Question

I’m using MEF (C# 4.0) for my plugin system. At times it is necessary to get a list of the derived types of all plugins (mainly for XML deserialization). Can this be done without instantiating the plugin?

This works but requires instantiation:

var cat = new DirectoryCatalog(path, "*.dll");
var container = new CompositionContainer(cat);
container.ComposeParts(this);
foreach (var plugin in Plugins)
{
    // Would be better if this could be done via Metadata!
    DoStuff(plugin.Value.GetType());
}
// ...
[ImportMany]
public Lazy<PluginBase, IPluginMetadata>[] Plugins
{
    get;
    private set;
}

Q: Is it possible to accomplish this via some ExportAttribute, or some other technique?

Thanks.

Answer 1

MEF cannot provide this information by itself. To understand why, consider the following export via a property:

[Export(typeof(PluginBase))]
public PluginBase MyPlugin
{
    get
    {
        if (someCondition)
        {
           return new FooPlugin();
        }
        else
        {
           return new BarPlugin();
        }
    }
}

However, you can still include the type in the export’s metadata (as below or via a custom export attribute that includes the metadata):

[Export(typeof(PluginBase))]
[ExportMetadata("Type", typeof(Foo))]
public class Foo : PluginBase
{
}

and add a IPluginMetadata.Type member.