This IConvention is an interesting one: “Let’s prefix all interfaces with a capital I.” Who thought of this? Why did they think it was a good idea?
Let’s say I want to implement the Command pattern in C#. I create the following interface…and to be true the fathers of C# I use the IConvention.
In my application I can now write Code like this.
Clearly, every object in the
IList of commands is an
ICommand is an
Interface because it starts with an I. That’s important to know because if it
didn’t have the I, it might be an abstract class or even a concrete class.
And if that were the case then…well…hmmm…it doesn’t really matter. I could
call the class
ICommand or I could call it
Command. From the point of view of the client code, maybe
Command it’s an interface or maybe it’s not.
So if it’s all the same, I might as well submit to the convention and call it
Wrong! Prefixing interfaces with I is a mistake and this is why…
Assume that I really do need a Command abstraction. Should it be an interface or an abstract class?
I can’t think of any logic to put in the base class and the Dependancy
Inversion Principles says interfaces are preferred. So I’ll make it an
interface and follow the IConvention naming it
After a while there are a dozen or so implementations of
ICommand in the application. New implementations are popping up
all the time. One day I realize that the application needs to know whether an
ICommand has executed or not.
Since this affects all implementations of
ICommand I can add
another method to the interface:
However, it’s not long before I realize that all the derived classes
Executed property with the exact same code.
They also need a boolean field. So to avoid duplicate code, I’ll use Template Method like so:
ICommand is not longer an interface. I can’t just
leave that I sitting there. Clients will think it’s still an interface. So
now I need to rename the class to
Command without the I.
!-ReSharper-! will help me there.
But now I’ve got the
Command class in a file named
ICommand.cs. Renaming the file is a bit more challenging since
it has to be changed in source control as well.
So I rename the file in the subversion repository, then remove the
ICommand.cs file from the Visual Studio project, and finally
add the new
Command.cs file to the project…whew. That silly I
sure causes a good deal of hurt.
I’ve stumble over this scenario more times than I care to mention. Experience has taught me that prefixing interfaces with an I is a choice that will come back and haunt me. So I don’t do it.
Here’s my dilema. Maybe you can help me. I’ve been translating Unclebob’s Agile Software Development book into C#. In the code examples I have heeded my experience and created interfaces without the I prefix. Reviewers don’t like this.
They keep telling me that I need to add the I prefix. I’m torn. I feel in my heart of hearts that using the IConvention does more harm than good. Yet, readers of the book will be familiar with the IConvention and may be confused by examples that don’t use it. What’s the right thing to do?