Mock Objects

TypeMock is a tool for creating Mock Objects for use in Unit Testing.

It says in the list of features...
"Mocking takes place on the fly, working on your production assemblies."

One of the benefits is that it...
"Saves time by eliminating refactoring and restructuring of your code, just to make it testable."

There is an explanation of how it works, and this explains that you do not have to change your code to make it testable.