MEF for Windows 8 Windows Store apps

In .NET 4.5 Beta, Windows 8 Windows Store apps could use a subset of MEF functionality from the full .NET Framework. To better align with the goals and scenarios of Windows Store apps, in .NET 4.5 RC MEF for Windows Store apps is consumed by installing the Microsoft.Composition package.

The MEF implementation in Microsoft.Composition is not code-compatible with the MEF functionality in .NET 4.5 Beta.

For most applications, a few simple steps will be required to move to the new MEF version. These are outlined step-by-step below. A full list of changes follows.

Project-level changes

Referencing the new assemblies

The new assemblies are referenced using the Visual Studio Package Manager dialog.
  1. Right-click on your Windows Store application project
  2. Select “Manage NuGet Packages”
  3. Select “Include Prerelease Packages”
  4. Search for Microsoft.Composition
  5. Click “Install”

Alternatively, the Package Manager Console can be used:

Install-Package Microsoft.Composition –Pre

Delivering MEF as a NuGet package enables a more responsive development process.

Namespace change

Use Visuals Studio’s Find and Replace in Files dialog to replace the text:




Rationale: MEF for Windows Store apps is optimized for this model and therefore makes changes to some fundamental MEF types like [Import] and [Export]. Changing the namespace ensures that types can be uniquely identified and correct information can be found more easily.

Part-level changes

Import/Export visibility

Imported and exported members must be publicly visible in order for them to be composed. For example:

    private ILogger Logger { get; set; }

Must be updated to:

    public ILogger Logger { get; set; }

The same is true of importing constructors and property exports.

Rationale: Requiring that imports and exports are public ensures that parts behave consistently on all platforms, and that code generation techniques can be used to improve throughput.

Unsupported features

Field exports and imports are not supported – use property imports and exports instead.

Rationale: Due to the requirement for public imports and exports, fields are less useful for this purpose. Ignoring them improves startup time for the composition engine.

Part creation policy

[PartCreationPolicy(CreationPolicy.NonShared)] is no longer required, as parts are non-shared by default.

Parts that need to be shared are marked with the [Shared] attribute.

Rationale: The default creation policy was previously CreationPolicy.Any, making it confusing to determine the scope in which a part could be used. Sharing and lifetime have been streamlined so that the sharing scope of a part is always explicitly visible – a part marked [Shared] will always be globally shared; a part marked [Shared("vm")] will always be shared within the named boundary “vm”, parts without an annotation are always non-shared. See also: [SharingBoundary] and ExportFactory<T>.


This interface has been replaced with the OnImportsSatisfied attribute.

For example, the following part:

    public class APart : IPartImportsSatisfiedNotification
        public void OnImportsSatisfied() { }

Can be rewritten as:

    public class APart 
        public void OnImportsSatisfied() { }

Rationale: MEF’s convention support is based on attributes. By marking the OnImportsSatisfied() method with an attribute, conventions can select this method. This was not possible with the interface-based approach.

Format of Metadata Views

Metadata views must be concrete types rather than interfaces. For example, the metadata view:

    public interface INamed
        string Name { get; set; }

Can be rewritten as:

    public class Named
        public string Name { get; set; }

Rationale: This requirement arises from the absence of the System.Reflection.Emit namespace in .NET for Windows Store apps. Without the functionality in this namespace, metadata views based on interfaces cannot be generated.

Hosting changes

Container configuration

On the hosting side, a smaller API with fewer configuration points is presented. Assemblies and part types are added to a ContainerConfiguration (rather than a catalog) and from there a container can be created and exports requested.

    var configuration = new ContainerConfiguration()

    using (var container = configuration.CreateContainer())
        var handler = container.GetExport<IMessageHandler>();

The ContainerConfiguration class provides a method-chaining API to add types and assemblies to the container. There is no catalog concept, nor composition batches, nor any “container hierarchy” or composition scoping APIs.

Rationale: The hosting APIs used in MEF in the full .NET Framework are designed to support open, extensible applications. This functionality is not required in Windows Store apps, and so the developer experience is simplified.

Conventions and RegistrationBuilder

In .NET 4.5 MEF introduces the RegistrationBuilder class. On Windows Store apps the same functionality is available via ConventionBuilder.

Rationale: ConventionBuilder is expected to evolve significantly in conjunction with the MEF implementation for Windows Store apps. Retaining the RegistrationBuilder name would make these differences confusing.

MEF in Windows Store apps vs. .NET Framework

The table below summarizes high-level feature compatibility across MEF for Windows Store apps and the full framework.

Functionality Windows Store apps Full
Catalogs No Yes
Recomposition No Yes
Rejection No Yes
Instance Exports Yes Yes
Static Exports No Yes
Property Exports Yes Yes
Field Exports No Yes
Importing Constructors Yes Yes
Property Imports Yes Yes
Field Imports No Yes
Method Exports No Yes
Internal Part Types Yes Yes
Non Public Imports/Exports No Yes
Export Metadata Yes Yes
Import Metadata Views Yes Yes
Part Metadata Yes Yes
Nested Containers No Yes
Composition Scope Definition No Yes
Lazy Imports Yes Yes
ExportFactory Imports Yes Yes
Import Many Yes Yes
Import Custom Collections No Yes
Contract Names Yes Yes
Import Sources No Yes
Cyclic Dependencies Yes Yes
Open Generics Yes Yes
IPartImportsSatisfiedNotification No Yes
Import Required Creation Policy No Yes
Interface Metadata Views No Yes
Concrete Metadata View s Yes Yes
Missing Metadata Ignored No Yes
Inherited Exports No Yes
PartCreationPolicyAttribute No Yes
CatalogReflectionContextAttribute No Yes
MetadataAttributeAttribute Yes Yes
PartNotDiscoverableAttribute Yes Yes
ICompositionService No Yes
CompositionOptions No Yes

Last edited Oct 18, 2012 at 6:33 PM by alokshriram, version 3


VcDeveloper Sep 2 at 5:15 PM 
Hi, I'm really having a hard time finding a Windows Store App using these features coupled with Prism. Can you link me to a good example please? And a plus would be using ConventionBuilder.

I would appreciate it very much. Thanks!...

nblumhardt Jun 8, 2012 at 10:26 PM 
Hi @luzkey - yes, CompositionContext.SatisfyImports() has the required functionality; it is an extension method, and is inherited by CompositionHost.

lukezy Jun 2, 2012 at 9:15 PM 
Is there something equivalent to SatisfyImportsOnce?