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

ICommand discovery with MEF

Sometimes you are in ViewModel X and you want to execute a command on ViewModel Y. You do not want to link the 2 because of some constrains that that might impose. How can you leverage MEFs capabilities to overcome such a situation?

Easy have the ViewModel Y expose the command as a property just like you would have it for binding from the View, but also add an Export attribute on the property and give it a name



Now from ViewModel X simple imports the ICommand by specifying that same name (yes you can have the string as a constant, also I would advice to use constants to avoid conflicts in strings)



MEF will automatically get the command from ViewModel Y into ViewModel X for you. This works very nicely with MEFedMVVM since MEFedMVVM resolves all ViewModels via MEF thus you do not need to do anything to resolve the ViewModel or anything. You simply decorate the properties for Export and Import and viola you can start drinking beer Smile

Happy coding Smile


Today I was looking at PRISM 4 and how it uses MEF as its DI Container and I thought, wouldn’t it be cool if you could use the 2 together?

Update: Please also check out this post to see how you can use the same composition container for both PRISM and MEFedMVVM so that stuff like IRegionManager, IEventAggregator etc can be injected also in MEFed ViewModels

Why would it be cool?

PRISM brings to the table

– Region Manager
– Modules infrastructure
– Many other utilities and services that you can consume

MEFedMVVM brings to the table

– ViewModel injection in XAML
– Design Time vs Runtime services (so that you can inject design time services when in blend)
– ContextAware services such as IVisualStateManager

Having the 2 working together would be awesome. The question is can they work together?

…teasing… suspense … ok enough Smile

The answer is yes and very easily…

How to do it

In PRISM you need to create a bootstrapper that will basically compose your application. MEFedMVVM also has a sort of Bootstrapper where you can specify how you want to compose the MEF composition. So as such all you need to do is to have your PRISM bootstrapper also tell MEFedMVVM how to do the composition.

Let’s start by creating a PRISM bootstrapper

We need a class that inherits from MefBootstrapper and we will need to override a couple of methods. Here is the code to do this

public class Bootstrapper : MefBootstrapper


    protected override void ConfigureAggregateCatalog()


        this.AggregateCatalog.Catalogs.Add(new AssemblyCatalog(typeof(Bootstrapper).Assembly));


    protected override void InitializeShell()



        Application.Current.MainWindow = (Shell)this.Shell;



    #region Overrides of Bootstrapper

    protected override DependencyObject CreateShell()


        return this.Container.GetExportedValue<Shell>();




Now lets enable MEFedMVVM

In order to do this we will simple need to implement the IComposer interface from MEFedMVVM and then return the AggregateCatalog property (that is given to us by PRISM)

#region Implementation of IComposer (For MEFedMVVM)

public ComposablePartCatalog InitializeContainer()


    //return the same catalog as the PRISM one

    return this.AggregateCatalog;


public IEnumerable<ExportProvider> GetCustomExportProviders()


    //In case you want some custom export providers

    return null;



In this case we will return null as the GetCustomExportProviders. This is a feature used if you have some custom ExportProvider you want MEFedMVVM to use.

The last step (which is the actual line of code to enable MEFedMVVM) is where we tell the MEFedMVVM LocatorBootstrapper to use this class as runtime composer.

protected override DependencyObject CreateShell()


    //init MEFedMVVM composed


    return this.Container.GetExportedValue<Shell>();


As you can see I have put that line of code in the CreateShell method so that the Composer is applied as early as possible so that all views can use MEFedMVVM.


Now you can start using MEFedMVVM as you would in a normal project… For example in the Shell you can say

<Window x:Class="MEFedMVVMAndPRISM.Shell"



        Title="Shell" Height="300" Width="300"



and the ShellViewModel would be


public class ShellViewModel


    public string Text { get; set; }

    public ShellViewModel()


        Text = "Hello from the ViewModel";



Of course here I am not really leveraging MEFedMVVM capabilities, yet the purpose of this post is not to show those capabilities but to show how you can use PRISM and MEFedMVVM together and take what is best from both. To read more on MEFedMVVM visit the codeplex site.

I create a small sample project to showcase both PRISM and MEFedMVVM working together.

Download sample


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 !

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 🙂