Question

The .NET standard of prefixing an interface name with an I seems to be becoming widespread and isn’t just limited to .NET any more. I have come across a lot of Java code that uses this convention (so it wouldn’t surprise me if Java used it before C# did). Also Flex uses it, and so on. The placing of an I at the start of the name smacks of Hungarian notation though and so I’m uncomfortable with using it.

So the question is, is there an alternative way of denoting that Something is an interface, rather than a class and is there any need to denote it like this anyway. Or is it a case its become a standard and so I should just accept it and stop trying to stir up ‘religious wars’ by suggesting it be done differently?

Answer 1

From the Framework Design Guidelines book:

Interfaces representing roots of a hierarchy (e.g. IList) should also use nouns or noun phrases. Interfaces representing capabilities should use adjectives and adjective phrases (e.g. IComparable, IFormattable).

Also, from the annotations on interface naming:

KRZYSZTOF CWALINA: One of the few prefixes used is “I” for interfaces (as in ICollection), but that is for historical reasons. In retrospect, I think it would have been better to use regular type names. In a majority of the cases developers don’t care that something is an interface and not an abstract class, for example.

BRAD ABRAMS: On the other hand, the “I” prefix on interfaces is a clear recognition of the influence of COM (and Java) on the .NET Framework. COM popularized, even institutionalized, the notation that interfaces begin with “I.” Although we discussed diverging from this historic pattern we decided to carry forward the pattern as so many of our users were already familiar with COM.

JEFFREY RICHTER: Personally, I like the “I” prefix and I wish we had more stuff like this. Little one-character prefixes go a long way toward keeping code terse and yet descriptive. As I said earlier, I use prefixes for my private type fields because I find this very useful.

BRENT RECTOR Note: this is really another application of Hungarian notation (though one without the disadvantages of the notation’s use in variable names).

It has very much become a widely adopted standard, and while it is a form of Hungarian, as Brent states, it doesn’t suffer from the disadvantages of using Hungarian notation in variable names.