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OO-Design Part 1: Increasing Reuse With Noun Interfaces

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Part 1 of the series OO-Design

The SOLID patterns are not enough to design a reusable and testable code base. Two concrete additions can significantly improve your codebase, while maintaining the SOLID principals.

The suggestions here do not run counter to traditional SOLID designs, they enhance them. SOLID is a fine place to start, but it lacks a critical half of good design: polymorphic data. We’ve found polymorphic data is far more common than polymorphic behavior, so programs designed without that concept often are far larger than they need to be.

Appropriate application of these concepts will reduce code and allow for easier unit testing


For this post, let’s distinguish between verb classes and noun classes. Unit testing often drives us to separate classes into data structures (the nouns) or behavior classes, (the verbs).

  • Noun class - has fields and properties filled with data, perhaps from an ORM.
    • Example: A Contact class with a name, billing and shipping addresses, a birthday, and a credit card.
  • Verb class - has functions and methods. Verb classes might have fields or properties, but usually those only contain other verb classes that are needed to compose work.
    • Example: A CustomerBiller class that creates an invoice and sends it to an address. Only needs a billing and shipping address and an amount.
  • Noun interface - an interface put on a noun class. Used to allow multiple nouns to be passed to a single verb. A very common use-case in business applications.
    • Example: ICustomer interface with a billing and shipping address only. Can be used by Contact, Company, Government, or any other entity that might like to buy something.
    • Allows a verb to operate on many types of data
  • Verb interface - an interface put on a verb class. Allows polymorphic behavior. Less common than noun interfaces.
    • Example: IChargeCustomer interface that allows different kinds of charging: one that generates invoices, one that charges credit cards, etc.
    • Allows several types of verbs to be grouped together

Two Changes

Two suggestions will guide your codebase to be easier to reuse and test.

  • Extract Noun Interfaces - Work to determine any missing noun interfaces. This inversion allows for easier reuse. Any data structure that “fits” can re-use that behavior.
  • Rely on Verb Functions Not Verb Interfaces - Replace Dependency Injected verb interfaces with function signatures, explained in the SimpleMock guide. By relying on the function signature as the default unit of abstraction, we remove test-only interfaces. This allows the dependent code to have only the functions it needs, rather than everything from the interface. When you really need polymorphic behavior, use a verb interface. Only use verb interfaces for polymorphic behavior, not for unit test mocking.

Here is an example of the patterns in C#, but it also works in Java:

// Verb Class
public class Notifier {
    public void Broadcast(string type, Type from, int id) { /* Broadcast code here ...*/ }

// Noun Interface
public interface SalesLead {
    boolean IsCustomer {get; set;}
    DateTime ConversionDate {set;}
    int Id {get;}
    Type From {get;}

public class User : SalesLead { /* User code here ... */ }
public class Company : SalesLead { /* Contact code here ... */ }
public class Government : SalesLead { /* Government code here ... */ }

// Verb Class
public class SalesRepresentative {
    // Rely on Verb Functions Not Verb Interfaces
    internal Action<string, Type, int> broadcast = new Notifier().Broadcast;

    // Extract Noun Interfaces
    public void ConvertToCustomer(SalesLead lead) {
      if(lead.IsCustomer) {
      lead.IsCustomer = true;
      lead.ConversionDate = DateTime.Now;
      broadcast("CustomerConverted", lead.From, lead.Id);

While this inversion of nouns and verbs seems counter to traditional OOP advice, it is actually a very object-oriented design. Noun Interfaces allow for polymorphic nouns. Polymorphic nouns allow are better suited to model a domain.

In our example, a User should not know how to convert itself to a Customer. A User would not know they had to broadcast their updated status, or that the date is important to record. To prove it models the domain more effectively, notice how it even follows a more English pattern: “tell the Sales Representative to convert this User”. This is much closer than the typical: “User convert yourself To Customer”.

Later, when we want to convert an Employee, a Company, or a Government, we know what sort of interface is required to make them into a SalesLead that can be converted. By relying on the interface, we are able to restrict SalesRepresentative to only have access to the data necessary to convert.

Case Study

We recently examined a 1.3 million line codebase used to collect and track interest rates and payments from a user. We found only four uses of verb interfaces for polymorphic behavior. On the other hand, there were hundreds of noun interfaces used to allow multiple shapes of data to be acted on by the same function.

An obvious example is IHaveIdentity

public interface IHaveIdentity {
    Guid Id {get;}
    string Name {get;}

This tiny, simple noun interface enables incredible re-use. We found the IHaveIdentity noun interface on many of our noun classes. It is used by hundreds of verb classes throughout the codebase. Verbs like MakeDropDownList, QueryById, QueryNamesMatching, and CreateAuditTrail. Those verb classes were easily reused, because any new noun class that fits the noun interface works automatically! If we had tried to put interfaces on those verbs, we’d have a lot of classes with no behavior other than just delegating to another class.


These patterns enable significant reuse of functionality. By defaulting to noun interfaces, you will find most of your verb classes can be re-used without any change. When you need actual polymorphic behavior, use a verb interface. For the more common case of unit test behavior replacement, the SimpleMock pattern makes for an easy way to keep your codebase clean and best model the domain.

Next in the series: OO-Design Part 2: Anemic Domain Models Are Healthy


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