Prefixing interfaces with I?

I am currently reading "Clean Code" By Rober Martin (UncleBob), and generally loving the musings of UncleBob. However, I got a bit confused, when I read that he avoids prefixing interfaces like "IPerson". He states "I don't want my users knowing that I'm handing them an interface".

Thinking in TDD/injection perspective, I will always be very interested in telling the "users" of my classes that I am handing on an interface. The primary reason is that I consider Interfaces contracts between the different "agents" of a system. An agent working with one corner of my system, should not know the concrete implementation of another agents work; they should only exchange contracts, and expect the contracts to be fulfilled without knowing how. The other, but also very important, reason is that an interface can be mocked fully, and thus making unit-testing much easier. There are limits to how much you can mock on a concrete class.

Therefore, I prefer to visualize that I am indeed handing on an interface... or taking an interface as argument. But since UncleBob is a heavyweight champ in our community, and I am just another flyweigth desk jockey, I would like to know if I am missing something.

Is it wrong for me to insist on I's in interfaces??


  • There are a number of conventions in Java and C# that we have grown comfortable with; but that are backwards. For example, the convention of putting private variables at the top of each class is quite silly from a technical point of view. The most important things about a class are its public methods. The least important things, the things we hide behind a privacy barrier, are the instance variables. So why would we put them at the top?

    The "I" in front of interfaces is another backwards convention. When you are passed a reference to an object, you should expect it to be an interface. Interfaces should be the default; so there is no point in doing something extra, like using an I prefix, to announce that you are doing what everyone expects you to do. It would be better (though still wrong) if we reserved a special marker for the exceptional condition of passing a concrete class.

    Another problem with using I, is that (oddly) we use it to communicate the implementation decision of using an interface. Usually we don't want implementation decisions expressed so loudly, because that makes them hard to change. Consider, for example, what might happen if you decided that IFoo really ought to be an abstract class instead of an interface. Should you change the name to Foo or CFoo, or ACFoo?

    I can hear the wheels turning in your head. You are thinking: "Yeah, but interfaces have a special place in the language, and so it's reasonable to mark them with a special naming convention." That's true. But integers also have a special place in the language, and we don't mark them (any more). Besides, ask yourself this: why do interfaces have a special place in the language?

    The whole idea behind interfaces in Java and C# was a cop-out. The language designers could have just used abstract classes, but they were worried about the difficulties of implementing multiple inheritance. So they made a back-room deal with themselves. They invented an artificial construct (i.e. interfaces) that would provide some of the power of multiple inheritance, and they constrained normal classes to single inheritance.

    This was one of the worst decision the language designers made. They invented a new and heavyweight syntax element in order to exclude a useful and powerful (albeit controversial) language feature. Interfaces were not invented to enable, they were invented to disable. Interfaces are a hack placed in the language by designers who didn't want to solve the harder problem of MI. So when you use the I prefix, you are putting a big spotlight on one of the largest hacks in language history.

    The next time you write a function signature like this:

    public void myFunction(IFoo foo) {...}

    Ask yourself this: "Why do I want to know that the author of IFoo used the word 'interface'? What difference does it make to me whether he used 'interface' or 'class' or even 'struct'? That's his business, not mine! So why is he forcing me to know his business by putting this great big I in front of his type name? Why doesn't he zip his declarations up and keep his privates out of my face?"