Why prefix C# interface names with an “I”

Question

What is the rationale behind this naming convention?

I don't see any benefit. The extra prefix just pollutes the API.

My thinking is inline with Konrad's response to this related question; the chosen answer of which is mostly what I am asking for here.

Answer 1

Why isn't this a function of syntactical highlighting instead of Hungarian notation? Why doesn't the IDE just italicize identifiers that refer to interfaces if it's so important to distinguish between classes and interfaces. I hate putting "" or "m" before fields, "C" before classes, etc. Even worse, it encourages programmers write really bad APIs such as:

public class List : IList

instead of a more reasonable:

public class LinkedList : List
public class ArrayList : List
public class HashList : List

Even the .NET common class authors fell into this trap. A class name should NEVER be the name of the interface with just the "I" removed. The class name should always tell the user how the class differs from other possible implementations of the interface(s). I vote for dropping the stupid "I" for that reason alone.

Also, when I use intellisense, I want to group things by functional area, not whether it's a class or interface. I never think, "I need an interface, not a class." I always think, "I need something that does X".

Answer 2

It makes it easily identifiable as an interface.

Answer 3

It is just a naming convention so everybody would know if it is an interface or something else it is not mandatory nor by the compiler nor by the IDE but All the interfaces i saw in my entire life starts with the letter I

Answer 4

Firstly I believe prefixing with I then description is wrong because it means implementations can have a shorter name. IList (intf) -> List. This is an anti-pattern as we all know we should be using intf and probably only concrete types when creating. Don't flame me this is a generalization but the premise is intf only impl rarely. The implementation name should describe how it's implementing the intf or what it's doing. Think intf List, LinkedList which implements List using a linked list. Who cares if it's longer as we should be using List most of the time. If we have a class implementing many intf we probably should not include all the intf as the shadows the real purpose of the class. IN the case something removed without the intf makes sense. Eg ppl call me by name not Person, Sibling, developer etc using my name is the best most descriptive name. I suppose if a class is impl a simple intf then call it Default Intf which makes it on ious this is the default implementation of Intf. Names of classes sHould in the end be human readable and almost a short phrase describing their purpose. Prefix codes etc are not great as we communicate with words not codes. Computers do t cAre what classes are called so why remains is that we name things so the names help us and our colleagues.

Answer 5

I also like it cause I can read it as "I verb-behavior" as in "ICanSave" or "IDoDoubleEntry" etc...

Answer 6

The "I" convention seems to be an old convention that wouldn't be relevant today. Current code editor provides lots of insight about the type you are using, so arguing that It's easier to identify the interface is like asking for a namespace to be prefixed by a "N" because you want to be sure that you will not confound it with a concrete class (prefix with a "C"?).

A convention doesn't mean that It's a good convention. Sometimes, It's just because people get to use it...

Take for example the C# documentation generator: It doesn't care about it... if your interface is not prefixed with a "I" you will still see your interface in the interface part of your documentation. Do you really think that having a prefix "I" for all your interfaces inside the interface section of your documentation is a relevant information and help you to better identify interfaces?