Let’s begin by implementing the Dependency Inversion Principle: our class should depend on an abstraction (i.e. interface).
An interface does not define any method bodies, but it forces any class implementing the interface to define them. If we make our class depend on an interface, then the dependency’s method body can change without breaking our class.
In our example, if we use an ISpeaker interface, we can make changes to the LoudSpeaker class as long as it still implements the interface. If it does, we don’t have to touch the Trainer class to handle the changes.
Instructions
We’ve provided the LoudSpeaker and QuietSpeaker classes and the ISpeaker interface. Both classes implement the interface.
In Trainer.cs, make the speaker field of type ISpeaker.
In the constructor in Trainer.cs, instead of instantiating a LoudSpeaker object, instantiate a QuietSpeaker object. It also has a Speak() method, which sets the argument to lower case and surrounds it with ellipses (...).
The Trainer behavior may change (as expected) but we didn’t have to change much of the Trainer code because it relies on the interface ISpeaker, not the specific LoudSpeaker or QuietSpeaker class.