aggregate Assembly that had the Deployment Catalog. I was doing this since the Main application was also using the ILogService as an import.

Why the Error?

After reading this post I immediately knew why the error was happening and how to solve it. Deployment Catalog will load all assemblies inside the XAP file thus all the assemblies that it references (The referenced assembly will be in the XAP file if you set Copy Local=true and by default VS does that for you). So in my case the Deployment Catalog was creating an Assembly Catalog for the CommonServices but I had already created that in my main application since I was importing ILogService in my main application.

The Solution

image The solution is easy set Copy Local to false for the referenced assemblies you do not want DeploymentCatalog to load at runtime. Your imports inside the assembly will still be staisfied since the Deployment Catalog is in the same Aggregate Catalog where you loaded the assembly as an AssemblyCatalog.

Hope this may help anyone having the same issue.

Enjoy your daily MEFing :)