Access interface methods without referring the class

Question

Say I have an Interface like this in a project called \"Interface\":

public interface TestInterface
{
    string Operation();
}

and class which

Answer 1

You have interface so that your app can have plug in's..

So basically you share your Interface dll to anyone who wants to make a plugin app for your app and then you can cast that new plugin class to the interface and invoke methods on it..

If you dont cast the class to the interface,how on earth are you going to make the plugin class work for your app..