MEF, Import by custom condition

Question

Hi, everyone.

I have some extensible type:

 

public abstract class Extensible
{
 protected IEnumerable<IExtension> Extensions { ... }
}

 

where IExtension is the interface of extensions (a.k.a. plugin, addin, etc).

There's may descendants of Extensible. Each descendant type has its own set of extensions, therefore I need to filter all available implementations of IExtension by some criteria, to initialize collection of extensions correctly.

 

If I would do this manually, I would use such logic: make a custom attribute, e.g. ExtensionAttribute, and pass the type of the concrete Extensible in attribute's constructor:

 

public class ExtensionAttribute : Attribute
{
 public ExtensionAttribute(Type extensibleType)
 {
  ...
 }
}

public class MyExtensible : Extensible
{
 ...
}

[Extension(typeof(MyExtensible))]
public class MyExtension : IExtension
{
}

Then I would load assemblies, iterate through types, search IExtension implementations and so on.

But how can I do this with MEF?

Here's variants, that have struck:

1) Make ExtensionAttribute a descendant of MEF's ExportAttribute. But how and when I can apply filter?

2) Make a "marker" interface for extensions, intended for the concrete Extensible, and use its type in ExportAttribute:

 

public interface IMyExtension : IExtension
{
}

[Export(typeof(IMyExtension))]
public class MyExtension : IMyExtension
{
}

 

But I don't want to make many superfluous types.

I addition, I can't understand, how to apply custom import condition, which cannot be described by metadata: e. g., "do not load extensions, which type name starts with "A" on Wednesdays". Yes, I know that example is very artificial, but nevertheless...

 

Answer 1

One good solution may be:

1-Create a class to define the Metadata atributes that you need to discriminate the kind of plugin:

 [MetadataAttribute]
  [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false)]
  public class YourPlugInMetadata : ExportAttribute, IYourPlugInMetadata <br/>  {
    public string KindOfPlugIn{ get; set; }
    public YourPlugInMetadata (string kindOfPlugIn) 
    {<br/>    }<br/>
    ...
  }

2-Decorate the plugins with the concrete information:

[Export(typeof(IXXXProvider))]
  [YourPlugInMetadata ("PlugINForYYYFeature")]
  public class YYYFeatureProvider : ...
  {
   ...
  }

And now you can query the metadata when you are composing the container with the Metadata property of the composed object.

---

Please mark posts as answers/helpful if it answers your question

Answer 2

Thanks, Andoni.

 

I've found more complete explanation here:

http://blogs.microsoft.co.il/blogs/bnaya/archive/2010/01/20/mef-for-beginner-metadata-part-8.aspx