When to pass a class and when to pass an interface

[tshad]

The is always been a little confusing. I understand how they work but not always sure when to use one over another.

For example:

http://www.codeproject.com/Articles/14642/Model-View-Presenter-with-ASP-NET

In a sample project I was playing with on MVP, controls are passed by interface to the presenter. But the presenter object is passed directly to the control.

Why would that be?

Why not just pass the control to the presenter?

Or have an interface for the presenter and pass that to the object?

Thanks,

Tom