Krzysztof Cwalina : MEF

MEF Primitives Explained

kcwalina — Tue, 09 Jun 2009 19:48:00 GMT

Daniel just wrote a really nice post explaining the basics of MEF primitives. I recommend this to all interested in the internals or in extending MEF.

Custom Programming Models for MEF (Provider Model Contrib)

kcwalina — Wed, 04 Mar 2009 01:12:00 GMT

The MEF composition engine operates on (composes) abstractions called ComposableParts. By default, parts are implemented as simple .NET classes annotated with MEF attributes (ExportAttribute and ImportAttribute). But, we envision that some parts will be implemented through variety of different mechanisms. For example, parts can be .NET types annotated with external files, DLR objects, XAML files, etc. We call such alternative means of specifying parts “custom programming models.”

In v1 of MEF, we focused on getting the default programming model super easy to use (most MEF users need to only understand a handful of types) and getting the ComposablePart abstractions right (making it possible to create custom programming models). We did not focus on making it easy to create custom programming models (quite advanced scenario).

Andreas stepped in and filled in that hole. He created a MEF contrib project which is a set of helper libraries that make it quite easy to create custom programming models. For all those who like to play around with internals of technologies like MEF, I recommend looking at the library.

MEF Preview #4 Released

kcwalina — Tue, 27 Jan 2009 19:53:00 GMT

We have just released a new update to MEF. I am super excited about this release as it represents something quite close to what we are going to ship in terms of public APIs. In the last milestone, we have done quite significant API cleanup, renamed many core types to what I think will be their final names, and finally we have done some namespace factoring work. Now, the primitives, host APIs, and parts’ APIs are all in separate namespaces.

MEF on CodePlex

kcwalina — Sat, 06 Sep 2008 00:00:00 GMT

We have just released an update to MEF. You can get it at http://www.codeplex.com/MEF

The changes are quite significant:

1. The preview ships with sources under a very permissive license (Ms-LPL).

2. We now support constructor injection. Feature that the community asked for.

3. We completely redesigned MEF’s extensibility points. The extensibility points are designed to support writing custom providers of composition data. For example, out of the box MEF requires composable parts to be attributed with attributes that provide metadata describing the composition. We got lots of feedback that this is not acceptable in many scenarios. The new extensibility points make it easier to extend MEF to externalize the metadata (to an XML file for example). Note, that the changes are just the first step toward the goal of making the extensibility easy and powerful. We will most probably keep making improvements in this space in the future, so feedback on the new extensibility points would be more then welcome.

4. We significantly cleaned up the container APIs. But as above, there is more clean up to come in the future.

Extensible Framework Design Studio Released

kcwalina — Sat, 30 Aug 2008 20:19:00 GMT

This summer we had a high school intern, Nick Moloney, who worked on incorporating MEF into FDS. The fruits of his labor are now on code gallery. You can download the extensible FDS here. Congratulations to Nick!

The current release has just a few extensibility points, but you should expect more in the future.

POCO Support for MEF

kcwalina — Mon, 07 Jul 2008 21:37:00 GMT

Jason, our technical evangelist, just posted a sample showing how MEF can compose plain old CLR objects.

MEF and System.AddIns

kcwalina — Fri, 13 Jun 2008 19:04:00 GMT

Several people asked about the relationship between MEF and the technology in System.AddIn namespace. The answer is that these two are independent and complementary features. MEF is a primarily a composition engine. System.AddIn is an add-in activation and isolation technology. MEF’s engine will be able to compose objects that are simple CLR object, COM and DCOM components (more precisely managed proxies to these components), remoting proxies, and finally System.AddIn add-ins.

The following is roughly how we see the basic “architecture” (conceptual diagram) of these technologies. As you can see on the diagram, the composition engine works on abstract representations of components currently called ComponentBinders. The composition engine is not concerned with how the parts are actually implemented or activated (COM, simple new operator, or System.AddIns activation). Its only concern is to inspect the binders for dependencies they need (imports) and objects they can give (exports), and provide “matchmaking” services for the binders (blue arrows on the diagram).

MEF CTP 1 Released

kcwalina — Thu, 05 Jun 2008 23:53:00 GMT

Several members of my team have already spilled the beans, but yes (!) we just released our first public preview of MEF. You can grab the bits from here and read a past post for a high level overview.

I am super excited about the release. It’s a very early preview and still lots of work remains, but the CTP is a significant milestone for us. Designing a general purpose extensibility framework is hard, and so we felt very strong about releasing such an early preview to the world to get early feedback from the community. Hopefully together we can create a set of APIs that scale to the variety of extensibility scenarios: from developers using DI and IoC to large applications with extensibility points and plug-ins.

In my previous post about MEF, several people asked about constructor injection and non-attribute based programming model. Constructor injection is not supported in this preview, but we did start working on the design of the composition engine that would allow it. The non-attribute based programming model (where connections are specified externally) is actually enabled, but not built-in. I will try to post a sample showing how to do it, but for those that want to experiment themselves: you need to implement a custom component binder. The built-in binder inspects types and looks for the attributes. You custom binder might want to read the same information (which type to inject where) in some external file.

Managed Extensibility Framework

kcwalina — Fri, 25 Apr 2008 19:45:00 GMT

Several months ago we formed what we call Application Framework Core team. The charter of the team is to play the same role in the application frameworks space (WinForms, ASP.NET, WPF, Silverlight) as the Base Class Libraries (BCL) team plays at the bottom of the platform stack.

The BCL team did a good job fulfilling the role of the team responsible for decreasing duplication and providing common abstractions for the low levels of the platform. Unfortunately, we did not have a similar team really focused on these sets of issues higher up on the stack. This resulted in some unfortunate duplication (like several data binding models for each of the application models, different dependency property system for WPF and WF) and lack of common abstractions (what undo APIs should my generic application plugin call?) for application model code. The Application Framework Core team is now in place to start addressing the problems.

One of the first concrete projects that we are working on and are ready to slowly talk about is what we call the Managed Extensibility Framework (MEF). We observed that there are more and more places in the .NET Framework itself and increasingly managed applications (like Visual Studio) where we want to provide, or already provide, hooks for 3^rd party extensions. Think about TraceListener plugins for the TraceSource APIs, pluggable rules for Visual Studio Code Analysis (and the standalone FxCop), etc. In the absence of a built-in extensibility framework (like MEF), our developers who want to enable such extensions often are forced to create custom mechanisms, thus duplication. We hope that MEF will both stop such duplication and encourage/enable more extensibility in the Framework and applications built on top of it.

We will blog more details about MEF in the upcoming months, but here are some early details (subject to changes, of course): MEF is a set of features referred in the academic community and in the industry as a Naming and Activation Service (returns an object given a “name”), Dependency Injection (DI) framework, and a Structural Type System (duck typing). These technologies (and other like System.AddIn) together are intended to enable the world of what we call Open and Dynamic Applications, i.e. make it easier and cheaper to build extensible applications and extensions.

The work we are doing builds on several existing Microsoft technologies (like the Unity framework) and with feedback from the DI community. The relationship with the Unity team is the regular relationship between the P&P group and the .NET Framework group where we trickle successful technologies and ideas from the P&P team into the .NET Framework after they have passed the test of time. We have done this with some features in the diagnostics, exceptions, and UI space in the past. The direct engagement with the DI community is also starting. We gave a talk on the technology at last week’s MVP Summit, and talked with Jeremy Miller (the owner of Structure Map) and Ayende Rahien (Rhino Mocks) . We got lots of great feedback from Jeremy and Ayende and I think their experience in the DI space and their feedback will be invaluable as the project evolves. Thanks guys! We are of course also looking forward to engaging others in the DI community.

And finally here is some code showing basic scenarios our framework supports:

Creating an Extension Point in an Application:

public class HelloWorld {

[Import] // import declares what a component needs

public OutputDevice Output;

public void SayIt() {

Output.WriteLine("Hello World");

}

// Extension Contract

public abstract class OutputDevice {

void WriteLine(string output)

}

1. Creating an Extension

[Export(typeof(OutputDevice))] // export declared what a component gives

public class CustomOutput : OutputDevice {

public void WriteLine(string output) {

Console.WriteLine(output);

}

2. Magic that makes composes (DIs) the application with the extensions.

var domain = new ComponentDomain();

var hello = new HelloWorld();

// of course this can be implicit

domain.AddComponent(hello);

domain.AddComponent(new CustomOutput());

domain.Bind(); // bind matches the needs to gives

hello.SayIt();

Expecting lots of questions, I will preemptively answer (J): we don’t yet know whether or when we will ship this. We do have working code and we are looking into releasing a preview/CTP of the technology. For now we would be very interested in high level feedback. What do you think hinders extensibility in frameworks and application? Where would you like the Framework to be more extensible? What DI framework features you need, like, want, and use on daily basis? i.e. is constructor injection required?

And lastly, we are hiring! :-)