Header Interfaces vs Role Interfaces

In some languages such as C++ you must define header interfaces. These constructs define how a public type is exposed in terms of its public interface. Other languages take a different route. C# or Java do not require headers but they are still very much in circulation. This unfortunately brings along some unwanted side effects.

Header Interfaces

Header interfaces are a one to one mapping of public methods that match the type it is defining. In other words, they are recreating the overhead of headers in languages that do not need them.

  • Header interfaces tend to break the Interface Segregation Principle.
  • Harder to switch objects via DI as you are forced to implement all members even if you do not use the whole interface.
  • Prone to breakages as the one to one mapping means any change is breaking.

Role Interfaces

Role interfaces define the role an object plays. Due to various roles having different responsibilities they are usually grouped by functionality. Role interfaces are usually combined with composition or interface inheritance.

The role of a Developer has now been introduced. This is a separate concept from the rest of the object.

  • Easier to follow the Interface Segregation Principle.
  • Closely related to the Liskov Substitution Principle - no need for partial implementations.
  • Less chance of breaking changes - interfaces can be removed or added easily.
  • Reduced scope - anything that fulfils the role of Developer can be provided as an argument.
  • DI frameworks may take more configuration if role interfaces are used. This may explain the bias towards header interfaces.