.NET developers should be familiar with the standard access modifiers (public, protected, internal, private). However, in loosely coupled code we can regard interface implementations as a fifth access modifier. This concept was originally introduced to me by Udi Dahan the only time I've had the pleasure of meeting him. That was many years ago and while I didn't grok it back then, I've subsequently come to appreciate it quite a lot.

Although I can't take credit for the idea, I've never seen it described, and it really deserves to be.

The basic idea is simple:

If a consumer respects the Liskov Substitution Principle (LSP), the only visible members are those belonging to the interface. Thus, the interface represents a dimension of visibility.

As an example, consider this simple interface from AutoFixture:

public interface ISpecimenContext
    object Resolve(object request);

A well-behaved consumer can only invoke the Resolve method even though an implementation may have additional public members:

public class SpecimenContext : ISpecimenContext
    private readonly ISpecimenBuilder builder;
    public SpecimenContext(ISpecimenBuilder builder)
        if (builder == null)
            throw new ArgumentNullException("builder");
        this.builder = builder;
    public ISpecimenBuilder Builder
        get { return this.builder; }
    #region ISpecimenContext Members
    public object Resolve(object request)
        return this.Builder.Create(request, this);

Even though the SpecimenContext class defines the Builder property, as well as a public constructor, any consumer respecting the LSP will only see the Resolve method.

In fact, the Builder property on the SpecimenContext class mostly exists to support unit testing because I sometimes need to assert that a given instance of SpecimenContext contains the expected ISpecimenBuilder. This doesn't break encapsulation since the Builder is exposed as a read-only property, and it more importantly doesn't pollute the API.

To support unit testing (and whichever other clients might be interested in the encapsulated ISpecimenBuilder) we have a public property that follows all framework design guidelines. However, it's essentially an implementation detail, so it's not visible via the ISpecimenContext interface.

When writing loosely coupled code, I've increasingly begun to see the interfaces as the real API. Most other (even public) members are pure implementation details. If the members are public, I still demand that they follow the framework design guidelines, but I don't consider them parts of the API. It's a very important distinction.

The interfaces define the bulk of an application's API. Most other types and members are implementation details.

An important corollary is that constructors are implementation details too, since they can never by part of any interfaces.

In that sense we can regard interfaces as a fifth access modifier - perhaps even the most important one.


Taking this a step further, I wonder if it would make sense to prefer explicit interface implementations, separating the interface from any additional functionality in the type in a way so that you could not invoke Resolve on a SpecimenContext directly, but only on objects that are typed as ISpecimenContext? I am not sure whether I like that idea or not, but it would help enforce the logical separation between the interface and its implementation.
2011-02-28 14:12 UTC

Thanks for this post.

I have to disagree.

Any public type or member becomes a part of your API. Consumer code will possibly link to them, they will become a part of your legacy, and you will need to provide backward-compatibility to any part of the public API.

If you don't want a type, constructor or method to be part of the API, you have to make this it internal.

Anyone is free to expose its entire API as a set of interfaces (as in COM), but the only thing that makes a semantic part of the API is whether it is public or not. Making a constructor public and then telling "oh you know, you can't use it in your code, it's an implementation detail" breaks the fundamental principles of object-oriented programming.

Interfaces exist to allow for late binding between the consumer and the provider of a set of semantics. Visibility exits to segregate contractual semantics from implementations details. These concepts are related but actually orthogonal: you can have internal and public interfaces too.

Bottom line: what makes your public API is the 'public' keyword.


2011-02-28 16:32 UTC
I must admit that I haven't fully thought through whether or not interfaces ought to be explicit or implicit, but my gut feeling is that implicit implementations (as shown in the example) are fine.

The thing is: when I unit test the concrete implementations I often tend to declare the System Under Test (SUT) as the concrete type because I want to exercise an interface member, but verify against a concrete member. If I were to use an explicit implementation this would require a cast in each test. If there was an indisputable benefit to be derived from explicit implementations I wouldn't consider this a valid argument, but in the absence of such, I'd tend towards making unit testing as easy as possible.

I think the argument that explicit interfaces help enforce the logical separation is valid, but not by a sufficiently high degree...
2011-02-28 20:49 UTC

Thank you for your comment.

Please note that the context of the blog post is loosely coupled code. When composing classes using Dependency Injection (DI), consumers will never see anything else than the interface members. Thus, the API from which we compose an application contains mainly the interfaces. These are the moving parts from which we can define interaction.

I agree that if you consider only a single concrete type at a time, all public (and protected) members are part of the API of that type, but that's not what I'm talking about. In DI it's implicitly discouraged to invoke public constructors of any Services because once you do that, you tightly couple a consumer to a specific implementation.
2011-02-28 20:57 UTC
I see from AutoMapper and the FDG references you're using the ILikePrefixes convention. I've been around the houses with it and have read most of the 'debates' on SO about it, but I keep coming back to the Growing Object Oriented Software Guided by Tests sidebar that says I is an antipattern and agreeing with it, even in .NET.

Any comment on the above? Ever tried going I-less?

Does it have any influence/overlap with your thoughts in the [excellent food-for-thought] article?
2011-03-01 01:15 UTC

Thank you for writing.

In my opinion, the most important goal for coding conventions is to reduce friction when reading (and writing) code. Thus, I generally try to write Clean Code, but another important guide is the POLA. When it comes to the debate around the Hungarian I in interface names, I think that the POLA weighs heavier than the strictly logical argument against it.

However illogical it is, (close to) 10 years of convention causes us to expect the I to be there; when it's not, it causes unnecessary friction.
2011-03-01 08:42 UTC
Harry Dev
If the "Builder" property is only used for testing why not use the "internal" access modifier instead, and allow the testing assembly access to this using the "[assembly: InternalsVisibleTo("XXX.Test")]" attribute?

If the "Builder" property really is an implementation detail there should be no reason to expose it. Although, as you say it does not pollute much since it is a readonly property.
2011-03-02 09:51 UTC
First and foremost I consider the InternalsVisibleTo feature an abomination. We should only unit test the public members of our code.

As I wrote in another answer on this page, the Builder property is most certainly part of the public API of the concrete SpecimenContext class. It doesn't pollute the class in any way because it's an integral part of what the SpecimenContext class does.

There's no reason to expect that the Builder property is used only for testing. It's true that it was driven into existence by TDD, but it makes sense as part of the class' API and is available to other potential consumers. In the rare cases that a third-party consumer wants to use the SpecimenContext directly, it can access the Builder property as well. It wouldn't be able to do that if the property was internal.

However, the Builder property in no way belongs on the interface because that would be a leaky abstraction, so while it doesn't pollute the class, it would pollute the interface.
2011-03-02 10:35 UTC
Ruben, I have to agree with Mark. The I prefix is a convention that is expected and I doubt anyone really considers it hungarian notation (even though it fits the definition).

If you're providing an API and there is a method expecting 'SomeType' most people will attempt to instantiate an instance of SomeType at which point they will receive red squiggles and will then have to do some investigation to determine what's going on. Even if it's only a matter of 5 seconds to figure it out, you've violated POLA, caused the developer to become confused because he now has to solve yet another problem loses momentum. There are many other potential confusing scenarios that can arise from not clearly marking an interface as such.

No one expects to see strings prefixed with 'str' but clearly identifying an interface is expected. There many "rules" that should be followed but with all rules, there are exceptions. Most of them are for the sake of developers. It takes me no time to see any type and recognize it as an interface which I already know I can do this or I can do that, because it's prefixed with an I. but unless i'm already familiar with a framework/API that does not use the I convention, I would need to spend time learning and trial/error. Waste time.

If for no other reason, then do it because Microsoft uses the I convention in the .NET framework and that is what .NET developers expect, even if it is "incorrect".
2011-03-02 16:29 UTC

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Monday, 28 February 2011 13:19:04 UTC


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