c# - Good Case For Interfaces

Question

I work at a company where some require justification for the use of an Interface in our code (Visual Studio C# 3.5).

I would like to ask for an Iron Clad reasoning that interfaces are required for. (My goal is to PROVE that interfaces are a normal part of programming.)

I don't need convincing, I just need a good argument to use in the convincing of others.

The kind of argument I am looking for is fact based, not comparison based (ie "because the .NET library uses them" is comparison based.)

The argument against them is thus: If a class is properly setup (with its public and private members) then an interface is just extra overhead because those that use the class are restricted to public members. If you need to have an interface that is implemented by more than 1 class then just setup inheritance/polymorphism.

Answer 1

Code decoupling. By programming to interfaces you decouple the code using the interface from the code implementing the interface. This allows you to change the implementation without having to refactor all of the code using it. This works in conjunction with inheritance/polymorphism, allowing you to use any of a number of possible implementations interchangeably.

Mocking and unit testing. Mocking frameworks are most easily used when the methods are virtual, which you get by default with interfaces. This is actually the biggest reason why I create interfaces.

Defining behavior that may apply to many different classes that allows them to be used interchangeably, even when there isn't a relationship (other than the defined behavior) between the classes. For example, a Horse and a Bicycle class may both have a Ride method. You can define an interface IRideable that defines the Ride behavior and any class that uses this behavior can use either a Horse or Bicycle object without forcing an unnatural inheritance between them.