MEF: Where should I put the CompositionContainer?
I have been using the Windsor IoC Container for my web-based application, to resolve the data access layer implementation the application should use.
The web application's UI will consist of pages, and each page consists of small units called portlets. (Their concept is somewhat similar to widgets.) These so-called portlets are basically web controls and can be configured in runtime for every page invidually.
The application will ship with some of these built-in, but I would like to enable extending it easily.
I figured out that this mechanism is exactly what MEF is built for. So I decided to implement the system in such a way that it discovers portlets using MEF. Then, I realized that it can also do what I currently use Windsor for, so I decided to ditch Windsor in favor of MEF.
Obviously, I will have to use the DirectoryCatalog, which scans for the .dlls in the app's bin folder and returns everything I need.
I read some tutorials, examples, and all questions regarding MEF in StackOverflow, as well. I figured that the easiest way to use MEF is through the PartInitializer which Glenn Block mentioned in his tutorials, but I realized that it is not in MEF. Actually, it is in the code I downloaded from CodePlex, but in a separate assembly, and only in source, not in binary form. (Does this mean that it isn't a part of MEF? Or what's the point in putting it to a separate project?) Then, I realized that it is for Silverlight, so it doesn't really help me. (Or should I just compile that against .NET 3.5, or include it in my project, and I'm good to go?)
So now I have a problem which is the following: where should I put the CompositionContainer in my application?
There is another thing I would like to consider: should I use only one CompositionContainer in the lifetime of the app, or I'm better off creating a container for every time when I need it?
Answer
Good questions.
In general in terms of questions about where to put the container, I recommend the following posts: http://blogs.msdn.com/nblumhardt/archive/tags/Container+Managed+Application+Design/default.aspx
In of MEF on the web, web-based apps are a bit tricker because of the request / response nature and scalability concerns. For web you would likely want to have a hierarchy of containers, one root one for the application which is shared, as well as child contianers per-request. The child containers should live and die with the request in order to conserve resources. The shared container contains services that are shared by all callers.
You might check out these articles for more insight into how to do this:
http://blogs.msdn.com/hammett/archive/2009/04/23/mef-and-asp-net-mvc-sample.aspx http://blogs.msdn.com/hammett/archive/2009/07/15/mef-and-asp-net-mvc-sample-updated.aspx http://mef.codeplex.com/wikipage?title=Parts%20Lifetime&referringTitle=Guide
As far as PartInitializer, I would avoid using something like it unless you have to. ASP.NET provides sufficient hooks in the pipeline through HTTP Handlers, modules and such to let automatically compose on creation.
The only place i would see using PI on the web would be possibly within a custom user control. PI ships as part of Silverlight 4 and is not available in the box for .NET 4.0. I have created a usable version for .NET 4.0 which you can find here: http://cid-f8b2fd72406fb218.skydrive.live.com/self.aspx/blog/Composition.Initialization.Desktop.zip
HTH Glenn