Export and Import Delegate using Type with MEF

Question

We are currently using MEF to import and export delegates by name.

public delegate IThing ThingGenerator();

[Export("ThingGenerator")]
public IThing MyThingGenerator() { ... }

[ImportMany("ThingGenerator")]
public IEnumerable<ThingGenerator> ThingGenerators { get; set; }

For maintainability, we would like to import and export using types. However, we cannot seem to make this work.

[Export(typeof(ThingGenerator))]
public IThing MyThingGenerator() { ... }

[ImportMany(typeof(ThingGenerator))]
public IEnumerable<ThingGenerator> ThingGenerators { get; set; }

In some forms we get errors stating the exported function doesn't match the delegate type. In other scenarios the types don't seem to get exported. Has anyone done this successfully?

Answer 1

You can actually import and export by type, only in your case you should try exporting and importing a typeof(IThing) instead of typeof(ThingGenerator).

Usually the exported parts implement some interface (which can be an empty interface, which acts like a tag), so you can identify what you are importing, but it works also with types.

Here is a nice video tutorial to help you get started with MEF (it's in SilverLight but works also for WPF, and should also work for Winforms) Part 1 Part 2

Check the part 2, it may be specially useful when he creates the custom export attributes with metadata.

Hope this helps, regards