Ceci n'est pas une interface

Likewise, extracting all the public functions from a class and surrounding them with

public interface ISomeClassName { 
 
}

Does not make an interface.

One of the terrible failings of Resharper is the Extract Interface dialog has a button for All Public functions.
This encourages this frankly misguided behaviour.


IEnumerable is an interface. It defines one thing and it defines it will. Wander into any code base and if you have a decent environment, you should be able to do some form of "Find Usages" which will show classes that implement IEnumerable. There will be quite a few. That's because it's REALLY useful.

If I take a Binary Tree, there may be many different functions, allowing me to add Ranges, delete Ranges, create Views, Find Items, Get Counts, etc etc. How do I benefit from extracting the entire public interface to create an IBinaryTree. It would likely turn out that the interface has so much in it that the only thing which can implement it is another Binary Tree.

Maybe I have a lump of code which wants a Binary Tree, but in fact it really only cares about a collection which has been sorted into a given order. SortedCollection (or if you must ISortedCollection) might be a perfectly reasonable interface.


When we look at the members that my code uses, it comes down to iterating through the list...


public interface ISortedCollection { 
   IEnumerator GetEnumerator();
}


So in fact my code just needs an IEnumerable. It does not need an IBinaryTree at all.*


If I pass in an IEumerable then I can actually use a List for my code after calling List.Sort().


Now we are down to what an interface is. An interface specifies some functionality that I want to use. Ideally the minimal functionality. It is so much less than a list of public functions in an existing class, and in being less, it is more.


Now my Unit Tests can be far simpler, I no longer need to have test which depend on the binary tree. I can use a simple Array which I define in sorted order. 


Small interfaces reduce unnecessary binding. 


* You could argue that a specific interface for Sorted data would allow me to catch, at compile time, the problem of passing unsorted data down here. This is true, but that's a whole other post.