Michel,

My 2 cents on this.
Try to use the decorator pattern by creating a custom class. I dont know if you care or not about the type of your custom class but I would do it this way: create a custom class that implements FileDialog(abstract class, to preserve type) and provide a private member of type OpenFileDialog to which you delegate to implement the FileDialog interfaces. Also provide a constructor that takes in an OpenFileDialog.

Vlad