托管可扩展性框架（MEF），为什么需要Imports？

Question

I wonder what the use of Import/ImportMany decorations are good for? I played with MEF and manage everything through the CompositionContainer. I decorated the exports with a custom MetaDataAttribute that derives from ExportAttribute. When I try to access plugin instances I can lazily access the meta data and the plugin implementation through Container.GetExports<T, IMetaDataAttribute>() .

So, why would I need to bother with Import decorations? I understand the core of MEF is the CompositionContainer and that is what I should really care about. But most examples on the web pass in Import decorated object instances. Why is that and what added value do they provide?

Here is an example of how I access my meta data and actual plugins:

    public static IEnumerable<IPluginAttributeView> GetMetaData<T>()
    {
        return Container.GetExports<T, IPluginAttributeView>().Select(e => e.Metadata);
    }

    public static T GetPlugin<T>(string pluginName) where T : class
    {
        var plugins = Container.GetExports<T, IPluginAttributeView>();
        var pluginByName = plugins.Where(e => e.Metadata.PluginName.Equals(pluginName)).FirstOrDefault();

        return pluginByName.Value;
    }

Am I missing something or lack understanding? Please help me understand.

Answer 1

The import attributes (and their equivalent RegistrationBuilder methods in MEF2) are very useful because they offer you automatic dependency injection. The container handles this for you. You can have many types decorated with export attributes and wiht members decorated with one of the import attributes and let the container compose them when you need them. So you just use the container to get the exports that no other type is importing. All the rest is handled for you by MEF. If you avoid the import attributes then you will have to inject all dependencies yourself.

If you look at most of the MEF examples out there you will note that most of them use the import attributes extensively. There are people that use MEF and don't even know that the GetExportXXX methods are provided by the CompositionContainer . It is up to you of course to decide what is best suited for your application(s). In your case (plug-in manager), the GetExports method might be enough. If you decide to add dependencies between plug-ins then the import attributes will become very valuable and your plug-in manager much simpler. Another nice feature of MEF for plug-in managers is recomposition, which as far as I know is available only via the import attributes (or MEF2's RegistrationBuilder ). This can add great value to your plug-in aware app. Either way you still have the extensibility feature that MEF is supposed to provide.