Managed Extensibility Framework

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Several months ago we formed what we call Application Framework Core team. 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? 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. 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 will blog more details about MEF in the upcoming months, but here are some early details subject to changes, of course: These technologies and other like System. AddIn together are intended to enable the world of what we call Open and Dynamic Applications , i. The work we are doing builds on several existing Microsoft technologies like the Unity framework and with feedback from the DI community. 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 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: WriteLine 'Hello World' ;. Magic that makes composes DIs the application with the extensions. AddComponent new CustomOutput ;. Expecting lots of questions, I will preemptively answer J: 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? And lastly, we are hiring! To answer one of your specific questions, I think constructor injection is very much required, and I explain why here: Also please provide good extensibility points to the DI container itself and make it easy to unit test. If you want to do it right, take a look at how the ASP. Listen to the community, look at other projects in this domain, ship early and often, make it easy to test, extend or even replace. The current approach to making extensions unloadable by using separate AppDomains results in too much of a performance-hit if there is a lot to serialize across the AppDomain boundary. I think that constructor injection is absolutely necessary I use Castle at the moment. If it is not available, I probably would not use MEF. It makes class much less maintainable, since there is not static way to check it got all dependencies. From your prospective, what makes dependency injection better than some of the existing patterns such as ProviderBase? For example, one could argue that the following example is comparable in flexibility but easier to understand and debug. Although more verbose, with a simple code generation tool perhaps based on T4 , this example could be as easy to implement as the example with attribute-based dependency injection. What do you think? Personally this is not the kind of dependency injection I am looking for. I am looking for a way to make HelloWorld and CustomOutput work together without any explicit code to bind them. The semantics of the 'extension' then deal with the fact that we have 0, 1 or more classes that apply for this case. The most generic way to supply an object-creation function is through a delegate. I think that Constructor Injection is absolutely required because it allows you to strictly define the object dependencies and abstracts you from a particular DI system. Use of attributes should be a last resort. Total transparancy is number one for me. I should be able to use existing code with a DI framework, no recompile or change needed. What also needs to be totally transparent is the errors you inevitably make in configuring the DI framework. Nothing worse than going through attributes or xml config to finally find a typo somewhere because the stacktrace gives you too little info. You need proper exception handling with human readable description of the reason why it doesnt work. Configuration should be possible through XML and programmatic interface. Configuration should obviously be extensible and replaceable with other implementations. As for programmatic interface I would like two flavours, standard. Even better would be if you could configure and connect the objects you want through some kind of class browser and select your output for configuration. Other than that I would like lifecycle management, hooks for creation and cleanup, a facility like extension model like Windsor Castle has, custom XML schema extensions like spring and. This seems very cool, but I have concerns regarding usability. How is it intended to be used in larger projects? Not saying you should ban CI — not at all! I just wanted to voice the opposite opinion in this torrent of praise for this one type of injection. I think standardizing on the optional attributes is a good thing. This way other frameworks can use these same attributes. Not sure about the. Bind idea — binding consumers and providers of services is often much more complicated than the 1: Also, DI is all about the configuration. What about setting properties using reflection? What about a standard container? The inheritance features for properties are just one very powerful feature that could be exploited for DI. In other word, we are looking into being complementary to System. Thanks for all the great feedback. It will really help us weigh the pros and cons of the two main design issues: We have had many discussion about these two internally, and there are interesting tradeoff we would need to make to add these two features. I will try to erite about the tradeoffs or ask somebody on the team to do it to start a focused discussion about these two. We want to use it for extensibility of the Framework itself and Visual Studio. We cannot use Unity or Spring for this. My biggest issue with this is I expect Console. WriteLine to write to the Console, not some other stream unless the console is being redirected to a file, which is standard behaviour. OutputDevice only implies the input will go to some device. Could be the console, a printer, a network stream, etc…. Primarily from a discoverability point of view — I want to be able to use a generic tool to examine an assembly and identify the dependencies. Without the attribute and being purely configuration based how do I know this? I now have to look elsewhere, making experimentation difficult. Perhaps you should think of Spring. Net as going to far in terms of complexity and performance hit — and trying for a target well inside that extreme. Also, why the 'bind'? Unless you are trying for a lazy injection or have order of injection issues, it would be great to avoid the 'bind' entirely. External configuration is great, but I have a couple of problems with it being the only source of information about the dependencies. This makes simple exploration more difficult. Configuration is separate from the assembly which has the dependencies, therefore without it, I have no way to determine what those dependencies are. Certainly configuration can be used in conjunction with this metadata to define who is providing an implementation for the dependencies. I feel another issue with external configuration is reuse. The problems arise when I want to write as an example a console application that reuses some of your framework. With some simple attributes on the classes to declare that this is an extension point, you now provide some discoverability. With \\\\\\\\\\\\\[Import\\\\\\\\\\\\\], I have some guidance. Another side effect is much better tool support. As an example, intellisense would be able to show a visual cue that Output depends on OutputDevice. Furthermore, code-analysis could show any classes I consume that have external dependencies. Reflector could also benefit from the attribute metadata. I think that Import should have one optional parameter, Required bool , so we can declare that if the service is not available when binding, building or resolving, to throw a some sort of DependencyNotFoundException. I will have to agree with the other commenters who want constructor injection. Initially Attributes were appealing and in many cases I still use them but for DI related functionality I much perfer constructor injection. Please do not bake in a dependency on attributes for this scenerio. The samples use attributes, but the same metadata can be theoretically provided externally. For example, form a configuration file. Constructor injection is harder. It would fundamentally change the architecture of MEF. This might take some more time to settle on. I will of course also describe the main tradeoffs here in the next couple of weeks. The single biggest factor for success on any project I have been apart of in the last 10 years at Microsoft. Explicit model of Dependency Injection is still preferable in most cases I guess … implicit model can have some problems:. Which implementation do we inject for which property? External configuration is acceptable although we then miss expressiveness of code. This is exactly what I did when I customized Prism for my needs. And by Provider I mean the one which inherits from ProviderBase, and is specified in a corresponding config section. Not the loose interpertation of the term as Prism currently implements. I expect all of these maps of dependencies and contributions to be fully dynamic right? Attributes are fun for the demo and simpler apps but are severely limiting for many scenarios. Users can be grouped into categories themselves grouped into organisational units or whatever tree structure you like. Il would be nice to be able to define a domain for the organisation unit, then inherit from the latter to define a domain for a category — with the possibility to override some of the options — and repeat the same pattern for particular users. Krzysztof recently announced on his blog that we have begun working on an extensibility framework for. There I work on the Managed. Today was a big day for my team because we released the bits for the first CTP of the Managed Extensibility. There is TechEd in Orlando, and this means, that there are a lot of new announcements. But we have some differences. First we develop a really compact DI container. The module also declares which inputs it needs, and we add a new concept that is the state of the input. Now each developer develops his module with the corresponding XML. The designer checks all the dependencies are correct, and then we compile this xml in a CF dll, for performance issues with codeDom. An then all dynamic injection is done in runtime between modules: We will see the progress of this project. Count with us for testing, and comments form Argentina! Several members of my team have already spilled the beans, but yes! NET 2, but quite frankly there are too many cooks in the kitchen right now and making a disaster of the beautify of. Core' as kludges to work around arbitrary 'red bits' rules. Image processing classes tied to the network layer? Threading issues abstracted away and hidden from developers? For example, TraceListener classes were simple and easy to use. A basic, obvious design pa ttern. If it was re-invented now, using this framework, it would be incredibly complex, require reflection, attributes, etc, etc — layers of unnecessary 'crud' over the design of the API. Clearly those names have no semantic meaning anywhere elese! Eric, I happy to engage in a discussion, as you might be pointing to good issues. The problem with the comment above is that it does not have enough concretes for me to respond, i. About three months ago, we were on the heels of shipping Prism Composite Application Guidance. Krzysztof, I have written a lot of classes that use constructor injection. I would ideally like a DI system that I can introduce with a minimum of code change. While eating my lunch I was running through google reader and started reading a post from Glen , this. We want reusable assemblies with dependencies to be 'reflective', i. We want to support classic application add-in scenarios possibly in addition to the callsic DI scenarios. In application add-in scenarios the add-ins need to be smart and know how to compose themselves i. In the classic Di scenarios, the developer does the composition. I have to say that my last year at Microsoft has been a little strange. Since the Acropolis project,. So MEF would need to dramatically evolve so that no longer requires all components to be instantiated and registered with the container up-front. There is much in common between this scenario and that of constructor dependency injection. Jeff, MEF does not require components to be registered upfront. We are very much on path to supporting 'late-binding' actually even more: NET 4 and will work on 3. AddComponent hello ; domain. SayIt ; Expecting lots of questions, I will preemptively answer J: April 25, at 6: April 25, at 7: April 26, at 1: April 26, at 3: April 26, at Van den Driessche Willy says: April 26, at 6: April 27, at April 27, at 3: April 27, at 7: April 28, at April 28, at 9: April 28, at 1: April 28, at 4: April 28, at 5: April 28, at 6: April 29, at 1: April 29, at 2: April 29, at 3: April 29, at 5: April 29, at 6: April 30, at 3: April 30, at 8: May 1, at May 1, at 7: May 2, at 6: May 4, at 4: The Real Glenn Block says: May 12, at 2: May 14, at May 14, at 7: May 19, at 1: June 1, at 6: June 2, at 1: June 4, at 3: June 4, at 8: June 5, at June 5, at 1: Just code - Tamir Khason says: June 5, at 4: Just Code - Tamir Khason says: June 5, at 2: June 5, at 5: June 7, at 3: June 10, at June 10, at 8: June 11, at 2: June 17, at July 16, at 1: July 16, at 2: July 17, at July 17, at 7: July 17, at 3: July 20, at 6: July 26, at August 30, at 1: September 23, at 3: September 23, at September 26, at 1: September 26, at 2: September 27, at 8: November 6, at 8: November 7, at November 20, at 3:

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