If I understand your requirement correctly, you may want to have a look at MEF - Microsoft's new Managed Extensibilty Framework.
It has a flexible add-on model using Import and Export attributes.
According to Microsoft: Using MEF, .NET applications can make the shift from being statically compiled to dynamically composed. If you are building extensible applications, extensible frameworks and application extensions, then MEF is for you
Here is a link to codeplex: Managed Extensibility Framework
And here is an article on MEF by it Project Manager: Building Composable Apps in .NET 4 with the Managed Extensibility Framework
I hope this is helpful.
Dan McMillan, May 12, 2010