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.

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).

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.

Our PDC talk has been posted on Channel9. http://channel9.msdn.com/pdc2008/PC58/.

Here is the talk summary:

Learn about guidelines that have helped the Microsoft .NET Framework grow into the most popular developer framework Microsoft has ever created. After ten years of use, we have an enormous amount of real customer data about what makes great framework design. Whether you are building your own framework or just want to get the most out of the .NET Framework, this is a must-attend talk! Join Krzysztof Cwalina and Brad Abrams, authors of the Dr. Dobbs award winning "Framework Design Guidelines" book, and get a sneak peek at the content from the 2nd edition (first available at PDC2008).

Brad and I just did a couple of video interviews that are now accessible online.

In the first one, we are talking about our PDC presentation (for those at the PDC, it’s at 4pm today). You can get it at 10 Years of Framework Design Guidelines (video).

The second interview is about just released 2^nd edition of the Framework Design Guidelines book. You can get the video at Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries (video)

Enjoy!

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.

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. 

I was updating FDG section on exceptions. I added one anntation that I thought I would post here as well:

These guidelines were just added as part of an update to the Framework Design Guidelines book (upcomming 2nd edition). Hope you find them useful.

Nullable<T> is a simple type added to the .NET Framework 2.0. The type was designed to be able to represent Value Types with “null” values.

Nullable<int> x = null;

Nullable<int> y = 5;

Console.WriteLine(x == null); // prints true

Console.WriteLine(y == null); // prints false

Note that C# provides special support for Nullable<T> in the form of language aliases for Nullable types, lifted operators, and the new coalescing operator.

int? x = null; //

long? d = x; // calls cast operator from Int32 to Int64

Console.WriteLine(d??10); // coalescing; prints 10 because d == null

þ CONSIDER using Nullable<T> to represent values that might not be present (i.e. optional values). For example, use it when returning a strongly typed record from a database with a property representing an optional table column. 

ý Do NOT use Nullable<T> unless you would use a reference type in a similar manner, taking advantage of the fact that reference type values can be null.

For example, you would not use null to represent optional parameters.

// bad design

public class Foo {

   public Foo(string name, int? id);

}

// good design

public class Foo {

   public Foo(string name, int id);

   public Foo(string name);

}

ý AVOID using Nullable<bool> to represent a general three-state value.

Nullable<bool> should only be used to represent truly optional Boolean values: true, false, and not available. If you simply want to represent three states (e.g. yes, no, cancel), consider using an enum.

ý AVOID using System.DBNull. Prefer Nullable<T> instead.

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).

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.

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! :-)

Almost 4 years ago, I blogged about Framework Design Guidelines Digest. At that time, my blog engine did not support attaching files and I did not have a convenient online storage to put the document on, and so I asked people to email me if they want an offline copy.

Believe it or not, I still receive 1-2 emails a week with requests for the offline copy. Now that I have a convenient way to put the document online, and the fact that I wanted to make some small updates, I would like to repost the digest. The abstract is below and the full document can be downloaded here.

This document is a distillation and a simplification of the most basic guidelines described in detail in a book titled Framework Design Guidelines by Krzysztof Cwalina and Brad Abrams. Framework Design Guidelines were created in the early days of .NET Framework development. They started as a small set of naming and design conventions but have been enhanced, scrutinized, and refined to a point where they are generally considered the canonical way to design frameworks at Microsoft. They carry the experience and cumulative wisdom of thousands of developer hours over several versions of the .NET Framework.

When I was coming back from Mix 2007, I was bored on the plane and so started to write a dev tool. What a geeky thing to do on a plane. :-)

The tool allows comparing two versions of an assembly to identify API differences: API additions and removals. Comparing versions of APIs comes very handy during API design process. Often you want to ensure that things did not get removed accidentally (which can cause incompatibilities), and as APIs grow, you want to review the addition without having to re-review APIs that were already reviewed. The tool, called Framework Design Studio (FDS) supports these scenarios.

Later on, I got lots of help from Hongping Lim (a developer on our team), and David Fowler (our 2007 summer intern). David ported the application to WPF, and Hongping basically took it from an early prototype stage to what it is today and made it possible to ship it externally.

Anyway, you can get the tool at the MSDN Code Gallery, the user guide is attached to this post, and lastly, here is the API diff output that the tool generates. Hope you find it useful.

Attached Media: application/vnd.openxmlformats-officedocument.word ( 393 ko)

I just wrote this pattern, but I am not sure if I should add it officially to the Framework Design Guidelines. It seems like a bit of a corner case scenario, though I do get questions about it from time to time. Anyway, let me know what you think. 

Different constructed types don’t have a common root type. For example, there would not be a common representation of IEnumerable<string> and IEnumerable<object> if not for a pattern implemented by IEnumerable<T> called Simulated Covariance. This post describes the details of the pattern.

Generics is a very powerful type system feature added to the .NET Framework 2.0. It allows creation of so called parameterized types. For example, List<T> is such a type and it represents a list of objects of type T. The T is specified at the time when the instance of the list is created.

List<string> names = new List<string>();

names.Add(“John Smith”);

names.Add(“Mary Johnson”);

Such Generic data structures have many benefits over their non-Generic counterparts. But they also have some, sometimes surprising, limitations. For example, some users expect that a List<string> can be cast to List<object>, just as a String can be cast to Object. But unfortunately, the following code won’t even compile.

List<string> names = new List<string>();

List<object> objects = names; // this won’t compile

There is a very good reason for this limitation, and that is to allow for full strong typing. For example, if you could cast List<string> to a List<object> the following incorrect code would compile, but the program would fail at runtime.

static void Main(){

List<string> names = new List<string>();

// this of course does not compile, but if it did

// the whole program would compile, but would be incorrect as it

// attempts to add arbitrary objects to a list of strings.

AddObjects((List<object>)names);

string name = names[0]; // how could this work?

}

// this would (and does) compile just fine.

static void AddObjects(List<object> list){

   list.Add(new object()); // it’s a list of strings, really. Should we throw?

   list.Add(new Button());

}

Unfortunately this limitation can also be undesired in some scenarios. For example, there is nothing wrong with casting a List<string> to IEnumerable<object>, like in the following example.

List<string> names = new List<string>();

IEnumerable<object> objects = names; // this won’t compile

foreach(object obj in objects){

   Console.WriteLine(obj.ToString());

}

In general, having a way to represent “any list” (or in general “any instance of this generic type”) is very useful.

// what type should ??? be?

static void PrintItems(??? anyList){

   foreach(object obj in anyList){

          Console.WriteLine(obj.ToString());

   }

}

Unfortunately, unless List<T> implemented a pattern that will be described in a moment, the only common representation of all List<T> instances would be System.Object. But System.Object is too limiting and would not allow PrintItems method to enumerate items in the list.

The reason that casting to IEnumerable<object> is just fine, but casting to List<object> can cause all sorts of problems is that in case of IEnumerable<object>, the object appears only in the output position (the return type of GetEnumerator is IEnumerator<object>). In case of List<object>, the object represents both output and input types. For example, object is the type of the input to the Add method.

// T does not appear as input to any members or dependencies of this interface

public interface IEnumerable<T> {

   IEnumerator<T> GetEnumerator();         

}

public interface IEnumerator<T> {

   T Current { get; }

   bool MoveNext();

}

// T does appear as input to members of List<T>

public class List<T> {

   public void Add(T item); // T is an input here

   public T this[int index]{

       get;

       set; // T is actually an input here

}

}

In other words, we say that in IEnumerable<T>, the T is at covariant positions (outputs). In List<T>, the T is at covariant and contravariant (inputs) positions.

To solve the problem of not having a common type representing the root of all constructions of a generic type, you can implement what’s called the Simulated Covariance Pattern.

Given a generic type (class or interface) and its dependencies

public class Foo<T> {

   public T Property1 { get; }

   public T Property2 { set; }

   public T Property3 { get; set; }

   public void Method1(T arg1);

public T Method2();

   public T Method3(T arg);

   public Type1<T> GetMethod1();

public Type2<T> GetMethod2();

}

public class Type1<T> {

   public T Property { get; }

}

public class Type2<T> {

   public T Property { get; set; }

}

Create a new interface (Root Type) with all members containing Ts at contravariant positions removed. In addition, feel free to remove all members that might not make sense in the context of the trimmed down type.

public interface IFoo<T> {

    T Property1 { get; }

    T Property3 { get; } // setter removed

    T Method2();

    Type1<T> GetMethod1();

    IType2<T> GetMethod2(); // note that the return type changed

}

public interface IType2<T> {

    T Property { get; } // setter removed

}

The generic type should then implement the interface explicitly and “add back” the strongly typed members (using T instead of object) to its public API surface.

public class Foo<T> : IFoo<object> {

    public T Property1 { get; }

    public T Property2 { set; }

    public T Property3 { get; set;}

    public void Method1(T arg1);

    public T Method2();

    public T Method3(T arg);

    public Type1<T> GetMethod1();

    public Type2<T> GetMethod2();

    object IFoo<object>.Property1 { get; }

    object IFoo<object>.Property3 { get; }

    object IFoo<object>.Method2() { return null; }

    Type1<object> IFoo<object>.GetMethod1();

    IType2<object> IFoo<object>.GetMethod2();

}

public class Type2<T> : IType2<object> {

    public T Property { get; set; }

    object IType2<object>.Property { get; }

}

Now, all constructed instantiation of Foo<T> have a common root type IFoo<object>.

var foos = new List<IFoo<object>>();

foos.Add(new Foo<int>());

foos.Add(new Foo<string>());

…

foreach(IFoo<object> foo in foos){

   Console.WriteLine(foo.Property1);

   Console.WriteLine(foo.GetMethod2().Property);

}

þ CONSIDER using the Simulated Covariance Pattern, if there is a need to have a representation for all instantiations of a generic type.

The pattern should not be used frivolously as it results in additional types in the library and can makes the existing types more complex.

þ DO ensure that the implementation of the root’s members is equivalent to the implementation of the corresponding generic type members.

There should not be an observable difference between calling a member on the root type and calling the corresponding member on the generic type. In many cases the members of the root are implemented by calling members on the generic type.  

public class Foo<T> : IFoo<object> {

   public T Property3 { get { ... } set { ... } }

   object IFoo<object>.Property3 { get { return Property3; } }

…

}

þ CONSIDER using an abstract class instead of an interface to represent the root.

This might sometimes be a better option as interfaces are more difficult to evolve (see section X). On the other hand there are some problem with using abstract classes for the root. Abstract class members cannot be implemented explicitly and the subtypes need to use the new modifier. This makes it tricky to implement the root’s members by delegating to the generic type members.

þ CONSIDER using a non-generic root type, if such type is already available.

For example, List<T> implements IEnumerable for the purpose of simulating covariance.

We have been incubating ideas about building a simple extensibility framework for some time. Now, as plans for the next version of the .NET Framework crystallize a bit more, we decided to productize the project. As a result, we have opened a job position (and most probably will be opening more) on the .NET Framework team. If you are interested, please see details here and send me an email at “kcwalina at microsoft.com.”

So, what is this extensibility framework? Initially, it will be a low level core .NET Framework feature to make it easy for applications to expose extensibility points and consume extensions. Think about what for example FxCop has to do define rule contracts and load rule implemented by the community. These are the basics, and we can talk about the broader and longer term vision when you come to Redmond for an interview :-)

This is a technical Program Manager position in Redmond, WA, and it’s basically exactly the job I did when I joined Microsoft. Besides working on the Framework features, all Program Managers on the core team have opportunities to work on API design and architecture projects.

Mircea, a program manager on my team, has worked on development of design guidelines for LINQ related features. The guidelines were reviewed internally and are now available on Mitch’s blog. We might still iterate on them a bit, but quite soon I plan to incorporate them into Framework Design Guidelines manuscript, so if you have feedback, it’s time to send it our way! :-)

Thanks!

My blog was relatively silent for several weeks. First, I was traveling to Europe for the TechEd, then was busy at work, then the holiday break. It's time to go back to more regular posting.

I will start with an announcement (or at least a more formal and broader announcement): after my TechEd presentation, the first question I got from the audience was whether we are working on a new edition of the Framework Design Guidelines. The answer is "yes", which I am super excited about. Right before the conference, we signed formal documents with the publisher and started wortking on the book. It's going to cover the new features in the .NET Framework 3.0, 3.5, and new advances in languages (e.g. LINQ) that are relevant to Framework design. BTW, I would appreciate feedback on what specifically you would like to get covered or clarified.

We are shooting to have the book ready around the end of 2008, but the publisher already sent me a draft of the cover art: